text:
\section{Introduction}
\label{intro}

Time-domain astronomy has developed rapidly in the past two decades.
The great progress has been driven by the advent of new instruments and
facilities dedicated to wide-field, deep and fast surveys, such as
Catalina Real-Time Survey (CRTS; \citealt{Drake2009}),
Palomar Transient Factory (PTF/iPTF; \citealt{Law2009}),
Panoramic Survey Telescope and Rapid Response System (Pan-STARRS or PS;
\citealt{Kaiser2004}; \citealt{Chambers2016}),
All Sky Automated Survey for SuperNovae (ASASSN; \citealt{Shappee2014}),
the Asteroid Terrestrial-impact Last Alert System (ATLAS; \citealt{Tonry2018})
and Zwicky Transient Facility (ZTF; \citealt{Graham2019}).
These remarkable projects have gradually make it possible to image
the entire sky every a few days and process the data in real time.
It is believed that
LSST will produce an unprecedented time-domain survey in the 2020s.

Supernova (SNe) are absolutely the major class of extragalactic transients
and its number has experienced explosive growth as expected
with aid of those above-mentioned surveys.
In 2018, there were more than 7,000 reported supernovae
\footnote{http://rochesterastronomy.org/sn2018/index.html}
although most have not been spectroscopically confirmed.
In addition to increasing in number of SNe, these surveys
have also accelerated the discovery
and in-depth studies of peculiar SNe, such as superluminous supernova
(SLSNe; e.g., ASASSN-15lh, \citealt{Dong2016};
see \citealt{Gal-Yam2019} as a recent review),
exotic SNe with multiple peaks (e.g., iPTF14hls, \citealt{Arcavi2017})
and even gravitationally lensed SNe (iPTF16geu, \citealt{Goobar2017}).
These unusual events could make great breakthrough in our understanding
of stellar explosion in extreme physical condition.

The other population of extragalactic variable sources which has aroused
great attention is the transient events associated with
the supermassive black holes (SMBHs) located in the centers of galaxies.
Among them, the tidal disruption event (TDE), in which a star is
torn apart by a SMBH's the tidal force, is of particular interest.
During the process, about half of the stellar mass may be ejected while
the rest of the stellar material is accreted on to the black hole,
producing a luminous flare of electromagnetic radiation lasting for months to
years (\citealt{Rees1988,Evans1989,Phinney1989}).
TDE discoveries require modern time-domain surveys
because their event rate is hundreds of times lower than SNe with a rate
of $10^{-4}-10^{-5}$/galaxy/year (\citealt{Wang2004,Stone2016}).
Hence, the number of TDEs  (or candidates) found to date is still very limited.
As a rare and special form of accretion, TDEs are nevertheless extremely
scientifically valuable as they offer us an ideal chance to
probe the existence, mass and spin of SMBHs in normal galaxies
(\citealt{Lu2017,Mockler2019,Pasham2019}).
Moreover, they can serve as a unique laboratory to study the dynamic process of
BH activity by witnessing the ignition and flameout of the accretion disk
(e.g., \citealt{Wevers2019}) as well as rapidly-launched jets
(e.g., \citealt{Bloom2011,Burrows2011,Mattila2018}).

Despite occasionally swallowing stars, SMBHs are believed to grow mainly by
accreting surrounding gas during their active galactic nuclei (AGN) phases.
Stochastic variability is ubiquitous in AGNs, among which a small fraction
are extremely variable with light curve featured as flaring
(e.g., \citealt{Graham2017}) or state changing (e.g., \citealt{Graham2020}) pattern.
Follow-up spectroscopic observations of the most variable AGNs suggest
they can even change their types on time scales of years,
characterized by appearance or disappearance of broad emission lines
(e.g., \citealt{Shappee2014,LaMassa2015,Runnoe2016,MacLeod2016,Yang2018}).
These objects are dubbed as changing-look (CL) AGNs, and their variation
is driven by the dramatic change of the accretion flow rather than the obscuration
(\citealt{Sheng2017,Hutsemekers2019}).
The most intriguing subclass could be the so-called "turn-on" AGNs,
which transition from a quiescent galaxy to a type 1 AGN within several months to years.
Although only few such systems have been confirmed (\citealt{Gezari2017,Yan2019}; see also \citealt{Frederick2019}),
they appear to not be extremely rare, and even one
poses challenges to canonical accretion disk theories.

In a nutshell, the known extragalactic transient sky is dominated by SNe and
transient SMBH accretion.
The investigation of latter has been facilitated recently by improved time-domain surveys.
Current large surveys have been exclusively performed in the optical band,
which are blind to transients that are either self-obscured (e.g. type~2 AGNs)
or located in the dusty regions (e.g. SNe).
Therefore, it is crucial to conduct surveys in bands free of dust obscuration.
In this sense, the mid-infrared (MIR) is the most promising band but it
was completely unexplored until the \emph{spitzer} Deep Wide-Field Survey, which
has now been incorporated into the Decadal IRAC Bo{\"o}tes Survey. This survey
is used to search for obscured SNe and to study the quasar variability in the MIR
\citep{Kozlowski2010,Kozlowski2016}
by taking advantage of the repeatedly surveyed region ($\sim9~\rm deg^2$) .
SPitzer InfraRed Intensive Transients Survey (SPIRITS, \citealt{Kasliwal2017})
is a more recent project dedicated to finding infrared luminous transients
by targeting 190 nearby galaxies, which has yielded out numerous hidden
SNe and dusty stellar outburst (\citealt{Jencson2019}).
Despite this, such projects generally focused on a small sample
of galaxies or a small sky region, so they are almost incapable
of capturing rare events like TDEs as well as "turn-on" AGNs.

Fortunately, the \emph{Wide-field Infrared Survey Explorer} (\emph{WISE})
mission (\citealt{Wright2010}) and its asteroid-characterization extension,
the Near-Earth Object Wide-field Infrared Survey Explorer Reactivation (\emph{NEOWISE})
mission (\citealt{Mainzer2014}), have opened a new window to map the MIR
dynamic sky benefited from its ideal survey mode (see \S\ref{wisemode}).
For instance, \citet{Sheng2020} have come up with an efficient method to
look for CL AGNs from MIR-variable quasars screened by
\emph{WISE}\ light curves (see also \citealt{Stern2018,Assef2018}) but
with the caveat that only "turn-off" CL AGNs can be selected because of
their parent sample of quasars.
\citet{Wang2018} used \emph{WISE} to systematically search for TDEs in
extremely variable normal galaxies and discovered IR echoes of TDEs,
immediately after the pioneering discoveries of IR echoes of TDEs
(\citealt{Jiang2016,Dou2016,vV2016}).
However, \citet{Wang2018} select variables using the variability flag
given by \emph{WISE}\ pipeline, which is on basis of data taken
between 2009 December and 2011 February.
As a result, the acquired galaxies have all entered the stage of dimming,
which make the further confirmation of their physical nature unrealistic.

To overcome the limitation from \citet{Wang2018},
we have designed a new project to search for and explore
MIR Outburst in Nearby Galaxies (MIRONG) specifically.
MIRONG uses both the public \emph{WISE}\ and \emph{NEOWISE}\ databases to search
for more recent outburst events in galaxies and enable multi-wavelength follow-up.
MIRONG may uncover a population of extragalactic transients which
have been overlooked by traditional optical surveys and ultimately
improve our understanding of the extragalactic dynamic sky.
The paper is organized as follows.
In Section 2, we describe the process of sample selection of MIRONG.
In Section 3-5, we characterize the sample from
properties of MIR light curves, event rate and luminosity function, and
their host galaxies, respectively.
We inspect the nature of MIRONG in Section 6 and discuss the
implications of our work in Section 7.
Finally, we end with a brief summary and discussion of future prospects in Section 8.
We assume a cosmology with $H_{0} =70$ km~s$^{-1}$~Mpc$^{-1}$, $\Omega_{m} =
0.3$, and $\Omega_{\Lambda} = 0.7$.

\section{Sample selection}
\label{sample_selection}

\subsection{Characteristics of WISE and NEOWISE Survey}
\label{wisemode}

\emph{WISE}\ has performed a full-sky imaging survey in four broad bandpasses
centered at 3.4, 4.6, 12 and 22~$\mu$m (labeled W1-W4) from 2010 January to August.
\emph{WISE}\ continued surveying the sky in its bluest three
bands during 2010 August and September. After that, its \emph{NEOWISE}\
hunted asteroids until 2011 February (\citealt{Mainzer2011}),
with only the W1 and W2 channels remaining operational
as the solid hydrogen cryogen used to cool the W3 and W4 instrumentation
has been depleted.
Following a 33 month hibernation period, the \emph{WISE}\ instrument recommenced survey
operations in 2013 December (\citealt{Mainzer2014}).
This post-hibernation mission is referred to as NEOWISE-Reactivation
(\emph{NEOWISE}-R) to hunt for asteroids that could pose an impact hazard to the Earth.

The \emph{WISE}\ survey strategy is very novel.
It has a field-of-view of $47^\prime\times47^\prime$ and a small (10\%)
overlap between adjacent fields in one orbit. The scan circle advances
by about 4$^\prime$ per orbit and 15 orbits can be fulfilled each day.
Therefor, for a typical sky region, the available exposures can be
segmented into a series of 1 day time intervals (referred to as visits),
with such visits occurring once every six months except for the gap
during the hibernation.
As a spontaneous outcome of the unique survey mode,
it becomes an unprecedented database to study the transient MIR sky.
There are typically 12 successive orbits covering a given source within
one day, with denser coverage toward higher ecliptic latitude
(\citealt{Wright2010,Hoffman2012}).
Such a high-frequency sampling allows us to probe the intraday MIR variability,
which was seldom explored in the past
(e.g., \citealt{Jiang2012,Jiang2018}).

The \emph{NEOWISE}\ survey is still ongoing at the time of submission of this paper.
Our sample selection is based on the \emph{WISE}\ and \emph{NEOWISE}-R data
from 2010 to the end of 2018, that is all of the available data when
we did this work.
They have yielded an average of 12 visits for each target,
so an investigation of MIR variability on year time scales
is absolutely achievable for a large sample as proved by previous works
(e.g., \citealt{Wang2018}).
We have noticed that the latest \emph{NEOWISE}\ data release (2020 March 26)
has come out just before our paper submission,
while it is a massive work to redo our work from the very beginning by including
the newly released date from 2018 December to 2019 December.
However, the newest photometry has been added in the analysis of
our final sample of MIRONG since the more complete light curves
will help us obtain more accurate quantities (e.g., peak luminosity).

\subsection{Parent Sample}

The Sloan Digital Sky Survery (SDSS) has spectroscopically observed
millions of galaxies with apparent Petrosian magnitude $r<17.77$
(\citealt{Strauss2002}).
We have checked the SDSS DR14 spectroscopic catalog (\citealt{Abolfathi2018})
and picked out those flagged with "GALAXY" class at $z<0.35$, that is 1,253,962
spectra in total. Note that some of them are repeated multi-epoch data
for same objects, there are 1,150,901 unique objects
differentiated by their celestial coordinates.
We choose spectroscopic galaxies instead of photometric because
we need to know our galaxies' redshift (z<0.35) and other properties.
The redshift cut is to ensure that the H\ensuremath{\alpha}\ region is located in the
wavelength range of SDSS spectrum, which will help us diagnose the spectral type
by emission-line ratios in the Baldwin-Phillips-Terlevich diagnostic diagram
(BPT diagram in short, \citealt{Baldwin1981}).
Second, the cut of z<0.35 will keep the dust emission within the \emph{WISE}\ bands
as it is expected to start at 2~$\mu$m.
Furthermore, the signal-to-noise ratio (S/N) of more distant galaxies are
generally too low to undertake variability studies accurately.
We designate these $\sim1$ million galaxies the parent sample.

\begin{figure*}
\epsscale{0.75}
\plotone{pattern_wiselc.eps}
\caption{
We show the different \emph{WISE}\ light curves patterns of the MIR-variable galaxies,
including the periodic oscillation (top left), long-term declination (top right),
stochastic variations (bottom left) and state transition (bottom right).
The recent flare-like patterns are presented separately in Figure~\ref{lc}.
Blue dots: W1 ($3.4\mu$m); red squares: W2 ($4.6\mu$m).
The raw single exposures are plotted in light blue and red, while the binned
data are plotted in dark blue and red.
As we explained in \S\ref{mirvar}, the case of one-year cycle
oscillation (top left) is not physical but caused by the
latent image artifact from a nearby bright star.
}
\label{pattern}
\end{figure*}

\subsection{MIR-variable Galaxies}
\label{mirvar}

We construct a sample of MIR-variable galaxies in
advance of the final selection of outbursts.
We retrieved the W1 and W2 profile-fit photometry of each galaxy
in the parent sample from the public AllWISE Multiepoch Photometry Table and
\emph{NEOWISE}-R Single Exposure (L1b) Source Table\footnote{
https://irsa.ipac.caltech.edu/cgi-bin/Gator/nph-scan?mission=irsa\&submit=Select\&projshort=WISE},
encompassing all exposures from 2010 up to 2018.
The photometry is measured by point spread function (PSF) profile fitting,
in which the PSFs have been estimated from
observations of many tens of thousands of stars.
The AllWISE Multiframe pipeline detects sources on the deep coadded atlas images,
and then measures the sources for all available single-exposure images
in all bands simultaneously while the \emph{NEOWISE}\ magnitudes are obtained by PSF-fit
to individual exposures directly.

The acquired single-exposure data are first filtered by the quality flags marked
in the catalogs. The bad data points with poor quality frame (\texttt{qi\_fact}<1),
charged particle hits (\texttt{saa\_sep}<5), scattered moon light (\texttt{moon\_masked}=1)
and artifacts (\texttt{cc\_flags}$\neq$0) have been removed.
In addition, we have also abandoned the photometry fitted with multiple PSF
components (\texttt{nb}>1 and \texttt{na}>0), that is performed when the source is fit
concurrently with other nearby detections or when a single object is split into
two components during the fitting process.
The surviving data are immediately binned every half year to increase
the S/N of the photometry, resulting in average 13 epochs for each target.
This binning strategy fits to the sampling rate of \emph{WISE}\ survey (see \S\ref{wisemode}).
We begin to perform a blind search of variable galaxies which satisfy
$\delta$W1$>$0.5 or $\delta$W2$>$0.5.
$\delta$W1 ($\delta$W2) are the difference between the maximum and minimum values
of W1 and W2 across the combined  ALLWISE+\emph{NEOWISE}\ light curves.
We have further requested $\delta$W1$>$0.3 or $\delta$W2$>$0.3
during the \emph{NEOWISE}\ phase to make sure the variability is
still obvious after 2013 December for ease of follow-up studies.
The latter condition has also excluded fake variable sources
caused by possible systematical offset between ALLWISE and \emph{NEOWISE}\ photometry.
The significance of the variability are required to be not lower than
5$\sigma$, that is
$(W?max-W?min)/\sqrt{(W?max\_{err})^2+(W?min\_{err})^2}>5$,
to ensure that the variability is still valid when taking uncertainties
into consideration.
Last, we have casted away faint sources by the cut
$W2min<14$ to obtain a magnitude limit sample.

\begin{figure*}
\epsscale{0.75}
\plotone{flareshow_wiselc.eps}
\caption{
Similar to Figure~\ref{pattern}, the \emph{WISE}\ light curves of four representative
outburst galaxies in our MIRONG sample. The estimated magnitudes at the
quiescent phase are shown with dashed blue and red lines.
The vertical green dashed lines mark the boundary of the quiescent
and outburst stage.
SDSSJ0103+1401 shows very fast rising followed by a relatively stable high state,
in contrast with SDSSJ0205+0004, which displays also fast rising but immediate
declining. The other two objects shows a slower rising, in which SDSSJ0120-0829
has manifested itself as a complete flare in terms of both rising and declining
(\citealt{Sun2020}).
}
\label{lc}
\end{figure*}

The above cut of variability amplitude has resulted in a sample consisting of
1,026 galaxies\footnote{The general information of the 1,026 variable galaxies
can be downloaded from this website:
\url{http://staff.ustc.edu.cn/~jnac/data\_public/wisevar.txt}},
allowing for us to check visually their MIR light curves one by one.
While stochastic variability is predominant,
peculiar variability patterns are also visible (see Figure~\ref{pattern}),
such as state transition, long-term declining and flare-like rising light curves.
We notice that very few of them ($11$ objects) show periodic oscillations
with a cycle of one year (see the example in the top left panel of Figure~\ref{pattern}).
These objects are affected by the latent image artifact from a nearby bright star
that appears in the preceding image in the scan\footnote{
The \emph{WISE}\ scan direction on the sky flips every 6 months as the orbit
precesses around the sky. Hence a particular point on the sky will
alternate being scanned north-to-south and south-to-north each 6 months.
This means that the bright star that is seen on the preceding image in one epoch
will be seen on the following image 6 months later.
As a consequence, the artifact affects with one year cycle.
The polluted photometry should have been flagged as "P" in
"cc\_flags" but the flag does not work well when the source is slightly
offset from the predicted position of the latent and
the expected size of the latent is poorly modeled.
}.
The photometry pipeline has often failed to flag them automatically
so that the one-year cycle variations may not be real.
In addition, we emphasize that our selection of the MIR variable
sources is conservative,
and the actual fraction of these galaxies may be much higher than $0.1\%$
as we inferred here.

\subsection{Sample of MIRONG: Galaxies with MIR Outburst}

For the purpose of this study, we further selected a subsample
of MIR outburst galaxies, which initially display a stable phase yet followed by a significant
brightening seen in the MIR light curves.
The emission in the quiescent state is served as the background that will be subtracted from the
outburst light curves.
We estimated the pre-outburst magnitudes
by adopting the median value of the data points in the quiescent state.
The beginning of outburst is set to the epoch when either W1 or W2 magnitude
shows a brightening over $3\sigma$ significance.
We adopted two criteria to qualify a flare:
(1) the maximum flux density after outburst has brightened by $>0.5$ mag in W1 or W2 with respect to the quiescent state ($\Delta$W1$>0.5$ or $\Delta$W2$>0.5$);
(2) the variability significance at the epoch of maximum flux
is larger than 5$\sigma$ in at least one band.
The above criteria are effective, with which we yield 148 MIR outburst sources.
Lastly, we caution that the \emph{WISE}\ spatial resolutions at W1 and W2 bands are of
$\sim6\arcsec$, so the photometry could suffer from the contamination of nearby sources.
Thus, we rejected those targets with companions (galaxy pairs or polluted by
foreground stars) within project distance closer than 6\arcsec\
by checking their SDSS images visually.
As a result, 137 objects are left, constituting the final outburst
sample (see basic parameters in Table~\ref{tb1_sample} and
four representative examples in Figure~\ref{lc}).
The median redshift is 0.102 (see Figure~\ref{zdis}).

To summarize, we have finally selected a well-defined sample of 137 MIR outburst
nearby galaxies (MIRONG) which display a brightening over 0.5 mag with respect to
the previous quiescent phase in the \emph{WISE}\ light curves.
These galaxies are all initially drawn from the SDSS spectroscopic catalog
with rich available information, e.g., properties
of host galaxies and nuclear activity (see \S\ref{host}),
which allows for further detailed study on the nature of transient
MIR emission in the center of galaxies.

\begin{figure}
\epsscale{1.1}
\plotone{zdis.eps}
\caption{
The histogram of redshift of our sample.
}
\label{zdis}
\end{figure}

\section{Properties of MIRONG}

\subsection{Optical Counterparts}
\label{opt}

It is natural to ask  whether these MIR flares have been detected by
various optical surveys.
First, we cross-matched our sample with (candidate)
SNe catalog (\citealt{GalYam2013})\footnote{http://rochesterastronomy.org/supernova.html,
noting that they are not all real SNe, but candidates including nuclear transients.}
discovered between 2010 and 2018 and yielded 11 objects.
In addition, we have also added four more objects that are reported in literature
but not listed in the SNe catalog, including two Gaia nuclear
transients (SDSSJ0854+1113, SDSSJ1647+3843; \citealt{Kos2018}),
one CRTS outburst event (SDSSJ1332+2036, \citealt{Drake2019}) and another
turn-on quasar discovered by iPTF (SDSSJ1554+3629, \citealt{Gezari2017}).
The basic information about the known 15 optical transients is presented
in Table~\ref{optical_match}.

Among these sources,
SDSSJ0936+0615, SDSSJ1531+3724, SDSSJ1540+0054 and SDSSJ1554+1636 are
spectroscopically confirmed SNe discovered by ASAS-SN.
SDSSJ0158-0052 is a candidate TDE discovered by Pan-STARRS and ASAS-SN,
which occurred in a Seyfert 1 galaxy with low mass black hole
(\citealt{Blanchard2017}), whose MIR flare has been successfully
explained by the dust echo of the AGN torus (\citealt{Jiang2017}).
SDSSJ1554+3629, SDSSJ0915+4814 and SDSSJ1133+6701 are newly reported turn-on AGNs
which have transferred from LINERs to quasars within a few of years
(\citealt{Gezari2017}; \citealt{Frederick2019}).
SDSSJ1620+2407 is a candidate TDE discovered by ATLAS, which shows markedly
newly-appeared broad Balmer lines and blue continuum (\citealt{Fraser2017}).
Apart from the nine studied objects above, the remaining six sources have only been alerted
by photometry without further spectroscopic observations.
Despite lack of reliable identifications, in contrast with the four SNe cases (>1\arcsec),
their close distance to the galactic center (<1\arcsec) suggests
that they are likely associated with the SMBH activity rather than stellar explosion.

For the remaining 120 galaxies for which no known optical counterparts
are matched, we have also examined the public
CRTS (\citealt{Drake2009}) and ASAS-SN (\citealt{Kochanek2017}) data.
The combined data give $V$-band light curves spanning from 2005 to 2018.
We did not find any optical flares, implying the faintness of optical
emission for most, if not all, MIR bursts galaxies.

\begin{figure}
\epsscale{1.1}
\plotone{offcenter.eps}
\caption{
The histogram of center offset distance ($\Delta d$) in unit of arcsec,
defined as the angular distance of the photometric center at the maximum epoch
and the quiescent epoch.
The $\Delta d$\ of SNe (highlighted in red) are all larger than 0\arcsec.5 in
contrast with the tiny values of other objects (median 0\arcsec.08).
}
\label{offcenter}
\end{figure}

\subsection{Constrains on the Physical Position of MIR flares}

The specific locations in the galactic region of the MIR flares
can give us strong clues to their nature. For example, an outburst
offset from the galactic center can be convincingly classified as SNe.
Otherwise the central SMBHs might be considered as the most likely origin.

The astrometry of \emph{WISE}\ catalog has been reconstructed with respect to the 2MASS
Point Source Catalog reference frame. The root mean square error of the position
is found to be less than 0.5\arcsec, for sources with SNR$>$20 in at least
one \emph{WISE}\ band, where the noise includes flux errors due to
zodiacal foreground emission, instrumental effects,
source photon statistics, and neighboring sources\footnote
{http://wise2.ipac.caltech.edu/docs/release/allsky/expsup/sec6\_4.html}.
The \emph{WISE}\ PSF profile-fit photometry we used is measured at the
intensity-weighted center, so the variation of the center position can tell us
whether or not the outburst is far away from the pre-outburst galactic center.
For instance, when an off-centered SNe has exploded and is
MIR bright, there should be a positional offset between the \emph{WISE}\ photometric center
of its host galaxy and the intensity-weighted center determined by the SNe outburst.

To test whether there significant positional offsets exist, we have calculated
the mean photometric center (RA, DEC) given by ALLWISE and \emph{NEOWISE}-R catalog
at the quiescent state and that from the maximum outburst epoch, respectively.
We defined the angular distance between them as the offset distance ($\Delta d$, in unit of arcsec).
The distribution of $\Delta d$\ is presented in Figure~\ref{offcenter}.
The majority of our galaxies show very small $\Delta d$\ with a median
value of $0\arcsec.08$.
We noticed that only the four known SNe (see \S\ref{opt}) have $\Delta d$\
larger than $0\arcsec.5$ (1.48, 0.99, 0.63 and 0.83 arcsec), which is expected
since their optical offsets are known as large as 2.93, 1.93, 1.20 and 1.00 arcsec, respectively.
It demonstrates that checking for the shift of
photometric center appears effective in recognizing a candidate outburst located
larger than 1\arcsec\ from the nucleus.

\begin{figure}
\epsscale{1.1}
\plotone{w12cm.eps}
\caption{
The color variation $\Delta\rm(W1-W2)$ versus W2 variability amplitude ($\Delta$W2)
of the MIR outburst galaxies.
The histogram of $\Delta\rm(W1-W2)$ and $\Delta$W2 are shown in the
right and bottom panels, respectively.
We have divided the whole sample into five equal-sized subsample sorted by $\Delta$W2
and plotted the median $\Delta\rm(W1-W2)$ of each subsample as red five-point stars.
The MIR variability shows a overall trend of redder when brighter.
}
\label{w12color}
\end{figure}

\begin{figure}
\epsscale{1.1}
\plotone{w2m.eps}
\caption{
The histogram of peak absolute W2 magnitude.
We plot the confirmed SNe in red and radio-loud AGNs in blue.
}
\label{w2peak}
\end{figure}

\subsection{The MIR Light Curves}

Since bulk of our MIR flares have no informed optical counterparts,
we instead try to characterize their properties by the MIR light curves themselves.
The whole sample has brightened by average 0.63 (median 0.53) and 0.96 (0.79)
magnitudes in W1 and W2, respectively, indicating that the
variability amplitude at W2 band is overall larger than W1.
In other words, the W1-W2 color displays a trend of redder-when-brighter
(RWB, see Figure~\ref{w12color}) with a median $\Delta\rm(W1-W2)=0.33$.
Only two Seyfert galaxies (SDSSJ0811+4054, SDSSJ1029+2526)
show the color changes in an opposite way.
The RWB evolution is consistent with the scenario found in CL AGNs, indicating
a higher hot dust contribution than the starlight-dominated
quiescent state when the AGN activity becomes stronger
(e.g., \citealt{Sheng2017,Yang2018}).

\begin{figure*}
\epsscale{0.9}
\plotone{dust.eps}
\caption{
The histograms of the fitted dust luminosity (panel $a$), temperature ($b$),
mass ($c$) and distance to the heating source ($d$), respectively.
All parameters are derived at the W2 maximum epoch.
The black line represents the pure blackbody case while the red shows the
fitting with dust absorption efficiency taken into consideration.
The numbers indicated in the
panels are medians and standard deviations of the distributions.
The four SNe sources have not been shown in the plots as they are
outliers in the low end whose values can be retrieved in Table~\ref{tb_dust}.
For the dust distance (panel $d$), we have also overplotted the distance given
by the rising time scale of the \emph{WISE}\ light curves (blue dashed line).
The right pointing arrows indicate that the black and red histograms
will shift by $\rm logf_d^{-1/2}$ if the real dust
covering factor ($\rm f_d$) is not unity.
}
\label{dustpar}
\end{figure*}

In order to investigate further the properties of the MIR outbursts,
we subtracted the background emission from the light curves in the flare state (Section 2.4).
The background-subtracted W2 absolute magnitude ($\mathrm{W2_m}$)
is presented in Figure~\ref{w2peak}, with the majority (95.6\%, 131 out of 137)
ranging from $-22.5$ to $-27$, corresponding to the
logarithmic monochromatic luminosity (log\ensuremath{L\mathrm{_{W2}}}) between 42 to 44.
It is conspicuous that while the low-luminosity outliers ($\mathrm{W2_m}\gtrsim-22.5$
or log\ensuremath{L\mathrm{_{W2}}}~$\lesssim42$) are all SNe, the radio-loud AGNs (see \S\ref{Jet})
dominate at the high-luminosity end.
We stress that the minimum absolute magnitudes (or maximum luminosity)
do not always tell us the true peak since some of them are still rising in the light curves.
If we naively suppose that the MIR outburst started from
the middle between the first brightening data point and the quiescent state,
median rising timescale to the (current) peak is 429 days, or 398 days in the
rest-frame of galaxies.

\subsection{Dust Properties}
\label{dust}

In this subsection, we try to acquire more physical quantities by
fitting the MIR emission with dust thermal emission scenario.
For dust grains with size distribution $N(a)$ ($a$ is the raidus of grain sphere),
density $\rho$ and absorption coefficient $Q_\nu$ at luminosity distance $d_L$,
the observed monochromatic flux at a given frequency can be calculated as below
(see also \citealt{Wang2019}).

\begin{align}
	f_\nu &=\frac{1}{4\pi d_L^2}\int_{a_{\rm min}}^{a_{\rm max}} N(a) 4\pi a^2 Q_\nu(a) \pi B_\nu(T)~da\nonumber \\
     &=\frac{3M_d}{4\rho d_L^2}\left<\frac{Q_\nu}{a}\right> B_\nu(T) \nonumber \\
\end{align}

where
\begin{equation}
\left<\frac{Q_\nu}{a}\right>=\frac{\int_{a_{min}}^{a_{max}}N(a) a^3(Q/a) da}{\int_{a_{min}}^{a_{max}}N(a) a^3 da};
\end{equation}

\begin{equation}
M_{\rm d}=\int_{a_{\rm min}}^{a_{\rm max}} N(a) \rho \frac{4}{3}\pi a^3 da
\label{bb}
\end{equation}

For simplicity, we assume that the dust grains follow a MRN size distribution
(\citealt{Mathis1977}; see also \citealt{Draine1984}) as
$N(a)\propto a^{-3.5}$ with $a_{\rm min}=0.01\mu$m, $a_{\rm max}=10\mu$m
and an average density of $\rho=2.7\rm g~cm^{-3}$ for silicate grains.

We begin the fit with blackbody model ($Q_\nu=1$).
However, the real dust emission is not perfect blackbody and the
absorption coefficient should be considered.
We adopt the silicate absorption coefficients
in \citet{Laor1993}, which gives $\left<\frac{Q_\nu}{a}\right>$=0.214
and 0.177 in W1 and W2, respectively.
The fluxes in W1 and W2 are then fitted with the modified blackbody model
to derive the dust temperature ($T_d$) and mass ($M_d$).
The luminosity is comparable to that obtained with a simple blackbody model,
while the $T_d$ is systematically lower and $M_d$ is about
one order of magnitude higher (see Figure~\ref{dustpar}).
We note that the $T_d$ are all below 1500~K, in agreement with
the suppression of the sublimation temperature of
silicate and graphite grains (\citealt{Barvainis1987,Mor2012}).
All the blackbody parameters to describe the dust emission at the epoch of luminosity maximum
can be found in Table~\ref{tb_dust} and their distributions are
presented in Figure~\ref{dustpar}.

We then attempted to estimate the distance of dust emission ($R_d$)
to the central radiation source.
By assuming spherical symmetry for the dust distribution
 the distance of dust emission can be expressed by
\begin{equation}
	R_d=\left(\frac{L_{\rm d}}{4\pi\sigma T_d^4}\right)^{1/2},
\label{rdust}
\end{equation}
As the dust distribution is uncertain, the above estimation may be
oversimplified but can be treated as a strict lower limit on the scale of dust distribution (median value of 0.06~pc).
In reality, the dust might not fully cover the central radiation source.
If the dust covering factor is $f_d$ (with unity as complete coverage),
the corresponding distance should be increased by a factor of $f_d^{-1/2}$.
Likewise, the $R_d$ for the case of modified blackbody model would be also larger,
yielding a median value of $0.20~pc$ ($f_d=1$) that is scaled by the dust mass.
Alternatively, the MIR rising time scale can be also used as a
distance indicator of the dust responsible for the peak emission.
In this way, we obtain a median value of $0.34~pc$
given the median rest-frame rising time scale of 398 days.
$f_d=0.04$ (or $f_d=0.34$) from the blackbody (or modified blackbody)
fit can generally reproduce the observed rising time scale.
Above all, we may conclude that the dust is located at the order of $0.1~pc$
(see distribution in panel $d$ of Figure~\ref{dustpar})
with covering factor at the order of 0.1.

\begin{figure}
\epsscale{1.1}
\plotone{flrate.eps}
\caption{
The accumulated redshift distribution of our sample. The curve
shows the comoving volume of SDSS footprint at the corresponding redshift
multiplied by $1.2\times10^{-6}$~$\rm Mpc^{-3}$.
}
\label{flrate}
\end{figure}

\section{Event Rate and Luminosity Function}
\label{erlf}

\subsection{Event Rate Estimation}
\label{evrate}

Regardless of their physical nature, we try to estimate the event rate of
the MIRONG selected by us with the same manner used in \citet{Wang2018}.
The accumulative number is roughly proportional to the comoving volume
up to $z\sim0.09$, then the increase
rate becomes slower and finally flattens (see Figure~\ref{flrate}).
It suggests that the sample can be taken as almost complete at $z<0.09$
but obviously under-representative at $z>0.09$ if there is no evident
redshift evolution in the event rate.
There are 61 objects at $z<0.09$, yielding a density of
$1.2\times10^{-6}~\rm Mpc^{-3}$ in the SDSS sky region.

On the other hand, we note that the SDSS main galaxy surveys are
originally designed to target galaxies with dereddened $r$-band magnitudes brighter than 17.77
(\citealt{Strauss2002}).
The cut ensures that galaxies with $r$-band absolute magnitudes of $M_r<-19.5$
are complete, which corresponds to the luminosity range for most galaxies in our sample
(Figure~\ref{hostcm}).
Considering only for galaxies with $M_r<-19.5$ at $z<0.09$ will result in
a density of $2.2\times10^{-4}~\rm gal^{-1}$.
Since our sample selection requires a brightening phase in the \emph{NEOWISE}\ stage
(spanning 5 years) and average rising timescale of about one year,
the event rate should be divided by a factor of four to obtain the rate per year.
The final event rate is about $5.4\times10^{-5}\rm~gal^{-1}~yr^{-1}$
and the corresponding density rate is
$3.0\times10^{-7}~\rm Mpc^{-3}~yr^{-1}$, that is basically
consistent with $10^{-7}~\rm Mpc^{-3}~yr^{-1}$ given by \citet{Wang2018}.

\subsection{Peak Luminosity Function}

\begin{figure}
\epsscale{1.1}
\plotone{zw2.eps}
\caption{
The host galaxy W2 absolute magnitudes (quiescent state) of our sample,
which decreases with the redshift.
The red dashed line denotes the limiting magnitude 14.5.
}
\label{zw2}
\end{figure}

We use the $V/V_{max}$ method (\citealt{Schmidt1968,Eales1993})
to calculate the MIR peak luminosity function of the outburst.
The formula is adopted as below
\begin{equation}
\Phi(L)d L = \sum_{L_i \in [L-\Delta L/2, L+\Delta L/2]} \frac{1}{V_{max} f_{comp}}.
\end{equation}
Here $V_{max}$ is the maximum comoving volume within which the galaxy
of interest is detectable at a given depth.
Taking 17.77 and 14.5 as the limiting magnitude at $r$-band and W2, respectively,
we computed the $V_{max}$ for each target.
In its simplest form, $\Phi(L)$ is the sum of $1/V_{max}$
for all objects in each luminosity bin.
The limiting magnitude of 14.5 at W2 band is determined from our cut $W2min<14$ and a
brightening amplitude of $>0.5$ mag (see \S\ref{mirvar}),
which is verified by the magnitude-redshift distribution of
the final MIRONG sample (see Figure~\ref{zw2}).
$f_{comp}$ is the completeness for sources locating in the SDSS footprint
that have been spectroscopically observed.
The spectroscopic effective area of the SDSS DR14 catalog is 9,376 $\rm deg^2$
(the full sky is 41252.96 $\rm deg^2$) and
about 92.8\% of the galaxies at $r<17.77$ have been included in the main
galaxy sample (\citealt{Laoz2018}), thus $f_{comp}=\frac{0.928\times9376}{41252.96}=0.211$.

The luminosity function in log-log space with log\ensuremath{L\mathrm{_{W2}}}\ bin 0.5
is shown in Figure~\ref{fllf}.
We estimate the statistical errors using the bootstrap method.
To this end, we generate $N=1000$ bootstrap samples, each of which consists of
objects picked randomly from the original sample.
The errors are then given by the standard deviation in the distribution of $\Phi$
measured from the bootstrap samples.
The Schechter function is typically used to characterize galaxy
luminosity functions (\citealt{Schechter1976}) in the form of
\begin{equation}
\Phi(L)d L = \left (\frac{\Phi^*}{L^*}\right ) \left (\frac{L}{L^*}\right )^\alpha e^{-L/L^*} d L  ~~,
\label{eqn:schechter}
\end{equation}
where $L$ is galaxy luminosity, $L^*$ is the characteristic luminosity
where the power-law form of the function cuts off and
the parameter $\Phi^*$ is the normalization.  We try to fit
the luminosity function at the high end with a single Schechter function,
yielding log$\Phi^*=-2.92$, log$L^*=43.50$ and $\alpha=-0.05$.
The sources at the very faint end of log\ensuremath{L\mathrm{_{W2}}}\ (<41)
are occupied by SNe and appear as a distinct population from the high end,
thus we have ignored them in the fitting.
The luminosity function drops quickly at the very high end.
The flattening at log\ensuremath{L\mathrm{_{W2}}}$\lesssim43$ could be due to the selection effect
that requires a brightening amplitude greater than 0.5 mag.
It is possible that there are more fainter outbursts with
relatively low brightening amplitudes (<0.5 mag), but missed by our selection.

\begin{figure}
\epsscale{1.1}
\plotone{fllf.eps}
\caption{
The W2 peak luminosity function of our MIRONG sample.
The data points are plotted with log\ensuremath{L\mathrm{_{W2}}}\ ranging from 40.5 to 44.5 (bin 0.5).
The faint end (log\ensuremath{L\mathrm{_{W2}}}$\in [40.5, 41]$) is totally contributed by
extremely low-redshift SNe (red circle).
The high end (log\ensuremath{L\mathrm{_{W2}}}$>42$) is fitted by a single Schechter function
(grey line).
The shifted grey dashed line depicts the luminosity function of the primary
emission which causes the  MIR outburst assuming a dust covering factor of 0.3.
We have also overlaid the observed
X-ray (blue squares, data from Fig. 6 of \citealt{Auchettl2018})
and optical (magenta triangles, data from Fig. 1 of \citealt{vV2018})
luminosity functions of TDEs for comparison.
}
\label{fllf}
\end{figure}

\section{Host Galaxy and Central Black Hole}
\label{host}

\subsection{Host galaxy properties}

The host galaxy property is crucial to understand
the nature of MIRONG.
It is well known that galaxies show a bimodal distribution in the
colour-magnitude diagram (CMD), which are mainly clustered into a red sequence and
blue cloud (e.g., \citealt{Strateva2001,Bell2004})
with a green valley in between.
We retrieved the apparent $ugriz$ Petrosian magnitudes from the SDSS DR14
and then corrected for the extinction using the dust maps of \citet{Schlegel1998}.
For a fair comparison of galaxies at different redshifts in the CMD,
we applied $k$-corrections to the observed magnitudes to $z=0.1$
(close to the median redshift of our sample) using the
$\tt IDL$ code $\tt KCORRECT$ (v4.3)~\footnote{http://kcorrect.org/} given by
\citet{Blanton2007} (see also \citealt{Blanton2003}).
To divide the SDSS galaxy sample into the red and blue class,
we used the following magnitude-dependent color cut:
\begin{equation}
  g-r=-0.027*M_r+0.14
\label{cmcut}
\end{equation}
Our sample concentrates on the densest region of the CMD diagram
(see Figure~\ref{hostcm}).
There are 82 galaxies categorized into the red sequence and the fraction (59.9\%)
is somewhat comparable to the ensemble SDSS spectroscopic galaxy sample (63.0\%).
The difference in $M_r$ is also tiny, with our sample only 0.24 magnitude brighter.
In other words, the host galaxies of the MIR outburst are not significantly
biased in terms of optical color and magnitudes.
For comparison, the host galaxies of optically-selected TDE are much
less luminous and most are dwarf galaxies.

\begin{figure}
\epsscale{1.1}
\plotone{hostcm.eps}
\caption{
The color-magnitude diagram ($g-r$ vs. $M_r$) of the MIRONG sample
(blue filled circles).
The magnitudes and color are displayed after correction for
Galactic dust extinction and $k$-correction to $z=0.1$.
We denote the four SNe host galaxies in red.
Contours show the density of low-redshift (z<0.2) SDSS DR7 spectroscopic galaxies.
The contour lines correspond to 5, 25, 100, 400, 2000, 5000 and 8000 galaxies
per bin of $\Delta(g-r)=0.1$ and $\Delta M_r=0.1$.
The magenta dashed lines is the assigned line of demarcation between the blue
clouds and red sequence. We have also plotted the optical TDEs
(\citealt{vV2020}) for comparison (green squares), in which the magnitudes are from
either SDSS or PanSTARRS.
}
\label{hostcm}
\end{figure}

\begin{figure*}
\epsscale{0.8}
\plotone{sedfit.eps}
\caption{
The SED fit of one galaxy (SDSSJ0000+1438) in our sample.
We have collected the NUV (GALEX), optical (SDSS), NIR (UKIDSS or 2MASS)
and MIR (\emph{WISE}) magnitudes and then performed the fitting with {\tt iSEDfit}.
The blue filled circles are actual photometry data used in the fitting and the
black open squares are predictions of the fitting model at the input bands.
The \emph{WISE}\ W1 and W2 bands (red open diamonds) have not been used in the
fitting but only plotted as a comparison since the PSF magnitudes
will underestimate the flux of extended sources (e.g., nearby galaxies).
Most of the photometry errors are tiny and thus are not visible in the plots.
}
\label{sedfit}
\end{figure*}

The SDSS data release provides also value-added catalog of the galaxy intrinsic
properties\footnote{https://www.sdss.org/dr15/spectro/galaxy/}.
For instance, the Portsmouth group has performed stellar kinematics and
emission-line flux measurements (\citealt{Thomas2013})
using the publicly available codes
Penalized PiXel Fitting (pPXF, \citealt{Cappellari2004}) and
Gas and Absorption Line Fitting (GANDALF v1.5; \citealt{Sarzi2006}).
GANDALF fits stellar population and Gaussian emission line
templates to the galaxy spectrum simultaneously to separate stellar continuum and
absorption lines from the ionized gas emission.
Stellar kinematics are evaluated by pPXF where the line-of-sight
velocity distribution is fitted directly in pixel space.
The fits account for the impact of diffuse dust
in the galaxy on the spectral shape adopting a \citet{Calzetti2001} extinction curve.
Outputs from this fitting process include stellar velocity dispersions (\ensuremath{\sigma\mathrm{_{\star}}}),
emission-line fluxes, equivalent widths and BPT classifications.
Note that for one source, SDSSJ1422+1609, the galaxy parameters are not available
in the catalog due to the lack of SDSS spectrum.

The Portsmouth group has also provided the measurements of stellar mass (\ensuremath{M\mathrm{_{\star}}})
through SED fitting with stellar population models (\citealt{Maraston2013}).
However, their fittings only considered the SDSS optical photometry
and may induce bias due to the narrow range of wavelength coverage.
Therefore, we tried more comprehensive SED fittings by including
the near IR and ultraviolet (UV) photometry, which is capable to better trace
the old and young stellar populations, respectively.
The UV data are taken by the \emph{Galaxy Evolution Explorer}
(\emph{GALEX}; \citealt{Martin2005}) with the near-UV (NUV) and far-UV (FUV)
filters.
For the NIR data, we adopted the $J, H, K$-band Petrosian magnitudes
given by the UKIRT InfraRed Deep Sky Surveys (UKIDSS; \citealt{Lawrence2007}).
For sources that are located outside the UKIDSS footprint,
we used the magnitudes from the Two Micron All Sky Survey
(2MASS; \citealt{Skrutskie2006}).

After gathering the UV, optical and NIR magnitudes as well as their errors,
we begin the broadband SED fitting utilizing {\tt iSEDfit}\footnote{http://www.sos.siena.edu/~jmoustakas/isedfit/},
which is a code to determine the \ensuremath{M\mathrm{_{\star}}}, SFRs, and other physical properties
of galaxies within a simplified Bayesian framework (\citealt{Moustakas2013}).
The fitting results are quite good (see an example in Figure~\ref{sedfit}) and the
resulted \ensuremath{M\mathrm{_{\star}}}\ are higher than those given by Portsmouth group by a median of 0.16 dex,
indicating a fraction of old stellar population probably missed by the SED fitting using only the
optical data.
Generally, the \ensuremath{M\mathrm{_{\star}}}\ distribution is consistent with the $M_r$.
Most galaxies ($92.0\%$) have stellar masses in the range $10^{10-11.5}$~\ensuremath{M_{\odot}}\ (median $10^{10.7}$~\ensuremath{M_{\odot}},
see Figure~\ref{mbhms}).
The least massive five galaxies with a mass lower than $10^{9.5}$~\ensuremath{M_{\odot}}\,
including the four SNe hosts, could be classified as dwarf galaxies
since their masses are even lower than the Large Magellanic Cloud
($2.7\times10^9$~\ensuremath{M_{\odot}}, van der \citealt{vander2002}).

\begin{figure*}
\epsscale{1.0}
\plotone{sdssj0158_spec.eps}
\caption{
We show the analysis of the SDSS spectrum of SDSSJ0158-0052 as an
illustration of the spectral decomposition. The left panel displays
the subtraction of starlight (grey) and AGN continuum (blue).
The sum of the starlight and continuum is plotted in red
and the residual is plotted in magenta.
The middle panel highlights the Gaussian fitting of H\ensuremath{\beta}-[O\,{\footnotesize III}]\ region,
in which the broad H\ensuremath{\alpha}\ component, narrow lines and total are shown in blue,
green and red respectively.
The right panel is similar but for H\ensuremath{\alpha}-[N\,{\footnotesize II}]-[S\,{\footnotesize II}]\ region.
}
\label{j0158}
\end{figure*}

\subsection{Nuclear Activity and Mass of SMBHs}
\label{smbh}

\begin{figure*}
\epsscale{0.8}
\plotone{bpt.eps}
\caption{
The locations of the MIR outburst galaxies (black filled circles) in the BPT diagram.
The lines separating the different regions are mainly drawn from
\citet{Kewley2006}
only with the Seyfert/LINER dividing line (magenta dashed line) in the left panel
from \citet{Cid2010}.
}
\label{bpt}
\end{figure*}

As we have mentioned in \S\ref{intro},
the transient accretion onto SMBHs is a major population
of extragalactic transients associated with galaxies.
It is thus useful to assess the pre-outburst nuclear activity of these galaxies,
which may shed light on the nature of the outburst.
The narrow-line ratios of galaxies, namely their location
at the BPT diagram, can be used as a diagnosis of the nuclear activity,
but only the broad emission lines can give unambiguous evidence of active SMBHs
and plausible estimated \ensuremath{M_\mathrm{BH}}.
In order to detect the potential broad lines, we have performed careful spectral
analysis on the SDSS spectrum of these galaxies.

Our spectral fitting procedures are as follows:
(1) subtracting the starlight and AGN continuum to obtain
the emission-line residuals;
(2) Gaussian fitting to emission lines including
broad components when necessary.
The spectrum is corrected for the Galactic extinction with the extinction
map of \citet{Schlegel1998} and the reddening curve of \citet{Fitz1999}.
We model the starlight component with the stellar templates of \citet{Lu2006},
which were built from the simple stellar population spectra
(\citealt{Bruzual2003}).
The AGN continuum is modeled as a power law.
After subtracting the starlight and AGN continuum component,
we try to fit emission lines with multiple Gaussians while only broad components
($\sigma>500~\ensuremath{\mathrm{km~s^{-1}}}$) for H\ensuremath{\alpha}, H\ensuremath{\beta}, Mg\,{\footnotesize II}\ and He\,{\footnotesize I}\ are allowed to vary.
Although emission lines in a fraction of galaxies can be fitted with a
broad H\ensuremath{\alpha}\ component superimposed on the narrow component,
only those broad H\ensuremath{\alpha}\ lines with S/N higher than 10 are considered as valid,
resulting in a final sample of 26 galaxies.
The robustness of our fitting results is demonstrated
in SDSSJ0158-0052 (see Figure~\ref{j0158}), which is a well-known low-mass AGN
candidate selected by broad H\ensuremath{\alpha}\ emission (\citealt{GH2007}; \citealt{Xiao2011}).
The \ensuremath{M_\mathrm{BH}}\ is subsequently calculated by empirical virial mass estimator
($\ensuremath{M_\mathrm{BH}}=fRv^2/G$) for single-epoch spectra using the formalism
presented in \citet{GH2007}.
This method postulates that the broad-line region (BLR) gas is virialized
with velocity dispersion characterized by the widths of broad lines and a distance
to the BH estimated from the conventional radius-luminosity relation
(e.g., \citealt{Kaspi2005,Bentz2013}).

Regarding the narrow-line sources, their nuclear activity can be alternatively
identified by their positions in the BPT diagram (see Figure~\ref{bpt}).
According to the classification of Portmouth group (with the broad-line
AGNs updated from our own fittings), our sample can be categorized into
37 (14 with broad lines) Seyferts, 23(4) LINERs, 35(2) starforming galaxies
and 41(6) composites.
Broad-line starforming galaxies are not common but indeed exist
(see Figure 8 of \citealt{Liu2019}).
However, the fraction in our sample looks somewhat high (2/35), which may
indicate some of the MIR outburst in starforming galaxies are driven by AGNs.
The total number of AGNs contained in our sample is 49 (37+4+2+6)
when taking count of both Seyferts and broad-line sources in other BPT types.
No clear evidence of intense AGN activity is found for other sources.
We caution that LINERs can be treated as weak AGNs powered by SMBHs.
In the absence of broad-lines, we have to estimate their \ensuremath{M_\mathrm{BH}}\ with
other approaches, such as the correlations with either the \ensuremath{\sigma\mathrm{_{\star}}}\ or mass of
galactic bulge (\ensuremath{M_\mathrm{bulge}}) established in local massive galaxies
(see \citealt{KH2013} for a review).
Our experiences and other works suggest that the velocity dispersion
after correction of the instrument broadening ($70$~\ensuremath{\mathrm{km~s^{-1}}}) is reliable
down to $\sim$50$-$60~\ensuremath{\mathrm{km~s^{-1}}}\ (e.g., \citealt{Zahid2016}; \citealt{Chilingarian2017}).
Hence we adopted only the \ensuremath{M_\mathrm{BH}}-\ensuremath{\sigma\mathrm{_{\star}}}\ relation when $\ensuremath{\sigma\mathrm{_{\star}}}>50$~\ensuremath{\mathrm{km~s^{-1}}},
leaving seven galaxies without \ensuremath{M_\mathrm{BH}}\ measurements because of low \ensuremath{\sigma\mathrm{_{\star}}}.
Although the \ensuremath{M_\mathrm{BH}}-\ensuremath{M_\mathrm{bulge}}\ relation (e.g., \citealt{McConnell2013}) has been
extensively used to estimate \ensuremath{M_\mathrm{BH}}, the SDSS resolution is generally too low
to isolate the bulge component from disk.
We thus used the relation between \ensuremath{M_\mathrm{BH}}\ and
total stellar mass (\citealt{Reines2015}) for the last seven objects.
Our final sample has a broad range of \ensuremath{M_\mathrm{BH}}\ with logarithmic mass
from 4.5 to 9.0 (median 7.3, see Figure~\ref{mbhms}).

\begin{figure}
\epsscale{1.15}
\plotone{mbhms.eps}
\caption{
The black hole mass (\ensuremath{M_\mathrm{BH}}) versus stellar mass ($M_*$) distribution of our sample.
The red solid circles are SNe and the blue solid circles are strong jetted sources.
The histogram of \ensuremath{M_\mathrm{BH}}\ and $M_*$ are shown in the right and bottom panels.
Overlaid in grey dotted and magenta dashed lines are the correlation between
\ensuremath{M_\mathrm{BH}}\ and bulge mass (\citealt{McConnell2013}) and total stellar mass
(\citealt{Reines2015}), respectively.
}
\label{mbhms}
\end{figure}

\section{Nature of the MIR flares}
\label{nature}

The above analysis of the properties of MIR flares and their host galaxies,
allow us to explore further their physical nature.
MIR flares can be generally attributed to non-thermal emission from jets or dust thermal emission
heated by different processes, such as supernova, AGN or TDE.
We will discuss each of these possibilities below.

\subsection{Infrared Luminous Supernova}
\label{SNe}

As mentioned in \S\ref{opt}, SDSSJ0936+0615, SDSSJ1531+3724, SDSSJ1540+0054
and SDSSJ1554+1636 are known hosts of SNe reported by ASAS-SN\footnote{
http://www.astronomy.ohio-state.edu/asassn/sn\_list.html}.
Interestingly, their peak MIR luminosity appears to be lowest,
with absolute W2 magnitudes fainter than -22 (or log\ensuremath{L\mathrm{_{W2}}}<42).
The reason for them to pass our selection threshold of variability amplitude
($>$0.5 mag) could be the dwarfness of their host galaxies (see Figure~\ref{hostcm}),
which makes the MIR outburst luminosity over luminous with respect to their host.
Indeed, the four SNe galaxies are the least massive ones
and occupy the low-mass end (log\ensuremath{M\mathrm{*}}$<9$).
In addition, these galaxies appear very young in terms of their blue colors (g-r<0.5).
Hence, the SNe in our sample, in a word, occupy the lowest MIR luminosity end, and
reside in dwarf starforming galaxies.

The spectroscopic follow-ups have classified the four SNe into three type~II
and one Ia-91T like sources.
Previous studies suggest that the IR emission of SNe IIn are statistically
more luminous and long-lasting than other types (e.g., type Ia)
due to the heating of preexisting dust in the circumstellar medium (CSM)
(\citealt{Fox2013,Tinyanont2016}).
The SNe Ia usually show very weak MIR emission and are not detectable in three years
after the explosion (\citealt{Tinyanont2016}) except the Ia-CSM subclass.
In the same way, some 91T-like SNe also display the interacting CSM
(e.g., \citealt{Harris2018}), which could be responsible for the observed MIR flare.
The luminous infrared transients uncoverevd by SPIRITS project
are mainly obscured core-collapse SNe with peak $4.5\mu$m
(\emph{Spitzer}, roughly \emph{WISE}\ W2) magnitudes between -14 and -18.2
(\citealt{Jencson2019}), which are significantly fainter than that in our sample.

\begin{figure}
\epsscale{1.15}
\plotone{lumevo.eps}
\caption{
The time-resolved W2 magnitudes with date zeropoint set as the time of
first MIR excess.
Our sample has been plotted in solid grey circles with the
radio-loud sources and SNe highlighted in blue and red, respectively.
We have also overplotted other IR luminous SNe (orange squares),
which are among the brightest known to us, for comparison.
The Ia-CSM light curve (green triangles) are drawn from \citet{Fox2013}.
}
\label{abslc}
\end{figure}

The evolution of absolute W2 magnitudes as a function of time is presented in
Figure~\ref{abslc}.
It is clear that the four SNe have the faintest MIR emission (see also the
peak magnitude distribution in Figure~\ref{w2peak})
and their duration is relatively short.
While the SNe occurred in SDSSJ1531+3724 (ASASSN-17jq) is relatively bright,
it is still fainter than non-SNe objects.
In our previous work (\citealt{Jiang2019}), we have checked for the \emph{WISE}\ light curves
of all SNe in the public catalog reported between 2008 and 2018.
Among them, the most luminous ones are all IIn (e.g., SN2010jl,
SN2013dz and SN2014ab) with luminosity $L\rm_{W2}\sim10^{42}$~erg~s\ensuremath{^{-1}},
which is comparable to that of ASASSN-17jq.
In summary, the MIR luminosity of our sample is systematically
higher than SNe by at least 2-3 magnitudes, thus the SNe scenario is
disfavored as the origin for the bulk of MIRONG.

\begin{figure}
\epsscale{1.1}
\plotone{sfradio.eps}
\caption{
The 1.4~GHz luminosity versus the SFR of radio detected sources in our sample.
The red line is the expected radio emission from star formation (Equation 3 in
\citealt{Davies2017}). One index offset from the expectation are plotted as
red dashed lines.
We have highlighted the radio-loud AGNs with blue squares and other
AGNs (Seyferts or broad H\ensuremath{\alpha}\ objects) with green triangles.
The grey dotted line represents the radio luminosity beyond which
can be considered as undisputed jet origin (\citealt{Kellermann2016}).
}
\label{sfr1.4}
\end{figure}

\subsection{Non-thermal emission from Jet}
\label{Jet}

We next examine the possibility of non-thermal emission produced by relativistic jets.
It has been proved that the synchrotron radiation of jets can contribute
significantly to the MIR emission as well as its variability
(e.g. \citealt{Jiang2012,Liao2019}).
We cross-matched our sample
with the catalog of Faint Images of the Radio Sky at Twenty cm (FIRST)\footnote{http://sundog.stsci.edu/cgi-bin/searchfirst} using a matching radius of 5\arcsec,
resulting in 22 objects detected with S/N higher than 5.
There are eight objects outside of the FIRST footprint, so
we matched them with the NRAO VLA Sky Survey (NVSS) catalog\footnote{https://www.cv.nrao.edu/nvss/NVSSlist.shtml} but found non-detections.

The radio detections do not necessarily suggest the association with jet activity, since
the star formation may also contribute to the radio emission.
We used the star formation rate (SFR) given by \citet{Chang2015},
which is derived
by fitting the broadband SDSS+\emph{WISE}\ SED using MAGPHYS (\citealt{daCunha2008}).
The SFR measurements for 128 out of 137 galaxies are found.
For the remaining sources, the SFRs were estimated from the 24~$\mu$m flux
\footnote{Here we adopted the \emph{WISE}\ W4 band (22~$\mu$m) flux as an approximation.}
(\citealt{Chary2001}).
Then we calculated the expected radio flux from star formation
using the correlation between 1.4~GHz luminosity ($L_{1.4G}$) and SFR
(Equation 3 in \citealt{Davies2017}).
As displayed in Table~\ref{radio} and Figure~\ref{sfr1.4},
the radio emission for star-forming and composite galaxies
are fully consistent with the expectation from the star formation.
Only SDSSJ1337+0035, which is a composite but shows extremely high radio power, is an exception.
We then used the radio-loudness parameter $R$ to quantify the radio intensity
for AGN sources (Seyferts or LINERs) plus SDSSJ1337+0035, which is defined as the ratio of the flux densities
between 6~cm and 4400~\AA\ (\citealt{Kellermann1989}).
Here the 6~cm flux is derived from the 1.4G~Hz flux assuming a spectral slope $-0.7$.
The 4400~\AA\ flux is converted from the 5100~\AA\ flux, which is derived
from the bolometric luminosity (\ensuremath{L\mathrm{_{bol}}}).
The latter is computed from the [O\,{\footnotesize III}]\ luminosity (\citealt{Lamastra2009}) assuming a bolometric correction of 8.1
(\citealt{Runnoe2012}).
The derived radio-loudness parameters are listed in Table~\ref{radio}.

Four galaxies (SDSSJ1046+1655, SDSSJ1211+4047, SDSSJ1337+0035, SDSSJ1348+1559)
stand out from the rest in above evaluations.
They show radio power 38-600 times higher than that predicted from SFR.
Meanwhile, their radio-loudness (>1000) is at least two orders of magnitude higher
than other sources.
Moreover, the radio luminosity for the four sources is higher than
$10^{23.6}~\rm W~Hz^{-1}$, above which a radio-loud AGN can be classified (\citealt{Kellermann2016}).
Therefore, they are likely radio-loud AGNs, for which the MIR flares
could originate from the non-thermal emission of pre-existing jets.
For those galaxies that are not radio-loud before the MIR flare,
we cannot rule out the probability of a new-formed relativistic jet
contributing the MIR emission.
Unfortunately, no quasi-simultaneous radio observations are available.
In fact, by examining the \emph{WISE}\ light curves in each epoch,
we did not found any evidence for intraday variability, which
could indicate the presence of a relativistic jet (\citealt{Jiang2012}).
Timely radio follow-ups would be helpful to further test this intriguing scenario.

\subsection{Dust Echo of Transient SMBH Accretion}

After removing the 4 known SNe and 4 radio-loud sources,
we now investigate the origin of MIR outbursts for the other 129 galaxies in our sample.
As we analyzed in \S\ref{smbh}, there are 49 galaxies
that show evidence of AGN activity in their optical spectra.
In addition, 16 galaxies can be classified as LINERs.
Therefore, the 128 galaxies (except SDSSJ1422+0609 for which the SDSS spectrum is not available)
can be grouped by two populations, including
65 AGNs (49 Seyferts and 16 LINERs) and 63 quiescent galaxies.

\subsubsection{Echoes of Turn-on AGNs}

The UV/optical variability on various time scales are well known to be an
inherent property of AGNs albeit the physical mechanism behind is not
fully understood (\citealt{Ulrich1997}).
The amplitude of optical variability is typically a few tenths
of magnitudes within time-scale of months, but also showing larger
variations over longer time-scales.
There is an increasing population of dramatically variable AGNs,
so-called CL AGNs, which exhibit flux rising/declining in continuum
($\Delta m>1$ mag) over several years and change accordingly spectral types
(e.g., \citealt{LaMassa2015,Runnoe2016,MacLeod2016,Yang2018}).
Specifically, the CL AGNs with rising/declining photometric light curves are termed "turn-on"/"turn-off".
In addition to state changes found in CL AGNs, major outbursts
have also been found in AGNs (e.g., \citealt{Graham2017,Trakhtenbrot2019}).

In the AGN unification, the dusty torus is a vital ingredient to address
a variety of phenomenons in different types of AGNs.
The torus will unavoidably absorbs part of the UV-optical photons
from the accretion disk and reprocesses them into the IR.
The picture has been widely accepted through the
spectral energy distribution (SED) fitting (e.g., \citealt{Fritz2006,Nenkova2008,Stalevski2012})
and IR reverberation mapping (e.g., \citealt{Suganuma2006,Koshida2014,Lyu2019}).
It is plausible that our discovered MIR flares are  due to dust echoes
of turn-on AGNs.
Such a scenario can be tested by obtaining new optical spectra
to see whether there is evident spectral evolution.
In fact, we have performed a detailed study of SDSSJ1115+0544, which has undergone
a brightening by 2.5~mag in $V$-band over $\sim120$ days,
then faded by 0.5 mag over 200 days, followed by a plateau lasting for $>600$ days.
The multi-epoch optical spectra over 400 days in the plateau phase
revealed newly formed and steady broad H\ensuremath{\alpha}, H\ensuremath{\beta}\ emission,
that is compatible with the characteristic of a turn-on AGN (\citealt{Yan2019}).
Intriguingly, together with the three objects (SDSSJ1554+3629,
SDSSJ0945+4814 and SDSSJ1133+6701) mentioned in \S\ref{opt}, the four
reported sources are all classified as LINERs in the BPT diagram, which may
suggest a uniform class of turn-on system (\citealt{Frederick2019}).

\begin{figure}
\epsscale{1.1}
\plotone{mbh.eps}
\caption{
The histograms of \ensuremath{M_\mathrm{BH}}\ for starforming (blue), composite (magenta),
Seyfert (green) and LINER (red) galaxies in our sample as classified
by the BPT diagram.
The bars errors are the Poisson fluctuations of
the number in each bin.
Noting that we have put all broad-line sources into the 'Seyfert' subset
whatever their BPT types are.
}
\label{mbh}
\end{figure}

\subsubsection{Echoes of Tidal Disruption Events}

Nearly half objects in our sample show no signs of AGN activity,
neither Seyfert nor LINER, in the SDSS spectra.
The giant nuclear flares of inactive galaxies are usually attributed to TDEs.
For Schwarzschild BHs, the TDEs are observable only when the BH masses
are lower than the Hills mass ($\sim10^8$~\ensuremath{M_{\odot}}).
Otherwise, the star will be swallowed wholly rather than produce an
electromagnetically luminous flare
because its tidal radius is within the horizon (\citealt{Hills1975,Rees1988}).
The \ensuremath{M_\mathrm{BH}}\ distribution of our sample peaks at $2.0\times10^7$~\ensuremath{M_\mathrm{BH}}\ with
only 17 (12.4\%) greater than $10^8$~\ensuremath{M_{\odot}}\ (see Figure~\ref{mbhms}).
If excluding Seyferts and LINERs, only three galaxies are more massive
than the Hills mass (see Figure~\ref{mbh}), hence the TDE interpretation for
the MIR bursts is possible.

Similar to AGNs, the UV/optical photons released by TDEs can be absorbed
by dust in the vicinity of BH and reprocessed into the IR band.
giving rise to an IR flare like an echo.
\citet{Lu2016} have calculated the light curve of the IR echo with
1-D radiative transfer model and showed that the dust emission peaks
at MIR (3-10$\mu$m). They have predicted that the
typical luminosity is between $10^{42}$ and $10^{43}$~erg~s\ensuremath{^{-1}},
depending on the dust covering factor (ranging from 0.1 to 1), which
is fairly comparable with  that inferred in our sample.
Such an IR echo has been detected in a handful of optical TDEs from the \emph{WISE}\ data,
(\citealt{Jiang2016,vV2016})
though the amplitude of IR variability is smaller ($\sim0.2$~mag) compared with
the sample in this work.
Furthermore, \citet{Dou2016} have reported the long-lasting IR echoes from
four coronal-line TDE candidates (\citealt{Wang2012,Yang2013}).
These findings suggest that IR echoes might be ubiquitous for TDEs occurred in
gas (dust)-rich nuclear environment.

On the other hand, the IR echo itself is suggestive of TDEs
in dust-rich environment.
\citet{Wang2018} have conducted a systematical search
and obtained a sample of 19 low-redshift (z<0.22)
non-Seyfert galaxies that display slow decline in the MIR emission,
reminiscent of those coronal-line TDEs (\citealt{Dou2016}).
Unfortunately, because they are already in the late fading stage,
the spectroscopic observations might be too late to identify the
TDE-like characteristics.
In contrast, our objects are observed at much earlier phase,
either still rising or just decaying, which are valuable
for timely multi-wavelength follow-ups.
By checking for optical counterparts (see \S\ref{opt}), we found that
SDSSJ0158-0052 (PS16dtm) and SDSSJ1620+2407 (ATLAS17jrp) have been indeed
alerted as TDE candidates \citep{Blanchard2017,Fraser2017}.
The identifications of TDEs in our sample are thus encouraged.
We caution that even for active galaxies in our sample, the TDE scenario is
possible and worthwhile to explore.
One universal manifestation accompanied with AGN TDEs
(e.g., \citealt{Tadhunter2017,Blanchard2017,Kankare2017})
is long-lasting and luminous MIR flares as a result of echoes of dusty torus
\citep{Dou2017,Jiang2017,Jiang2019}.

Given the faintness in the optical bands for most MIRONGs (\S\ref{opt}),
they could be promising candidates of dust-obscured TDEs
that are still poorly explored.
If not due to obscuration, their optical emission should be otherwise
intrinsically weak, which is especially possible in the TDE scenario.
The electromagnetic output of TDEs was initially thought to be dominated
by X-ray or FUV emission from accretion, making the origin of
optical emission remains hotly disputed (\citealt{Roth2020}).
Therefore, some objects in our sample could belong to classical TDEs, which are
luminous in X-ray/FUV but are faint in optical band.
In this sense, the IR search of TDEs is superior to other individual bands
as long as the BH is set in dusty environment.

\begin{figure*}
\epsscale{1.0}
\plotone{lirmbh.eps}
\caption{
Left panel: the correlation between peak MIR luminosity ($L_d$) and \ensuremath{M_\mathrm{BH}}.
Right panel: the correlation between the inferred dust mass ($M_d$) and \ensuremath{M_\mathrm{BH}}.
Overlaid in red are the linear least-squares fitting.
The $L_d$ and $M_d$ values are derived by dust with absorption efficiency
(see Table~\ref{tb_dust}).
}
\label{lirmbh}
\end{figure*}

Based on the sample of 19 galaxies, \citet{Wang2018} have found that the peak MIR luminosity is well
correlated with the \ensuremath{M_\mathrm{BH}}. However, the correlation (excluding SNe) appears less significant in our sample presented
here (see Figure~\ref{lirmbh}), though a Spearman rank analysis gives a correlation coefficient
of $\rho= 0.377$, corresponding to a null hypothesis probability of $p=8\times10^{-6}$.
If we examine the dust mass instead of luminosity,
the correlation is even less obvious ($\rho= 0.116$, $p=0.001$).

\section{Discussion}

Similar to \citet{Wang2018}, our tentative conclusion is that the
MIRONGs discovered by us are most likely associated with transient SMBH accretion,
either in the form of TDE or turn-on AGN activity.
Only a small fraction of them are due to IR-luminous SNe or non-thermal
emission from relativistic jets.
The conclusions are supported by several lines of evidence:
1) the outburst locations prefer to be in the region very close to
the galactic center; 2) the maximum MIR
luminosity is higher than the brightest SNe by 1-2 orders; 3)
only few have radio-loud jet emission before the outburst;
4) the luminosity function broadly agrees with results of optical
and X-ray TDEs (or nuclear transients).

In order to differentiate the diverse nature of these MIRONG,
multi-wavelength follow-up observations have been conducted,
including optical spectra (Y. B. Wang et al. in preparation),
NIR photometry (H. Liu et al. in preparation), radio (\citealt{Dai2020})
and X-ray observations.
So far, we have successfully obtained a sizable sample of likely TDEs
or turn-on AGNs, which will be reported in the following series of papers.
These events are generally not detected in the optical band and
thus largely overlooked by previous works, hence present a significant advance
over previous studies of similar events.

\subsection{TDE Demography and Missing Energy Puzzle}
\label{obsTDE}

The observations of TDEs are initially restricted to the serendipitous discoveries
of X-ray flares in the centers of galaxies, yielding a very limited number of
candidates.  The number has grown remarkably in the past decade
with the advent of dedicated optical time-domain surveys
(\citealt{Komossa2015,vV2020}).
However, there is still a large discrepancy in the event rate between
observation and theory (\citealt{Stone2020}).
The observations usually find a rate of $\sim10^{-5}~{\rm gal}^{-1}~{\rm yr}^{-1}$
(\citealt{Donley2002,vV2014}),
which is at least an order of magnitude
below the theoretical estimation in realistic galactic nuclei
(\citealt{Wang2004,Stone2016}).
Although dynamical mechanisms to suppress TDE rates have been proposed,
they seem unlikely to work in practice (e.g. \citealt{Lezhnin2015}).
One solution may be a very broad intrinsic TDE luminosity function of
which we have so far only seen the high end at given band
(e.g., \citealt{Blagorodnova2017,vV2018}) in view of the
complicated radiative emission mechanisms (\citealt{Roth2020}).
The TDE demography at any single band can be thus seriously underestimated.

Moreover, the rate discrepancy becomes more specific and definitive when
optical TDEs are found being overrepresented in post-starburst
(or so-called E+A) galaxies with the rate
elevated by a factor of $\sim100$ (e.g. \citealt{Arcavi2014,French2016}).
This preference can not be fully accounted for by selection effects
in comparison with control samples (\citealt{Law-Smith2017}).
Such an enhancement is difficult to understand in theory, even though
their hosts show high central stellar densities (\citealt{French2020}).
This would require violent dynamical processes to occur but it
seems impossible in most post-starburst galaxies.

Another observational discrepancy is known as the missing energy puzzle, namely
a huge shortage of the observed energy relative to predictions.
In the simplest picture, if half stellar material is accreted onto the BH,
the expected energy released is $\sim$$10^{52}${--}$10^{53}\,$erg.
The observed total energy of optical TDEs is only $\sim 10^{51}$~erg.
Where is the missing 90{--}99\% of the energy?
Some models argue that the real accretion efficiency is actually quite low
due to advection or outflow (e.g., \citealt{Svirski2017,Dai2018}).
\citet{Lu2018} suggest that the missing part may be in the
unobserved extreme-UV (EUV) band and/or in the form of relativistic jets.
The jet mode is disfavored as the rareness of jetted TDEs
but the EUV option coincides with the theoretical
peak and the Rayleigh-Jeans-like optical/NUV SED.
If that is true, at least in several TDEs, the issue then goes to
how to measure the EUV energy which is invisible directly.

Two recent studies may shed light on an avenue to solve or
alleviate the puzzles above.
\citet{Tadhunter2017} has reported a candidate TDE in F01004-2237
from a sample of 15 nearby ultra-luminous infrared galaxies (ULIRGs),
suggesting an event rate as high as $\sim10^{-2}~{\rm gal}^{-1}~{\rm yr}^{-1}$.
The ULIRGs are generally starburst systems with a large amount of dust
which is able to even veil energetic outbursts like TDEs.
In the meantime, the energy absorbed by dust will manifest the
orginal TDE emission as a luminous IR echo.
The picture is nicely corroborated by the huge MIR flare detected in
F01004-2237 (\citealt{Dou2017}), whose integrated IR energy till now is
$>3\times10^{52}$~erg.
The other work claims a dust-enshrouded TDE in the western nucleus (B)
of the merging galaxy pair Arp~299 (\citealt{Mattila2018}).
This event is almost silent in optical and X-ray, but displays long-lasting
echoes in the NIR and MIR with a total radiated energy $>1.5\times10^{52}$~erg.
The missing energy puzzle is almost settled in both systems when the IR energy
is taken into account.

The two case studies also hint that perhaps the TDEs in post-starburst systems
are only the tip of the iceberg and the real TDE rate in
starburst galaxies could be immense (\citealt{Guillochon2017NA}).
It is widely believed that the galaxy mergers or interactions are
capable of driving galactic-scale gas inflow to the nuclear region
and triggering intense star formation.
Even for an isolated disk galaxy, its center could be a concentrated region of
gas and dust ornamented by enhanced star formation.
Indeed, a considerable fraction of galaxies in the MIRONG sample are
starforming systems as indicated by their SDSS fiber spectrum, whose nucleus
are easily obscured as a result of intense star formation process.

Interestingly, the MIRONG event rate estimated by us
is $5.4\times10^{-5}~{\rm gal}^{-1}~{\rm yr}^{-1}$ (see \S\ref{evrate}),
 which is comparable to that of optical TDEs.
It would be of a high value if a large percentage of them
are eventually confirmed as real TDEs.
Furthermore, if we naively assume a typical dust covering factor
as AGN torus, the inferred luminosity function of the MIRONG
is comparable with the observations
of X-ray (\citealt{Auchettl2018}) and optical TDEs (\citealt{vV2018})
at high end (see Figure~\ref{fllf}).
The faint-end discrepancy is at least partly caused by the imperfect correction
for the sample completeness of various selection approaches.
We emphasize again that our selection criteria is somewhat strict
(>0.5 mag brightening). In the future we will extend the sample by a relaxed
requirement to obtain a more complete view of the event rate
and the luminosity function at faint end.
The new population that is largely missed by optical surveys, due to
either dust obscuration or intrinsically faintness, may offer us a
promising approach to solve the problems of TDE demographics and missing energy.

\subsection{Implication on the Turn-on and Duty Cycle of AGNs}

CL AGNs have gradually become a rather popular phenomenon.
The early searching has been focused mainly on known quasars,
which naturally yield out more "turn-off" AGNs.
The systematical discovery of "turn-on" cases have to start from a
much larger galaxy sample, consisting of both Seyfert and normal galaxies.
Galaxies which show rapid transformation from a quiescent galaxy to a type~1 AGN
within several months to years are of great interest.
So far, only two unambiguous systems (iPTF~16bco and SDSSJ1115+0544)
have been reported (\citealt{Gezari2017,Yan2019}).
We refer them to \emph{bona fide} turn-on AGNs, which are
different from the normal CL AGNs that usually changed from
type~2/1.9/1.8 to type~1.
Some CL AGNs have shown to change their types
back and forth frequently (e.g., \citealt{Denney2014,McElroy2016,Oknyansky2019}),
implying the existence of a persistent but
unstable accretion disk, e.g., susceptible and sensitive to gas feeding.
In comparison, the \emph{bona fide} turn-on AGNs can serve as a more genuine
laboratory than normal CL AGNs to explore the ignition mechanism of SMBHs, e.g.,
the rapid formation of accretion disk as well as the origin of
concomitant multiwavelength emission (e.g., X-ray, radio).

It is worthwhile to note that both iPTF~16bco and SDSSJ1115+0544 are
present in our MIRONG sample.
It suggests that more similar systems would present in our sample
in view of the quiescence of many galaxies before the outburst, and
the selection of "turn-on" AGNs might be effectively using the MIR
light curves.
The dust covering factors ($f_d$) of quiescent SMBHs are around $10^{-2}$
as revealed by the IR echoes of optical TDEs,
which is at least one order of magnitude lower
than that in AGNs (\citealt{vV2016,Jiang2020}).
We estimated the $f_d$ of turn-on AGNs
with the similar method used in \citet{Jiang2020}, which is $f_d=\ensuremath{L_\mathrm{dust}}/\ensuremath{L\mathrm{_{bol}}}$.
The \ensuremath{L\mathrm{_{bol}}}\ of iPTF~16bco and SDSSJ1115+0544 after state transformation
are $\sim10^{45}$~erg~s\ensuremath{^{-1}}\ (\citealt{Gezari2017})
is $\sim4\times10^{44}$~erg~s\ensuremath{^{-1}}\ (\citealt{Yan2019}), respectively.
Meanwhile, their peak dust luminosity (\ensuremath{L_\mathrm{dust}}) inferred from \emph{WISE}\
W1 and W2 photometry (fitted by a blackbody radiation)
is $\sim10^{44}$~erg~s\ensuremath{^{-1}}\ and $3\times10^{43}$~erg~s\ensuremath{^{-1}}, respectively.
The inferred $f_d$ ($\sim0.1$) for "turn-on" AGNs appears to fall within the regime
connecting normal galaxies and AGNs.
The result is interesting, and may shed important light onto the triggering mechanism of AGNs,
which is perhaps regulated by the availability of interstellar medium in the vicinity of SMBHs.

If "turn-on" AGNs occur only when a large amount of gas and dust accumulate
around the SMBHs, the dust echo appears inevitable, explaining the high efficiency of
searching in MIR.
Particularly in the extremely dusty environments, the ignited SMBHs
can be severely obscured and only be identified in the IR (\S\ref{obsTDE}).
Uncovering more \emph{bona fide} turn-on AGNs are valuable
not only for understanding the AGN accretion physics, but also
for the probe of AGN duty cycle as well as the impact on their host galaxies.
The duty cycle, namely the accreting phase of a SMBH (i.e., AGN phase) is
considered to be roughly a few times $10^7$~yr (\citealt{Combes2000,Haehnelt1993}).
However, the turn-on time scale is found within one year
(\citealt{Gezari2017,Yan2019}),  indicating that the event rate could be
as low as $\sim10^{-7}$.
Such a low expected rate seems contrary to the discovery of
two and possibly more cases in our sample.
Perhaps the traditional duty cycle of $10^7$~yr is only suitable for the total
active time but not appropriate for flare events which may have different time scales.
The current and future surveys may be able to constrain the time scale distribution
with a large number of newly discovered "turn-on" AGNs.
The specific mode of AGN duty cycle may have distinctive influence on
their host galaxies and advance our understanding of galaxy evolution.

\section{Summary and Prospects}

The combined \emph{WISE}\ and \emph{NEOWISE}\ light curves, which have a time baseline of
about one decade with a cadence of half year, have provided us
a unique dataset for MIR time-domain study.
Starting from  $\sim1$ million SDSS galaxies, we have selected out 137 galaxies which have
displayed MIR outburst with amplitude $>0.5$ magnitude with respect
to the pre-outburst quiescent phase.
Only a small fraction (15/137) of these outbursts have been reported
by optical surveys including four SNe, two TDE candidates, three turn-on AGNs
and six unclassified objects.
The remaining sources are likely associated with the dust echoes
of transient accretion events of SMBHs, as suggested by
their proximity to host galaxy centers, their high MIR luminosity,
weak radio emission and MIR luminosity distribution, {\it i.e.} luminosity function.
We are undertaking multi-wavelength follow-up observations to identify the
nature of these MIR outburst in nearby galaxies (MIRONG).
The MIRONG unveiled by our study demonstrate the importance and necessity of MIR time-domain survey.
For example, they may pave a way to solve the perplexed issues in the
current study of TDEs and changing-look AGNs.

Since the \emph{NEOWISE}\ survey is on-going, we expect more data points will be
accumulated in the future.
Additional data will help us
diagnose the nature of MIR outbursts and obtain better measurements of dust properties.
In addition, more MIR transients will be discovered with the updated database.
At the post-WISE era, the Near-Earth Object Camera (NEOCam)
is a planned mission to discover and characterize asteroids and comets at
two MIR channels simultaneously, which will cover 68\% of the extragalactic sky at
the wavelengths of $4.0-5.2~\mu$m and $6.0-10.0\mu$m, respectively (\citealt{Ross2019}).
The survey depth of NEOCam is quite similar to \emph{NEOWISE}\ yet its cadence is
even better with on average 30 visits per year.
Moreover, the already selected SPHEREx mission (\citealt{Dore2016})
that is scheduled to launch in 2023, will be an excellent complement to NEOCam at near-IR band,
aiming to obtain spectra over $0.75-5\mu$m across the full sky.
It will scan the entire sky four times during its nominal 25-month mission life, though
it is not as deep as \emph{WISE}. Therefore, the prospects for the study of MIR transients
are still very bright.
 \\

We are grateful to the anonymous referee for his/her careful reading
and nice comments, which have greatly improved the paper.
We thank Dr. Roc Cutri for many useful suggestions about the use of \emph{WISE}\
and \emph{NEOWISE}\ archival data all the time
and Dr. John Moustakas for his nice help for the usage of {\tt IDL/iSEDfit}.
This work is supported by the Chinese Science Foundation
(NSFC-11833007, 12073025, 11421303, 11733001),
Joint Research Fund in Astronomy (U1731104) under cooperative agreement
between the NSFC and the CAS, Anhui Provincial Natural Science Foundation
and the Fundamental Research Funds for the Central Universities.
This research makes use of data products from the
\emph{Wide-field Infrared Survey Explorer}, which is a joint
project of the University of California, Los Angeles,
and the Jet Propulsion Laboratory/California Institute
of Technology, funded by the National Aeronautics and
Space Administration. This research also makes use of
data products from {\emph{NEOWISE-R}}, which is a project of the
Jet Propulsion Laboratory/California Institute of Technology,
funded by the Planetary Science Division of the
National Aeronautics and Space Administration.
This research has made use of the NASA/ IPAC
Infrared Science Archive, which is operated by the
California Institute of Technology, under contract
with the National Aeronautics and Space Administration.
This research has made use of the NASA/IPAC Extragalactic Database (NED),
which is operated by the Jet Propulsion Laboratory,
California Institute of Technology,
under contract with the National Aeronautics and Space Administration.

redpajama_set_name: RedPajamaArXiv


CHRIS WOOD sprung Bristol from the clutches of the relegation zone with a second-half penalty worthy of its place as a one in a lifetime occurrence.
The Robins collected maximum points at the City ground for the first time in 57 years and leapfrog Coventry out of the Championship bottom three.
It was only fitting that a Forest side that believes it has invited relegation trouble on itself suffered because of its own inadequacy.
While the hosts played as a side in control of its destiny in the first half, they only packed empty threats.
And Wood punished them from the penalty spot after 54 minutes, giving up a goal that cuts the difference between Bristol and Forest back to four points.


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text:
Le patin clap (également appelés clap skate, clapskates, slap skates, slapskates, du néerlandais klapschaats ) est un type de patin à glace utilisé en patinage de vitesse. Contrairement aux patins traditionnels où la lame est rigidement fixée à la chaussure, les patins clap ont la lame attachée à la chaussure au moyen d'une charnière à l'avant. Ceci permet à la lame de rester plus en contact avec la glace, et ainsi la cheville peut être étendue vers la fin de la course. Aussi, il y a un mouvement plus naturel qui permet de distribuer ainsi l'énergie de la jambe de manière plus efficace.

Les patins clap ont été mis au point à par la Faculté des Sciences des mouvements humains de l'Université libre d'Amsterdam, dirigée par Gerrit Jan van Ingen Schenau, bien que l'idée d'un patin clap soit beaucoup plus ancienne puisque des dessins datant des années 1900 sont connus.

Le patin clap est utilisé pour la première fois durant la saison de patinage 1984/1985. Il a cependant fallu attendre la fin des années 1990 pour que l'idée soit prise au sérieux. Lors de la saison 1996/1997, l'équipe féminine néerlandaise commence à utiliser les patins qui leur permettent un grand succès. Le reste du monde du patinage a rapidement emboîté le pas, ce qui provoque un torrent de records du monde battus lors des saisons suivantes, notamment lors des Jeux olympiques d'hiver de 1998 à Nagano au Japon.

Références

Liens externes

 Histoire et contexte des patins clap
 THE NEW DUTCH "SLAPSKATES": Will They Revolutionize Speed Skating Technique?

Patinage de vitesse

redpajama_set_name: RedPajamaWikipedia


This detached holiday home is located in the village of L Escala on the edge of the forest. The holiday home is nicely furnished and has an open kitchen so that you can keep company while you cook. Prepare a lovely meal on the barbeque during your holiday while you enjoy the Spanish sun.L Escala is the most southerly village on the bay of Rosas. Here you can enjoy the peace and quiet or undertake some of the activities which L Escala and the environs have to offer. Whether you love sports such as diving. windsurfing. cycling or golfing. or are on the hunt for culture. Visit the famous Dali museum in Figueres only a 30 minute drive away.


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Reversal fuel, electricity hike, labour leaders tell FG - ITREALMS
ITREALMS ... making leadership SENSE with digital news!
The second meeting between the Federal Government and members of the organised Labour ended in deadlock as Labour Leaders insisted that FG reverses the fuel, and electricity hikes for any meaningful talk to exist, ITREALMS reports.
ITREALMS recalls that the Thursday meeting was the second on efforts by FG to forestall the avowed industrial action due to commence on Monday, September 28, 2020 by labour movement on the recent increase in the prices of petrol and electricity.
The Secretary to the Government of the Federation, Boss Mustapha who is leading the FG team, comprising of the Minister of Labour and Employment, Chris Ngige, Minister of State for Labour and Employment, Festus Keyamo, the Minister of Information and Culture, Lai Mohammed, the Minister of Power, Sale Mamman and the Minister of State for Petroleum Timipre Sylva.
Leaders from the Nigeria Labour Congress (NLC) and the Trade Union Congress (TUC).
Welcoming participants to the meeting, Ngige noted that the meeting will address the reasons behind the planned strike, assuring them that the government has no plan to make anyone go through hardship.
He said that they have received the ultimatum by the NLC for the strike, "and as a government, today's meeting will try to arrest the situation".
He promised that the meeting will be frank but have the general interest of the public.
The SGF in his response said that no government decision taken is intended to cause any harm to the workers.
He explained that he had the privilege of serving in the transition committee after President Buhari was first elected and that the report of that committee recommended the need for deregulation of the petroleum sector and energy sufficiency.
According to him, the government considered that decision as low hanging fruit to boost the economy, noting that the time was not ripe for it.
He, however, added that now it has become inevitable for the government to take the very painful decision, in the best interest of the country and the effects of the pandemic on the economy.
The SGF expressed concern with the organised Labour, saying he understands the hardship that certain decisions of government places on the people, but believe that within such decisions, are opportunities that as a nation, we can explore to help stabilize the system for while enduring the momentary pains.
The NLC President, Ayuba Waba said that labour as usual always makes itself available for dialogue as strike is usually the last option, pointing out that the NLC is a pan Nigerian organisation with a remarkable sense of patriotism, insisting that the recent increases have eroded the little gains made with the passage of minimum wage.
The TUC president Quadri Olaniye said that if there has been constructive engagement with Labour movement before now, some of the problems may have been solved, stressing that 30,000 minimum wage was agreed last year, only to face an increase in electricity tariff, fuel price increase, Value Added Tax (VAT) increase.
The meeting however, adjourned till Monday, September, 28, 2020.
This is coming as court in Abuja has stopped the proposed strike due to commence on Monday, September 28, this year.
Uboshe Uboshe/Editor, with additional reports by ChannelsTv.

redpajama_set_name: RedPajamaCommonCrawl


text:
Dean's Reign Will Be Fun to Watch
In making Howard Dean its new national chair, it's easy to imagine the Democratic Party thinking, "We've lost the White House and lost the Congress; what else do we have to lose?"
In making Howard Dean its new national chair, it's easy to imagine the Democratic Party thinking, "We've lost the White House and lost the Congress; what else do we have to lose?"
Usually the party chairmanship goes to a political technician, someone with backroom skills to mediate among the contending factions, and, of course, someone with fund-raising skills like Dean's prodigiously successful predecessor, Terry McAuliffe.
Dean was an aggressive, even abrasive presidential candidate and indeed may still harbor presidential ambitions. His campaign memorably came to a screaming halt in Iowa, but the spectacular circumstances of its demise should not detract from just how fast and how far he rose from political obscurity.
He was a governor from Vermont, a small state of the political and geographical mainstream. He was green in the mechanics of a national campaign. He did not realize that not having his wife Judy, a limelight-shunning physician, campaign with him raised unnecessary questions.
The miracle is that he got as far as he did with an unusual combination of enthusiastic young supporters _ the Deaniacs _ and a highly successful Internet fund-raising operation. Politically, he said in a dig at his fellow candidates, he represented "the Democratic wing of the Democratic Party."
That party needs help. Its traditional power bases, labor and big-city machines, are no longer what they were. It is losing ground in the South and West. It has just lost = fumbled away – two very winnable presidential elections. And it faces an aggressive, energized Republican Party with an instinct for the jugular.
Said the party's new leader: "If you told me one year ago that I would be standing here today as your choice for chairman of the Democratic National Committee, I wouldn't have believed you. And neither would a lot of other people."
A lot of other people would include the usual party power brokers for whom Dean was not a first choice. But they couldn't agree on any other choice. Almost by default, Dean is now his party's chief spokesman. Unfettered by congressional or statehouse duties, he has the bully pulpit and it should be fascinating how he uses it.
(Contact Dale McFeatters at McFeattersD(at)SHNS.com. Distributed by Scripps Howard News Service, https://www.shns.com)


text:
Zonda earns her crown in the Maryland Million Ladies
Matthew Schera's Zonda put forth a powerful performance in the $125,000 Maryland Million Ladies to defeat two-time Ladies winner My Sistersledge on Saturday, Oct. 19 at Laurel Park.
Making her fifth attempt to find a stakes victory out of eight career starts, Zonda moved to the front to trail pacesetter Something Magical, dueling briefly before surpassing her as they led the pack into the final turn. My Sistersledge moved up along the outside to present a challenge, but just couldn't extend enough to catch Zonda down the stretch, who managed to preserve a one and a half length win over the previous year's winner, with Jameela Stakes winner Ghoul's Night Out finishing third.
The James L. Lawrence, II trainee completed the one and one-eighth miles on the turf, rated firm, in 1:50.46, showing a much improved performance over her fifth-place finish in the one and a half mile Bald Eagle Derby at Laurel last month. Since winning her career debut in May, the 3-year-old filly has finished fifth in the Light Hearted Stakes, fourth in the Christiana Stakes at Delaware Park, and fourth in the Fasig-Tipton Virginia Oaks at Colonial Downs. Her most recent win prior to the Ladies was an allowance at Colonial Downs in August, where she finished a neck ahead of Princesschope.
A daughter of Scat Daddy who was purchased for $155,000 as a yearling by Schera in the 2017 Fasig-Tipton New York Saratoga Select sale, Zonda's record has improved to 3-1-0, with $159,407 in total earnings.
Bred by Caroline Stautberg's Willow Oaks Stable LLC, Zonda is out of the graded stakes placed daughter of Lemon Drop Kid, Winning Season. Out of 10 foals produced, Winning Season has five winners from six starters, with Zonda her first blacktype winner. Winning Season is the daughter of multiple graded stakes winning mare Topicount, a daughter of Private Account, who won the 1988 Rare Perfume Stakes-G2 and the 1990 Monmouth Park Budweiser Breeders' Cup Handicap-G3, also finishing second in the 1988 Acorn Stakes-G1 and 1989 John A. Morris Handicap-G1.
To view the replay of the Maryland Million Ladies, click here


text:
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redpajama_set_name: RedPajamaGithub