text:
\section{Introduction}
A breakthrough of GRB observation, made by {\it Swift} satellite in
2008, is the discovery of the very bright burst GRB 080319B which
was accompanied by a naked-eye optical flash \citep{Racusin08b}. The
optical observation was going on even before the onset of the
$\gamma$-ray burst because TORTORA was monitoring the same region of
the sky at that moment \citep{Cwiok08, Karpov08}. The X-ray
telescope (XRT) onboard {\it Swift} satellite slewed to the source
about $60$ sec after the trigger of the burst and recorded a quickly
decaying but extremely bright X-ray component. These continuous
observations collected fruitful data \citep{Racusin08b,Bloom08} and
rendered GRB 080319B one of the best-studied bursts so far. Although
no very high-energy emission was directly detected from GRB 080319B
the unique spectrum of this burst and its afterglow suggest that it
has been accompanied by a very strong GeV-TeV emission that would
have already been detected by AGILE if not occulted by earth at that
moment. Based on a model in which the prompt optical and soft
$\gamma$-ray emission are respectively the synchrotron and the first
order inverse Compton (IC) radiation components of the internal
shocks, Kumar \& Panaitescu (2008), Racusin et al. (2008) and Fan \&
Piran (2008) suggested that the second order IC of the internal
shocks would peak in GeV-TeV energy range and the isotropic energy
might be high up to $\sim 10^{55}$ erg (see however Piran, Sari \&
Zou 2008 and Fan, Zhang \& Wei 2009). Because of the tight
overlapping of the prompt emission with the reverse/forward shock
regions, some soft $\gamma$-rays will be up-scattered by the reverse
shock electrons and some prompt optical photons will be up-scattered
by the forward shock electrons, i.e., the so-called external inverse
Compton (EIC). As a result, two additional GeV-TeV emission
components with a duration $\sim 100$ s are expected (Fan \& Piran
2008). In this work, we discuss these possibilities in more detail.
Moreover, we show that the early ($60-2000$ s) forward shock
synchrotron and the synchrotron self-Compton (SSC) emission in the
energy range $20{\rm MeV}-300{\rm GeV}$ is as powerful as the high
energy emission detected in GRB 080916C \citep{Tajima08}. A
schematic plot of the expected GeV-TeV signals from GRB 080319B is
shown in Fig.\ref{fig:sketch}.

\begin{figure}
\includegraphics{f1.eps}
\caption{Schematic light curves for the different component of the
high energy emissions: {prompt} SSC, reverse shock EIC, forward
shock EIC and external shock SSC respectively.} \label{fig:sketch}
\end{figure}

Since its successful launch on June 11 2008, the Fermi satellite has
detected the prompt $>10$ GeV emission in GRB 080916C
\citep{Tajima08,Omodei08b}, and the GeV emission following a short
burst GRB 081024B \citep{Omodei08}. As GRB 080514B
\citep{Giuliani08}, GRB 080825C \citep{Bouvier08}, and some other
events detected by the Compton Gamma Ray Observatory (CGRO)
satellite in 1991-2000 \citep{Hurley94,Gonz03}, the high energy
emission of both GRB 080916C and GRB 081024B lasted longer than the
prompt soft $\gamma$-rays. The detection of high energy signals
sheds some lights on the bulk Lorentz factor of the ejecta, the
radiation mechanisms, the physical composition of the outflow and
the prolonged activity of the central engine. This is particularly
the case if the simultaneous X-ray/optical emission data are
available (see Fan \& Piran 2008 for a recent review). In this work
we'll outline the origins of the GeV emission from GRB 080514B, GRB
080916C and GRB 081024B, based on the (preliminary) public data.

The paper is structured as follows. In Section 2, we calculate the
possible prompt and afterglow GeV-TeV emission of GRB 080319B. In
section 3, we interpret the high energy emission detected in GRB
080514B, GRB 080916C and GRB 081024B. In section 4, we summarize our
results with some discussions.

\section{Possible GeV-TeV emission from GRB 080319B}
GRB 080319B \citep{Racusin08b} was most notable due to its huge
total energy and especially its extremely luminous prompt optical
emission that could be seen with naked eyes \citep{Cwiok08,
Karpov08}. This burst was located at a redshift $z=0.937$ space
\citep{Vreeswijk08} and duration was $T_{90}\sim $57s. The peak
energy of the {$\nu F_{\nu}$ spectrum} was $E_{\rm p} \simeq
675\pm22$ keV, and the photon indexes below and above $E_p$ were
$-0.855^{+0.014}_{-0.013}$ and $-3.59^{+0.32}_{-0.62}$ respectively.
Choosing standard cosmological parameters $H_0=70 {\rm km}\,{\rm
s}^{-1}\,{\rm Mpc}^{-1}, \Omega_{\rm M}=0.3,
\Omega_\Lambda=0.7$(corresponding to a luminosity distance $D_L \sim
1.9\times 10^{28}$cm), we have a peak luminosity $L_{\rm peak} \sim
1.0\times 10^{53} {\rm erg}\,{s}^{-1}$ and an isotropic energy
$E_{\rm iso} \simeq 1.3 \times 10^{54}$ erg
\citep{Racusin08b,Bloom08,ga08}. \citet{Karpov08} reported the
optical V-band ($\sim 6 \times 10^{14}$Hz) light curve in the prompt
phase (from $\sim$ -10 s to $\sim$ 100 s). Variability was evident
and there were at least 3 or 4 main pulses in the light curve. The
peak V-band reached magnitude of 5.3, corresponding to a flux
density $\sim 28.7$ Jy, and {isotropic equivalent} energy $E_{opt}
\sim 2 \times 10^{52}$erg if we take $\sim 20$ Jy as the average
flux density. The variability and the very sharp decline of the
prompt optical emission support an internal origin of these optical
photons, though the underlying physical process is not clear yet
\citep[see][for a discussion of various possible models]{zps08}.

Afterglow modeling can in principle constrain the total kinetic
energy and the initial Lorentz factor of the GRB ejecta, and the
physical parameters of the external shocks (Sari, Piran \& Narayan
1998; Chevalier \& Li 2000; Panaitescu \& Kumar 2001). The behavior
of the afterglow of GRB 080319B suggests a free wind medium
\citep{kp08,Racusin08,Wu08}. A self-consistent modeling of the X-ray
and optical afterglow data favors a two-component jet model
\citep{Racusin08,Wu08}. Moreover, the shock parameters of the narrow
and wide ejecta components need to be very different, as found in
GRB 051221A \citep{Jin07}. Following \citet{Racusin08} and
\citet{Wu08}, we take the isotropic kinetic energy of the narrow
ejecta (represented by the subscript $``{\rm n}"$) $E_{\rm k,n}\sim
3\times 10^{55}$ erg\footnote{An $E_{\rm k,n}$ high up to $\sim
10^{55}$ erg is rather unusual. Similar result has only been
reported in the afterglow modeling of GRB 060418 \citep{JF07}.
However we believe that such a huge value is possible for GRB
0980319B because the XRT flux at $t\sim 70$ s is as bright as $\sim
10^{-7}~{\rm erg~s^{-1}~cm^{-2}}$, which is the brightest X-ray
afterglow detected so far and is even much brighter than most prompt
X-ray emission of {\it Swift} GRBs. On the other hand both the
spectral and the temporal behaviors of the early ($60-2000$ s) X-ray
emission strongly favor a fireball model in the slow cooling phase,
which requires small $\epsilon_{\rm B,n}$ and $A_*$. As a result, we
do need an $E_{\rm k,n} \sim 10^{55}$ erg to reproduce the
observation data (see footnote \ref{foot:2}).}, the wind parameter
$A_* \sim 0.01$, the fraction of forward shock energy given to the
electrons $\epsilon_{\rm e,n}\sim 0.1$, the fraction of forward
shock energy given to the magnetic field\footnote{We do not take
$\epsilon_{\rm n,B} \sim 10^{-6}$ as in \citet{Racusin08} (see
section \ref{sec:EIC} below) because the peak flux density of the
forward shock synchrotron emission is $F_{\nu,\max}^{\rm syn} \sim 9
 \epsilon_{\rm B,n,-4}^{1/2} E_{\rm k,n,55.5}^{1/2} A_{\star,-2} D_{L,28.3}^{-2}  t_3^{-1/2}
{\rm mJy}$. At $t\sim 60$ sec, the X-ray (at 1 keV) flux $\sim 20$
mJy \citep{Bloom08} disfavors an $\epsilon_{\rm B,n}$ as small as
$\sim 10^{-6}$. On the other hand, an $\epsilon_{\rm B,n} \sim
10^{-6}$ will give rise to a too large cooling Lorentz factor
$\gamma_c \sim 10^{10}(1+Y_{\rm ssc})^{-1}$, where the forward shock
SSC parameter  $Y_{\rm ssc}\ll \sqrt{\epsilon_{\rm
n,e}/\epsilon_{\rm B,n}}$ since the SSC emission of such energetic
electrons should be in Klein-Nishina regime and thus be effectively
suppressed.\label{foot:2}} $\epsilon_{\rm B,n}\sim 10^{-4}$, the
power-law distribution index $p_{\rm n} \sim 2.4$, and the
half-opening angle $\theta_{\rm j,n} \sim 0.2$ degree. We do not
discuss the wide jet component because it plays a less important
role in producing GeV-TeV afterglow emission. The average Lorentz
factor of the narrow jet outflow ($\Gamma_{\rm i}$) before getting
decelerated by a stellar wind medium is very high. A lower limit can
be set by the Lorentz factor of the forward shock at $\sim 70$ s,
when the X-ray afterglow began to decline normally, i.e.
\citep{BM76,DL98},
\[
\Gamma \simeq 600 E_{\rm
k,n,55.5}^{1/4}A_{*,-2}^{-1/4}(t/70{\rm s})^{-1/4}[(1+z)/2]^{1/4}.
\]
So a choice of $\Gamma_{\rm i}\sim 1000$ is rather reasonable.
Throughout this work we adopt the convenience $Q_{\rm x}=Q/10^{\rm
x}$ in units of cgs

In the leading fireball model for GRBs \citep[see][for
reviews]{Piran04,Mesz02,Zhang07}, the synchrotron and IC radiation
will give rise to a high-energy component that will be emitted along
with the prompt sub-MeV photons and the afterglow
radio/optical/X-ray emission \citep{fp08}.
Depending on the seed photons' origins, IC can be
SSC or EIC. Below we'll show that for GRB
080319B both processes plausibly played an important role in
producing GeV$-$TeV emission. This suggests that similar bursts will
provide promising sources for the Fermi high energy satellite.

\subsection{Prompt GeV-TeV IC emission}
\citet{zps08} showed that the SSC models in which the soft
$\gamma$-rays are the IC component of the optical photons cannot
explain the observations. The major obstacle is the resulting high
synchrotron self-absorption frequency and then the X-ray spectrum
that is inconsistent with the observation. If we ignore this
problem, there is a solution with a Compton parameter $Y \sim 1$ and
a stochastic Lorentz factor $\gamma_{\rm e} \sim 100$. Then the 2nd
IC peaks at $2 \gamma_{\rm e}^2 E_{\rm p} \sim 15$ GeV, and the
number of the detectable photons is $Y E_\gamma S_{\rm det}/4\pi
D_L^2 h \nu_{\rm 2nd,IC}$ corresponding to detected $\sim 130$
photons by LAT, with $S_{\rm det} \sim 10^4 {\rm cm}^2$ at GeV
energies. Other models with larger $Y$ lead to even stronger signals
\citep{kp08,Racusin08,fp08}.

{As discussed in \citet{zps08}, too much energy should be hidden
either in the high energy component if using the SSC model, or in
the low energy component of electrons if we assume the prompt
optical emission and $\gamma$-rays were from synchrotron emission by
two components of electrons in the same region. It indicates that
the two different bands of prompt photons should come from different
geometrical regions.} Below we consider these two different regions
{(possibly but not necessarily two sets of different internal
shocks) within the same outflow cone}, denoted by the subscripts
``${\rm opt}$" and ``$\gamma$" respectively. Strong high energy
prompt emission is still possible, and it can be estimated even
though the {details of the internal shocks are} still unclear. The
possible high energy emission consists of four components: self-IC
scattering in the optical emission region; self-IC scattering in the
$\gamma$-ray producing region; optical photons IC scattered in the
$\gamma$-ray producing region; and soft $\gamma$-rays IC scattered
in the optical emission region. Note that because of the steep
decline of the high energy slope ($\beta \sim 2.6$) extrapolation of
the soft $\gamma$-ray emission gives only a very weak signal.

\subsubsection{Prompt optical emission region}
The observed optical flux density limits the temperature of this
region (see the Appendix for the derivation):
\begin{equation}
f_{\rm \nu,opt} \leq \frac{2\pi \nu^2_{\rm opt} (1+z)^3 \Gamma_{\rm
opt} k T_{\rm opt}}{c^2} \left(\frac{R_{\rm opt}}{\Gamma_{\rm opt}
D_L}\right)^2,\label{eq:main1}
\end{equation}
where $\Gamma_{\rm opt}$ is the bulk Lorentz factor, $k$
is Boltzmann constant, $T_{\rm
opt}$ is the temperature (while the minimal temperature $T_{\rm
opt,\min}$ corresponds to the equality), and $R_{\rm opt} $ is the
emission region radius. Thus
\begin{equation}
k T_{\rm opt,\min} = 6 \times 10^{-5} \Gamma_{\rm opt,3} R_{\rm
opt,16}^{-2}~ {\rm erg}.
\end{equation}
Noticing that the bulk Lorentz factor in the afterglow is
high \citep{Racusin08}, we take a fiducial  value of
$\Gamma_{opt}\sim 10^3$ for the
prompt phase. Considering the variability of the light curves
and the deceleration radius, which constrains the radius should not
be too large, then the choice $10^{16}$cm is reasonable. The
corresponding typical stochastic Lorentz factor of the electrons is
\begin{equation}
\gamma_{\rm e,opt,\min} \sim k T_{\rm opt,\min}/(m_{\rm e} c^2) \sim
75 \Gamma_{\rm opt,3} R_{\rm opt,16}^{-2}, \label{eq:gamma_e,min}
\end{equation}
where $m_{\rm e}$ is the rest mass of the electron.

The first order IC is in the soft $\gamma$-ray band. As mentioned
before the prompt soft $\gamma$-rays are unlikely to be the first
order IC component of the optical emission. So the first order IC
radiation of the electrons emitting optical photons would be much
smaller than the detected soft $\gamma$-rays.
Correspondingly, the 2nd order IC radiation in GeV-TeV energy range
is unimportant as it falls below the IC radiation that arises when the prompt soft
$\gamma$-rays cross the optical emission region.

\subsubsection{$\gamma$-rays IC scattered in the prompt optical emission region}
\label{sec:gamma-opt}
If the soft $\gamma$-rays pass through the prompt optical
emitting electrons, the ``optical depth" for electrons is approximately
$
 \sigma_T \frac{N_{\gamma} \delta t_{\rm opt}/T_{90}}{4\pi R^2}
{\iffalse
= 6.65e-25  \frac{1.3e60 }{4\pi 1e32 R_{16}^2} \\
\fi} \sim 3 N_{\gamma,60}R_{16}^{-2} \delta t_{\rm opt,-0.5}, $
 where $\delta t_{\rm opt} \sim 0.3~R_{\rm
opt,16}\Gamma_{\rm opt,3}^{-2}$s is the typical variability
timescale of the prompt optical emission. For each collision the
electron loses energy $\sim \gamma_{\rm e,opt}^2
h\nu_{\gamma}/\Gamma_{\rm opt} < \gamma_{\rm e,opt} m_e c^2 $ as
long as $\gamma_{\rm e,opt} < \Gamma_{\rm opt}$.

Assuming that almost all electrons carried by the GRB outflow
contributed to the prompt optical emission, which should be an upper limit,
we estimate the
number of electrons that participate in a typical optical pulse (with a
variability timescale $\delta t_{\rm opt}$):
\begin{equation}
N_{\rm e,p,opt} \sim  {E_{\rm k,n}\delta t_{\rm opt}\over
\Gamma_{\rm i}m_{\rm p}c^2T_{90}}
 \sim 10^{53}E_{\rm k,n,55.5}\delta t_{\rm opt,-0.5}\Gamma_{\rm
 i,3}^{-1}.
 \label{eq:N_eopt}
\end{equation}

Using this value we estimate the optical depth for soft
$\gamma-$rays being scattered by the
electrons emitting the prompt optical emission as $\tau \sim
\sigma_{\rm T}N_{\rm e,p,opt}/(4\pi R_{\rm opt}^2)\sim 5 \times
10^{-5}E_{\rm k,n,55.5}\delta t_{\rm opt,-0.5}\Gamma_{\rm
i,3}^{-1}R_{\rm opt,16}^{-2}$. The total number of the IC photons
detectable by LAT is thus
\begin{equation}
N_{\rm det,\gamma-\rm opt} \sim {\tau N_{\gamma} S_{\rm det} \over
4\pi d_L^2} \leq  100 E_{\rm k,n,55.5}{\delta t_{\rm
opt,-0.5}}\Gamma_{\rm i,3}^{-1}R_{\rm opt,16}^{-2}N_{\gamma,60},
\label{eq:opt-gamma}
\end{equation}
where $N_{\gamma}$ is the total number of prompt soft $\gamma-$rays.
The typical energy of the IC photons
is greater than $E_{\rm IC,\gamma-opt}\sim 2\gamma_{\rm
e,opt,\min}^2 E_p \sim 8 \Gamma_{\rm opt,3}^2 R_{\rm opt,16}^{-4}$
GeV. The corresponding total energy of these photons is
$\sim 5 \times 10^{53}$ergs.

\subsubsection{Soft $\gamma$-ray emission region}
Since there may be no suitable IC model for the soft $\gamma$-rays,
we assume that these soft $\gamma$-rays are the synchrotron emission
 at a radius $R_{\rm \gamma}$. To match the
peculiar spectrum of the soft $\gamma$-rays, the cooling Lorentz
factor $\gamma_{\rm c} \sim (1+z) \frac{6\pi m_{\rm e}
c}{\sigma_{\rm T} \Gamma B^2 \delta t_{\gamma}}$ should be
comparable to the typical Lorentz factor of the electrons
$\gamma_{\rm m}$ \citep{zps08}. $\sigma_{\rm T}$ is the Thompson's cross
section and $\delta t_{\gamma} \sim 0.1$s \citep{Margutti08}
is the variability timescale of the
soft $\gamma$-rays. The condition $E_{\rm p}\sim \Gamma_{\gamma} 2
\gamma_{e,\gamma}^2 \frac{q_{\rm e} B_\gamma}{2 \pi m_{\rm e} c} /(1+z)$
gives
\begin{equation}
B_\gamma \sim 16 \Gamma_{\gamma,3}^{-1/3} \delta t_{\gamma,
-1}^{-2/3} {\rm Gauss},
\end{equation}
where $q_{\rm e}$ is the electron's charge. The typical Lorentz
factor of the emitting electrons is thus
\begin{equation}
\gamma_{\rm e,\gamma} \sim 6 \times 10^4 \Gamma_{\gamma,3}^{-1/3}
\delta t_{\gamma,-1}^{1/3}.\label{eq:gamma_egamma}
\end{equation}
{This value is relatively too high for internal shocks. However,
here we don't need it come from the internal shocks necessarily. The
other energy dissipation mechanisms may produce high $\gamma_e$.}
The SSC will be deep in the Klein-Nishina regime, and pair avalanche
effect might exist \citep{psz08}, additional component of high
energy photons would peak at energy $\Gamma_\gamma^2 (m_e
c^2)^2/(h\nu_\gamma) \sim 400 \Gamma_{\gamma,3}^2$ GeV, where $h$ is
the Plank constant.

Using $f_{\rm \nu,\max} = (1+z) N_{\rm
e,\gamma} \Gamma_{\rm \gamma} m_{\rm e} c^2 \frac{\sigma_{\rm T}
B}{3 q_{\rm e} 4\pi D_L^2}$, we get the number of electrons for each pulse
$N_{\rm e,p,\gamma} \sim 9 \times 10^{49} \Gamma_{\gamma,3}^{-2/3}
\delta t_{\gamma,-1}^{2/3}$, and the total number of electrons
is then $N_{\rm e,\gamma} \sim N_{\rm e,p,\gamma} T_{90}/\delta t_\gamma
\sim 5 \times 10^{52}\Gamma_{\gamma,3}^{-2/3} \delta t_{\gamma,-1}^{-1/3}$.

The corresponding optical depth for Thompson scattering is $\tau \sim
\sigma_T N_{\rm e,\gamma}/(4\pi R_\gamma^2) \sim 5 \times 10^{-8}
N_{\rm e,p,\gamma,50}R_{\gamma,16}^{-2}$.
\footnote{By the time a single photon passes through a sub-shell, this sub-shell
expands by a factor of $\sim$2 in radius. Therefore subsequent scattering in
other sub-shells will be negligible and when considering the optical depth
a single sub-shell should be taken into account.}
 And the Compton parameter in
KN regime is $Y \sim \gamma_{e,\gamma}^2 \tau /[\gamma_{e,\gamma}
h\nu_\gamma/(\Gamma m_e c^2)]^2 \sim 0.03  \Gamma_{\rm \gamma,3}^2
N_{\rm e,p,\gamma,50}R_{\gamma, 16}^{-2}$\citep{psz08}.
The total energy of the avalanche loaded pairs is in the order
of $2 Y E_\gamma$ even all the first produced very high energy photons
are cooled into steady pairs.  The number of
detectable photons by LAT is then
$\sim 0.1 R_{\gamma,16}^{-2}$. It is thus undetectable
even without taking into account the large optical depth ($\sim 10$)
of the universe to such energetic photons (Stecker et al. 2006).

\subsubsection{optical photons IC scattered in $\gamma$-rays region}
If the optical photons are produced in smaller radii than the soft $\gamma$-rays (
$R_\gamma \geq R_{\rm opt}$), they would be
 IC scattered in the $\gamma$-rays region. In this case, the electrons will be
 cooled to a random Lorentz factor $\gamma_{\rm e,\gamma,c} < 1.8\times 10^{4}
 \Gamma_{\rm \gamma,3}^3R_{\gamma,16}L_{\rm opt,50.7}^{-1}<\gamma_{\rm e,\gamma}$ \citep{fp08},  where
 $L_{\rm opt}>5\times 10^{50}{\rm erg~s^{-1}}$ is the luminosity
 of the prompt optical emission,
 suggesting that all the electrons were cooled by the IC scattering.
 The typical energy of the IC scattered photons is
$E_{\rm IC,opt-\gamma}\sim 2\gamma_{\rm
e,\gamma}^2 h \nu_{opt} \sim 14 \Gamma_{\gamma,3}^{-2/3} \delta t_{\gamma,-1}^{2/3}$GeV.
Since the electrons lost almost all the energy, the number of the
detectable photons by LAT is
\begin{equation}
N_{\rm det,opt-\gamma} \sim {E_{\rm e,\gamma}S_{\rm det}\over 4\pi
D_L^2 E_{\rm IC,opt-\gamma}} \sim 240 \Gamma_{\gamma,3}^{-1/3}
\delta t_{\gamma,-1}^{-2/3}
\end{equation}
where $E_{\rm e,\gamma}\approx \Gamma_{\gamma}\gamma_{\rm
e,\gamma}N_{\rm e,\gamma}m_{\rm e}c^2$ is the total energy carried
by the electrons emitting soft $\gamma-$rays.

{\it This discussion is valid only for $R_{\rm \gamma}\geq R_{\rm
opt}$ that is less likely}. As long as $R_{\rm opt} \gtrsim {\rm a
~few \times} R_{\rm \gamma}$, the prompt optical emission cannot
cool the accelerated electrons emitting soft $\gamma-$rays,
because the photons from $R_{\rm opt}$ reached $R_\gamma$ in a time
$\sim (R_{\rm opt}-R_{\rm \gamma})/c \sim 3\times 10^{5}~R_{\rm
opt,16}~{\rm sec}$ when the photons at $R_\gamma$ had been
disappeared long before. For the same reason,  there would be no
high energy photons produced by the optical region electrons as
presented in section \ref{sec:gamma-opt} (i.e., $N_{\rm
det,\gamma-opt}=0$) if $R_{\rm \gamma}>R_{\rm opt}$.

\subsection{Very early EIC emission}\label{sec:EIC}
Whatever the mechanism is, the prompt emission should have an
internal origin, in view of the high variability of the light curves
and the very sharp decline at $t>T_{90}$. External reverse-forward
shock formed very quickly. Consequently, the prompt photons passing
through the reverse/forward shock regions were IC scattered by the
shock accelerated electrons. As a result, two additional GeV-TeV EIC
components were present.

\subsubsection{EIC in reverse shock region}
 \citet{Racusin08} and \citet{Wu08} argued that the reverse shock
 emission of the narrow jet component had not been seen. Its physical
 parameters are thus unknown. {In some optical flash modeling, the
$\epsilon_{\rm B}$ (or/and $\epsilon_{\rm e}$) of RS is found to be
much larger than that of the FS (Fan et al. 2002; Zhang, Kobayashi
\& M\'esz\'aros 2003; Kumar \& Panaitescu 2003; cf. Nakar \& Piran
2005). However, if such a phenomena is popular very bright optical
flashes would be frequently detected (McMahon, Kumar \& Piran 2006),
inconsistent with current optical afterglow observations. For the
particular burst GRB 080913B, Racusin et al. (2008) argued that the
RS of the wide jet component has an $\epsilon_{\rm B} \sim 0.1$,
much larger than that of the corresponding FS. However, if
$\epsilon_{\rm B} \sim 0.1$ holds for the RS of the narrow core too,
the resulting optical emission would be too strong to match the data
(X. F. Wu. 2008, private communication). On the other hand, assuming
that these two parameters are the same as those of the forward
shock, it is straightforward to show that the RS optical emission of
the narrow core is $\sim 0.3$ Jy at the crossing time, outshone by
the simultaneous prompt emission and consistent with the data. So
below we simply assume that the shock parameters of the FS and RS
are the same for the narrow jet component.}

The reverse shock emission must have
 overlapped the prompt gamma-rays and optical emission. Therefore
 the electrons accelerated by the reverse shock front were
 cooled by the prompt emission and gave rise to an EIC radiation
 component \citep{b05,fzw05}.

The number of electrons in the reverse shock region is
\begin{equation}
N_{\rm e,r} \simeq \frac{E_{\rm k,n}}{\Gamma_{\rm } m_p c^2} \simeq
3 \times 10^{55} E_{\rm k,n,55.5} \Gamma_{\rm 2.8}^{-1}.
\end{equation}

The typical radius of the reverse shock can be estimated as
\begin{equation}
R_{\rm r} \sim 2\Gamma^2 c T_{90}/(1+z) \sim 5\times 10^{17}
\Gamma_{2.8}^2 ~{\rm cm}.
\end{equation}

The optical depth of the prompt photons being scattered by the electrons was
\begin{equation}
\tau_{\rm r} \sim \sigma_{_{\rm T}}{N_{\rm e,r}\over 4 \pi R_{\rm
r}^2} \sim 7 \times 10^{-6}E_{\rm k,n,55.5} \Gamma_{\rm
2.8}^{-1}R_{17.7}^{-2}.
\end{equation}

On the other hand, the total number of the prompt soft $\gamma$-rays
that reached us (per area) can be estimated as \citep{fp06}
\begin{equation}
N_{\rm tot,\gamma} \sim  {\beta_{_{\gamma}}-1 \over
\beta_{_{\gamma}}}{{\cal F} \over h\nu_{\rm \gamma,p}},
\label{eq:N_tot}
\end{equation}
where ${\cal F} \sim 10^{-4}~{\rm erg~cm^{-2}}$ is the energy
fluence of the prompt $\gamma$-rays and $\beta_\gamma \sim 2.6$ is
the high energy spectral index of the prompt $\gamma$-ray emission.

The number of the reverse shock EIC photons detectable by the Fermi satellite
and their typical energy can be estimated as
\begin{equation}
N_{\rm det,r} \sim \tau_{\rm r} N_{\rm tot,\gamma} S_{\rm det} \sim
3,
\end{equation}
and
\begin{equation}
h\nu_{_{\rm EIC},\rm r} \sim 2 \gamma_{\rm m,r}^2 E_{\rm p} \sim
13~{\rm GeV}~({\gamma_{\rm m,r} \over 100})^2,
\end{equation}
where $\gamma_{\rm m,r}$ \footnote{$\gamma_{\rm m,r} \sim
\epsilon_{\rm e,n} (\bar\gamma-1) m_{\rm p}/m_{\rm e} (p_{\rm
n}-2)/(p_{\rm n}-1)$, where $m_{\rm p}$ is the rest mass of the
protons, and $\bar\gamma$ indicates the internal energy density in
the shocked region.  For the mildly Relativistic reverse shock we
have $\bar\gamma-1 \sim 1$ \citep[see][for details]{zwd05}, which is
the case for this burst.
}
 is the minimal Lorentz factor of the electrons
accelerated in the reverse shock front. The electrons are in slow
cooling phase since the cooling Lorentz factor is \citep{fp08}
\begin{equation}
\gamma_{\rm c,r} \sim 10^{3} \Gamma_{2.8}^3 R_{\rm
r,17.7}L_{\gamma,52.7}^{-1}>\gamma_{\rm m,r}.
\end{equation}
The Compton parameter $Y_{\rm EIC,r} \sim \gamma_{\rm m,r}^2
\tau_{\rm r} \ll 1$. So the energy of this EIC component was much
smaller than that of the prompt soft $\gamma$-rays.

Here we do not take into account the cooling caused by the
synchrotron radiation because $U_B \sim  \varepsilon_B 4 \Gamma^2 A
R^{-2} m_p c^2 \sim 3.3\times 10^{-3}  \varepsilon_{\rm n,B,-4}
\Gamma_{2.8}^2 A_{\star,-2} R_{17.7}^{-2}{\rm erg}\,{\rm cm}^{-3}$,
which is much smaller than $U_{\gamma} \sim \frac{L_{\gamma}}{4\pi
R^2 \Gamma^2 c} \sim 1.3 \Gamma_{2.8}^{-2} R_{17.7}^{-2}{\rm
erg}\,{\rm cm}^{-3}$.

Some prompt optical photons will be up-scattered  by the reverse
shock electrons and will be boosted to an energy $\sim
2\gamma_{m,r}^2 h\nu_{\rm opt} \sim 10$ keV, which is too low to be
of interest.

\subsubsection{EIC in forward shock region}
The prompt emission will
cool the forward shock electrons as well \citep{fzw05,wp06}.
However, for the prompt $\gamma$-rays, this EIC process is
unimportant since it is in the Klein-Nishina regime. Because
the large radius lowers the optical depth, pair
avalanche does not exist in this case. Here we focus
on the EIC radiation of the prompt optical emission. The
energy density of the emitted prompt photons is $U_{\rm opt} \sim
\frac{L_{\rm opt}}{4 \pi R^2 \Gamma^2 c} \sim 0.05~L_{\rm
opt,51}\Gamma_{2.8}^{-2} R_{17.7}^{-2}{\rm erg}\,{\rm cm}^{-3}$,
which is larger than $U_{\rm B}$. So the cooling of the forward
shock electrons is dominated by the EIC process.

The number of the electrons swept by the forward shock is
\begin{equation}
N_{\rm e,f} \simeq 4 \pi A R \simeq 1.8 \times 10^{52} A_{\star,-2}
R_{17.7}.
\end{equation}
The optical depth of the prompt photons for being scattered is thus
\begin{equation}
\tau_{\rm f} \sim \sigma_{_{\rm T}}{N_{\rm e,f}\over 4 \pi R^2} \sim
3\times 10^{-9}A_{\star,-2} R_{17.7}^{-1}.
\end{equation}

Noticing that we don't know the spectrum in the optical band, {we
can only evalute the lower limit by taking the observed optical
emission as the peak.} The total number of the optical photons
reaching us (in unit area) can be estimated as
\begin{equation}
N_{\rm tot, opt} \sim  {{\cal F}_{\rm opt} \over h\nu_{\rm opt}}\sim
10^6 {\rm cm}^{-2}, \label{eq:N_tot_opt}
\end{equation}
where ${\cal F}_{\rm opt}$ is the fluence of the prompt optical
emission.

For Fermi, the detectable number of the forward shock EIC radiation
can be estimated as
\begin{equation}
N_{\rm det,f} \sim \tau_{\rm f} N_{\rm tot,opt} S_{\rm det} \sim 30.
\end{equation}
{Usually for an integration time $t_{\rm int}\lesssim 10^{5}$ sec,
LAT needs 5 high energy photons to claim a significant detection
\citep[e.g.,][]{zm01,fzw05}. With a duration of 120 s, and for the
typical energy of $\sim 10$ GeV, this detection corresponds to $8
\times 10^{-8} {erg/s/cm^2}$. We plot such a threshold in
Fig.\ref{fig:eic} and find out that the forward shock EIC emission
component is detectable in $\sim 100$ s, longer than the prompt soft
gamma-ray emission.}

The typical energy of these forward shock EIC photons is
\begin{equation}
h\nu_{_{\rm EIC},\rm f} \sim 2 \min\{\gamma_{\rm c,f}^2,\gamma_{\rm
m,f}^2\} h\nu_{\rm opt} \sim 10~{\rm GeV},
\end{equation}
where $\gamma_{\rm c,f}\sim 10^5 \Gamma_{2.8}^3R_{17.7}L_{\rm
opt,51}^{-1}$ and $\gamma_{\rm m,f} \sim 4\times 10^4 \Gamma_{2.8}$.

The Compton parameter $Y_{\rm EIC,f} \sim \gamma_{\rm m,f}^2
\tau_{\rm f} \sim 10$. As the emitted energy of the optical photons was
$3\times 10^{52}$ergs (isotropic), the total energy of the EIC
photons by forward shocked electrons is $\sim 3\times 10^{53}$ergs.

In the rest frame of the forward shock, the seed optical photons
have a typical energy $\sim \gamma_e h\nu_{\rm opt}/\Gamma < m_e
c^2$ for $\gamma_e<10^{8} \Gamma_{2.8}(h\nu_{\rm opt}/2{\rm
eV})^{-1}$. So the EIC scattering in the forward shock front is well
in the Thompson regime. The resulting spectrum for $\nu<\nu_{_{\rm
EIC},\rm f}<1$ TeV is expected to be not steeper than  $F_\nu
\propto \nu^{-p_{\rm n}/2} \sim \nu^{-1.2}$. On the other hand, the
absorption depth for a 30 GeV photons from a redshift $z\sim 1$ is
only about 1 \citep{stecker06}. So we expect that, if Fermi worked
at that moment, it could have detected some photons as energetic as
$\sim 30$ GeV.

Though very bright optical flashes from GRBs are very rare, a few
such events are still possible during Fermi's 10 years of operation.
Since the EIC component from the forward shock
region can give rise to a significant detection for a Fermi-like
satellite, here we use the numerical code by \citet{fan08} to
a more detailed estimate. For simplicity, we approximate the prompt optical
emission flux by $F=10^{-7}(t/10)^6~{\rm erg~s^{-1}~cm^{-2}}$ for
$t<10$ sec, a constant plateau lasting till $t\sim 60~{\rm sec}$,
and $F=0$ afterward. The optical spectrum is set as a typical Band
function \citep{band93}, for which (the break energy, the low energy
spectral index, the high energy spectral index) are taken as (2 eV,
-1, -2.25), respectively. Notice that it is also a lower limit, as
we take the V band as the peak. As shown in Fig. \ref{fig:eic}, the
forward shock EIC emission lasts about twice that of the
prompt emission. This is because the duration of the high-energy
emission is affected by the spherical curvature of the blast wave
\citep{b05} and is further extended by the highly anisotropic
radiation of the up-scattered photons \citep{fp06,wp06}. We also
find out that the total energy of the EIC emission is about 10 times
that of the prompt optical emission, consistent with our analytical
estimate.

\begin{figure}
\includegraphics[width=0.5\textwidth]{f2.eps}
\caption{The numerical light curve of GRB 080319B forward shock EIC
emission, and the prompt optical prototype is also shown. {The dash
dot-doted line represents the detection of five $\sim 10$ GeV
photons in $120$ sec by LAT onboard Fermi satellite.}}
\label{fig:eic}
\end{figure}

\subsection{The late GeV-TeV emission of the external forward shock}
The high energy emission of the external forward shock has been
extensively discussed in literature since 1994 (M\'esz\'aros \& Rees
1994; Dermer, Chiang \& Mitman 2000; Sari \& Esin 2001; Wang, Dai \&
Lu 2001; Zhang \& M\'esz\'aros 2001; Fan et al. 2008).
GRB 080319B is distinguished from most bursts
by its huge $E_{\rm k,n}$ and by the
large contrast between $\epsilon_{\rm n, e}$ and $\epsilon_{\rm
n,B}$, both indicating a very strong high energy radiation
component.

In the very early afterglow phase ($t\leq 60$ s), the Lorentz factor
of the forward shock is almost a constant. The typical Lorentz factor
of the shocked electrons is $\gamma_{\rm m} \sim 4\times 10^4
\epsilon_{\rm e,n,-1} \Gamma_{2.8}$.

After that, the forward shock forms a self-similar profile and its
Lorentz factor can be estimated as
\begin{eqnarray}
\Gamma \sim 310 (1+z)^{1/4} E_{\rm k,n,55.5}^{1/4}
A_{\star,-2}^{-1/4} t_3^{-1/4}.
\end{eqnarray}
The typical Lorentz factor of the shocked electrons is
\begin{eqnarray}
\gamma_{\rm m} \sim 2 \times 10^4 (1+z)^{1/4} \epsilon_{\rm e,n,-1}
E_{\rm k,n,55.5}^{1/4} A_{\star,-2}^{-1/4} t_3^{-1/4}.
\end{eqnarray}

At this stage, the forward shock is in the slow cooling phase
\citep{Racusin08}, and $\nu_{\rm m} < \nu_{\rm X} <\nu_{\rm BAT}
<\nu_{\rm c}$, where $\nu_{\rm BAT} \sim 10^{20}$ Hz is the
frequency of the BAT detector onboard {\it Swift} satellite and
$\nu_c$ is the cooling frequency \footnote{For $\nu>\nu_{\rm c}$,
the synchrotron radiation spectrum is $\propto \nu^{-p_{\rm n}/2}$.
On the other hand, the maximum synchrotron radiation frequency
$h\nu_{\rm M} \sim 30\Gamma/(1+z)~{\rm MeV}$ \citep{cw96} is up to a
few GeV for $\Gamma \geq 300$. It is straightforward to show that
the fluence of the high energy afterglow emission in the energy
range of LAT is in order of $10^{-5}~{\rm erg~cm^{-2}}$, comparable
to the fluence of the soft X-ray/$\gamma$-ray afterglow emission.
Such a conclusion is almost independent of the afterglow models.}.
On the other hand, $\Gamma m_e c^2/\gamma_m \sim m_e c^2/100 \sim
10^{18}$Hz $< \nu_c$, implying that the SSC emission of the
electrons with a Lorentz factor $\sim \gamma_c$ is in extreme
Klein-Nishina regime and it is effectively suppressed. So we expect
that the SSC emission will peak at an energy
\begin{eqnarray}
h\nu_{\rm p}^{\rm SSC} &\sim &  \Gamma \gamma_m m_e c^2
\nonumber \\
& \sim & 2.3 ({1+z \over 2})^{1\over 2} \epsilon_{e,-1} E_{\rm
k,n,55.5}^{1/2} A_{\star,-2}^{-1/2} t_3^{-1/2} {\rm TeV},
\end{eqnarray}
\iffalse
\begin{eqnarray}
h\nu_{\rm p}^{\rm SSC}& \sim & \Gamma \gamma_m m_e c^2
\nonumber \\
& \sim&
\left\{
\begin{array}{l}
6.2 \epsilon_{e,-1}\Gamma_{2.7}^2 {\rm TeV} \\
1.6 (1+z)^{1/2} \epsilon_{e,-1} E_{\rm k,n,55.5}^{1/2}
A_{\star,-2}^{-1/2} t_3^{-1/2} {\rm TeV},
\end{array}
\right.
\end{eqnarray}
\fi for {the late afterglow.} The SSC emission of the forward shock
in the very early afterglow phase overlap with the GeV-TeV emission
of the {prompt phase} and is very likely to be outshone. Below we
just discuss the SSC emission of the forward shock in the normal
decline phase ($t>60$ sec).

To check our estimate, we calculate numerically with Fan et al.'s code (2008)
the forward shock emission spectrum. As shown in
Fig.\ref{fig:essc}, the SSC emission peaks at TeV energies, with
a fluence $\sim 6 \times 10^{-6} {\rm erg\, cm^{-2}}$, and
an isotropic energy $\sim 3 \times 10^{52} {\rm erg}$.
The detection of the TeV emission is beyond the scope of the Fermi
satellite. Ground based Cherenkov telescopes, like MAGIC and
H.E.S.S, may be suitable to detect these energetic signals. However,
before reaching us, these TeV photons would have been absorbed by
the infrared background photons, and such emission could be seen
only from rare very nearby sources.
\iffalse
resulting in $e^\pm$ pairs with a
Lorentz factor $\sim 10^6$. These $e^\pm$ pairs will up-scatter on
the microwave background photons and boost them to GeV band. The
detection prospect of these secondary GeV photons is mainly
determined by the poorly known $B_{_{\rm IG}}$, the strength of the
intergalactic magnetic fields. Usually $B_{_{\rm IG}}<10^{-18}$
Gauss is required to give rise to detectable signals (see Fan \&
Piran 2008 and the references therein). In this work we do not
discuss it further.
\fi

We find in Fig.\ref{fig:essc} that for a Fermi-like satellite the
MeV-GeV synchrotron radiation of the forward shock may give rise to
a detectable signal. In our calculation, we take a maximal Lorentz
factor of the shocked electrons $\gamma_{\rm M} \sim 4\times 10^7
B^{-1/2}$ \citep{cw96}, where $B$ is the magnetic field generated in
the shock front. This leads to the synchrotron GeV cutoff (see
Fig.\ref{fig:essc}). As a numerical example, following
\citet{fan08}, we take a real effective area of LAT and integrate
the spectrum over the frequencies to estimate the number of
detectable photons. For this particular example, the LAT onboard
Fermi can detect $\sim 400$ ( $>20$ MeV), $\sim 20$ ( $>1$ GeV), and
$\sim 0.1$ ( $>100$ GeV, without the correction due to the
absorption by the infrared background photons) high energy photons.
{This would be a very exciting detection.}

\begin{figure}
\includegraphics[width=0.5\textwidth]{f3.eps}
\caption{The integral of $\nu F_\nu$, including the synchrotron +
SSC components, in the time interval 60 sec $-$ 2000 sec.}
\label{fig:essc}
\end{figure}

\section{Origins of GeV emission of some recent GRBs}
Recently high energy emission has been detected by AGILE: GRB 080514B
\citep{Giuliani08}, and by Fermi: GRB 080825C \citep{Bouvier08}, GRB 080916C
\citep{Tajima08} and GRB 081024B \citep{Omodei08}.
 We can apply the above considerations for GRB 080319B
to all these bursts, though the very early afterglow data
are unavailable and the constraints on the model are not very tight.

\emph{GRB 080514B}: the burst light curve shows a multi-peaked
structure with a duration of $\sim 7$s \citep{Golenetskii08}. The high energy
emission lasted about 2 times longer than the MeV emission
 and the most intense high energy emission arrived
at $\sim 10$ sec after the trigger \citep{Giuliani08}. We interpret
such an intense high energy flash as the EIC emission in the reverse
shock region. In this case, some seed photons (the prompt MeV
emission) are upscattered by the reverse shock electrons and are
boosted to an energy $\lesssim 1$ GeV \citep{b05,fzw05}. The main
advantage of this model is that the duration of the high energy
emission is longer than that of the prompt soft $\gamma$-ray
emission by a factor of 2, consistent with the observation.
There are also 2 high energy photons detected at $\sim 26$ s.
They may be the synchrotron or SSC emission of the forward shock.
The possibility that they are the SSC emission of an underlying
X-ray flare (Wei, Yan \& Fan 2006; Wang, Li \& M\'esz\'aros 2006;
Galli \& Guetta 2008; Fan et al. 2008) cannot be ruled out. The
lack of the simultaneous XRT observation makes it difficult to draw further
conclusion.\\

\emph{GRB 080916C} was a long burst with a duration $\sim 60$s. The
time averaged spectrum, from 8 keV up to 30 MeV, of the main
emission is best fitted by a Band function with $E_{\rm p} = 424\pm
24$keV, $\alpha = -0.91\pm 0.02$, and $\beta = -2.08 \pm 0.06$. The
fluence (8 keV $-$ 30 MeV) is $1.9\times 10^{-4} {\rm erg/cm^2}$
\citep[Swift]{van08} \citep[slightly different in Konus-Wind
observation, ][] {Golenetskii08}. More than 10 photons are observed
above 1 GeV during the prompt phase
 \citep{Tajima08}
and the high energy emission lasted longer than the soft $\gamma-$rays
 \citep{Abdo09}.
This was a very bright burst with a hard spectrum. A simple extension
of the keV$-$MeV spectrum to higher energy range gives $N({\rm 30
MeV-1 GeV})\sim 700$, $N({\rm 1-10 GeV})\sim 100$ and $N({\rm >10
GeV})\sim 9$ by LAT (on-axis case), enough to match the observation
\citep{Tajima08,Omodei08b}. This fact suggests that the synchrotron
radiation of the internal shocks plays an important role in
producing high energy prompt emission.

{The redshift of GRB 080916C is estimated to be $\sim 4.5\pm 0.1$
\citep{Greiner09}. The peak luminosity is thus as high as $\sim
5\times 10^{53}~{\rm erg~s^{-1}}$. The typical variability timescale
is suggested to be as long as $\sim 2$ s \citep{Abdo09}. With these
information,} the detection of $>10$ GeV prompt emission gives a
tight constraint on the initial bulk Lorentz factor of the GRB
outflow, i.e. (Lithiwick \& Sari 2001; Fan \& Piran 2008; Gupta \&
Zhang 2008),
\[\Gamma_{\rm i}>400 ({h\nu_{\rm
cut}\over 10{\rm GeV}})^{p\over 2(p+4)}L_{\gamma,54}^{1\over
p+4}\delta t_{\rm 0}^{-{1\over p+4}}.
\]
Using the maximal synchrotron radiation frequency of the shocks
$h\nu_{\rm M}\approx 30\Gamma/(1 + z)~{\rm MeV}$ \citep{cw96}, we
find that if the high energy emission up to $\sim 10$ GeV is
attributed to the synchrotron radiation of internal shocks, the
initial Lorentz factor should satisfy:
\[ \Gamma_{\rm i}\geq 1800
{1+z\over 5.5}{h\nu_{\rm cut} \over 10~{\rm GeV}}.
\]
A $\Gamma_{\rm i}$ much higher than $2000$ is unlikely. So this
strongly suggests that the internal shocks can accelerate high
energy particles (both protons and electrons) very efficiently and
the highest energy of electrons is limited by the loss via
synchrotron radiation. The energy distribution index of the
shock-accelerated electrons ($p\sim 2.4$) is also close to that
predicted in the theory. This is a very encouraging news for the
people interested in the ultra-high energy particle acceleration in
GRBs. However, we'd like to caution that it is the only case among
the 70 events observed so far by Fermi \citep{Abdo09}. It might be
too early to say more at this moment.

The internal shock synchrotron radiation cannot account for the
delayed high energy emission \citep{Abdo09}. The possible mechanisms
that can produce this emission are (i) the EIC emission from the
reverse-forward shock regions, (ii) the SSC emission of the forward
shock and (iii) SSC emission of the weak internal shocks powering an
extended X-ray emission component that is below the threshold of
GBM.

{The spectrum of the $\geq 100$ MeV emission in the time interval
$\sim 200-1400$ sec is $F_\nu \propto \nu^{-1.8\pm 0.5}$
\citep{Abdo09}. Such a soft spectrum imposes a tight constraint on
the models. In the standard afterglow model, the late time infrared
and X-ray afterglow \citep{Greiner09} can only be interpreted as the
forward shock emission of an ejecta expanding into a weak stellar
wind. Like in GRB 080319B, an $A_* \sim 0.01$ is needed to have a
cooling frequency above the XRT energy range at $t\geq 1$ day (Gao
et al. 2009, in preparation). An electron energy distribution index
$p \sim 2.2$ can reproduce both the infrared to X-ray spectrum
$F_\nu \propto \nu^{-0.63}$ and the X-ray (infrared) afterglow
decline $\propto t^{-1.29\pm 0.09}$ ($t^{-1.40\pm 0.05}$). The
spectrum of the SSC or the EIC emission\footnote{For the EIC model,
the flare photons should peak in far ultraviolet band, as suggested
in \citet{fp06}, otherwise the typical energy of the scattered
photons will be well above $\sim 100(1+z)$ MeV and we need a very
strong X-ray flare to account for the detected high energy photons.}
of the forward shock should have a spectrum not steeper than
$\nu^{-p/2} \sim \nu^{-1.1}$, and can only marginally match the
data. So we prefer the possibility (iii). For the X-ray emission
powered by the prolonged activity of the central engine, the SSC
emission can peak at an energy $\lesssim 550[(1+z)/5.5]$ MeV
\citep[see section 5.1 of][for details]{fan08}. In this case, the
electron energy distribution index is irrelevant to that of the
afterglow electrons and can be as large as $\sim 3$, as found in the
spectrum analysis of X-ray flares \citep{Butl07}. As a result, the
soft spectrum of the delayed $>100$ MeV
emission may be interpreted.}\\

\emph{GRB 081024B} was a short burst with a duration $\sim 0.4-0.8$s
\citep{Connaughton08,Hanabata08}. The LAT saw the emission from this
source up to 3 GeV, in the first 5 seconds after the trigger.
Here we consider two possible interpretations. One is that the
delayed emission is the SSC component of an extended/prompt soft
X-ray emission. Following Fan \& Piran (2008; see their
eqs.(47-49)), the typical frequency of the internal shock SSC
emission can be estimated as
\[
h\nu_{\rm m}^{\rm ssc} \sim 75~{\rm MeV}~(E_{\rm p}/0.3~{\rm
keV})^2R_{\rm int,14}L_{\rm X,49}^{-1/2}(1+Y_{\rm ssc})^{1/2},
\]
where $R_{\rm int}$ is the radius of the continued but weak internal
shocks that power the underlying prompt X-ray emission with a
luminosity $L_{\rm X}$, and $Y_{\rm ssc}$ is the SSC parameter of
the internal shocks. This model requires a unmagnetized outflow
launched by the continued activity of the central engine, in
contradiction with most models proposed so far (see Zhang 2006 for a
review). If confirmed, a stringent constraint on the nature of the
extended emission following short GRBs will be established. So, in
principle, the cooperation of {\it Swift} and Fermi satellite can
reveal the nature of the late outflow powering the extended
emission. The other possible origin of the delayed high energy
emission is the SSC emission of the forward shock. It is
straightforward to show that the outflow with an initial Lorentz
factor $\Gamma_{\rm i} \sim 400$ gets decelerated in the
interstellar medium with a number density $\sim 1~{\rm cm^{-3}}$ in
$\sim 5$ sec. The typical SSC emission frequency of the forward
shock can be estimated as \citep{se01,fp08}
\[
h\nu_{\rm m}^{\rm ssc} \sim 25 ~{\rm GeV}~\epsilon_{e,-1}^4
\epsilon_{\rm B,-2}^{1/2}\left[{13(p-2)\over 3(p-1)}\right]^4E_{\rm
k,51}^{3/4}t_{1}^{-9/4}.
\]
One may be able to distinguish between the above two scenarios by
analyzing the spectrum.
If the delayed high energy emission is the
SSC component of extended but weak internal shocks, the $0.1-3$ GeV
spectrum is expected to be steeper than $\nu^{-1}$. If the delayed
high energy emission is the SSC component of external forward shock,
the $0.1-3$ GeV spectrum is expected to be
$\nu^{-1/2}$ unless $p\sim 2$. The forward shock synchrotron
radiation can also give rise to GeV emission. It is, however, difficult to
say more concerning this possibility because the early afterglow
physics of short GRBs is still poorly understood.

\section{Conclusions and discussions}

\begin{table}
\caption{The expected emission of high energy photons {(mostly
inverse Compton scattering)} from different origins for GRB 080319B,
which should be detectable by LAT onboard Fermi satellite.}
\begin{tabular}{l|l|c|c|c|c}
\hline
Seeds & region for& duration (s) & typical photon  & detectable\\
 & electrons  & & energy (GeV) &  photons
\\ \hline
$\gamma$-rays & prompt opt & $\sim$ 60 &  $\gtrsim 8$ & $\leq 100$
\\ \hline
opt & prompt $\gamma$-ray & $\sim$ 60 &  $\sim$ 15 & $\sim 240$ $^\dagger$
\\ \hline
$\gamma$-rays &  reverse shock & $\sim 10^2$ & $\sim$ 13 & $\sim$ 3
\\ \hline
opt &  forward shock & $\sim 10^2$ & $\sim$ 10 & $\sim$ 30
\\ \hline
afterglow & {(Synchrotron)} & $\sim 10^3$ & 0.01-0.1 &  $\sim 400$
\\ \hline
afterglow & external shock & $\sim 10^3$ & $\sim 10^3$ & $\sim$ 0.04 $^\ddagger$
\\ \hline
\multicolumn{5}{p{0.45\textwidth}}{\footnotesize $^\dagger${This
case is less likely, and possibly alternates with the former case.}}
\\
\multicolumn{5}{p{0.45\textwidth}}{\footnotesize
$^\ddagger${Supposing an instrument with effect area $10^4 {\rm
cm^{2}}$ and without considering the absorption on the way to the observer.}}
\\ \hline
\end{tabular}
\label{tab:con}
\end{table}

High-energy emission provides a new window into prompt
emission/afterglow physics and can provide an independent test of
models. Motivated by this, we calculate the possible high-energy
prompt/afterglow emission in GRB 080319B that was distinguished by a
naked-eye optical flash and by an unusual strong early X-ray
afterglow. Two possible GeV-TeV emission components may be related
to the naked-eye optical flash. The first is the Inverse Compton
scattering of the prompt optical photons by electrons producing the
soft $\gamma$-rays. The second is the very early EIC emission from
the forward shock region when the prompt optical emission overlaps
the shock front. The difference is their duration. The former is
expected to be simultaneous with the prompt soft $\gamma$-ray
emission while the latter lasts longer (see Fig.\ref{fig:eic}). The
synchrotron radiation of the forward shock can give rise to a
significant detection, too (see Tab. \ref{tab:con} for a summary).
This component may be more common than the two that depend on a
strong optical flash as which is quite rare. The detection prospect
of the forward shock synchrotron radiation by LAT is fairly good.
For the {\it Swift} GRBs detected so far, GRBs 060105, 061007,
070419B and 080721 have a $0.3-10$ keV flux $\sim 10^{-8}~{\rm
erg~s^{-1}~cm^{-2}}$ at $t\sim 100$ s after the trigger
(http://www.swift.ac.uk/xrt$_{-}$curves/; Evans et al. 2007). Though
about one order of magnitude lower than that of GRB 080319B, they
are strong enough to produce a GeV synchrotron emission detectable
by LAT as long as the synchrotron spectrum can indeed extend to an
energy $\sim 30\Gamma/(1+z)~{\rm MeV}$. The forward shock SSC
emission of these very bright events may be {more} suitable for the
ground-based Cherenkov telescopes, like MAGIC or H.E.S.S.

In section 3, we discussed the {possible} physical origin of the
high energy emission of GRB 080514B, GRB 080916C and GRB 081024B. We
find that these detections can be {generally} understood by the
synchrotron and inverse Compton radiation of the internal shocks or
external shocks. For example, the delayed sub-GeV flash detected in
GRB 080514B may be the EIC emission from the reverse shock region
and the prompt GeV-emission of GRB 080916C may be dominated by the
synchrotron radiation of the internal shocks. The ``long lasting"
high energy emission detection in the short burst GRB 081024B may be
attributed to the SSC emission of the decelerated forward shock or
the internal shocks powering an extended X-ray component which is
below the threshold of GBM. {However, as lack of detailed
observations, it is defficult to draw a firm conclusion.}

Finally we focus on the common feature that the high energy emission
usually lasts longer than the prompt soft $\gamma$-rays, as detected
in GRB 080514B, GRB 080916C and GRB 081024B. Such a phenomena,
peculiar in pre-afterglow era, {may be explained as}: (1) The
synchrotron and the SSC emission of the long lasting forward
external shock can contribute to the high energy emission
significantly. (2) The GRB central engines usually do not turn off
abruptly. The SSC emission of the continued but weak internal shocks
may peak at GeV energies. (3) If a (mildly) relativistic reverse
shock formed, the prompt optical/X-ray/$\gamma$-ray photons overlap
the external shock fronts tightly and cool the accelerated electrons
effectively. This process will produce a GeV emission component with
a duration about twice that of the prompt photons. For a
sub-relativistic reverse shock, the prompt soft $\gamma$-ray photons
exceed the external shock fronts quickly. Its effect on cooling the
reverse/forward shock electrons can be ignored.
 However in such a case the
electrons/protons accelerated in reverse shock contain just
$\lesssim 10\%$ of the total energy of the GRB ejecta
\citep{NP04,Mimica08} and cannot play an important role in producing
high energy emission. (4) The EIC in the late afterglow phase caused
by X-ray flares can also give rise to GeV emission. However the
luminosity is lowered since its duration has been significantly
extended. Usually LAT is unable to catch such a weak signal.

\section*{Acknowledgments}
YZF thanks Bing Zhang and Daming Wei for discussions.
This work is supported in part by the Israel Science Foundation (for
TP), a grant from the Danish National Science Foundation, a special
grant of Chinese Academy of Sciences, National basic research
program of China grant 2009CB824800, and the National Natural
Science Foundation of China under the grants 10673034 (for YZF) and
10703002 (for YCZ).

redpajama_set_name: RedPajamaArXiv


Marjorie knows that James has had affairs in the past but she believed him when he said it would never happen again. Then the old warning signs start to reappear. He stays late at work and seems distracted when he's at home. That doesn't stop her breath catching in her throat when she sees the letter inked into his diary. A red A - seemingly innocuous and yet devastating. Undeniable.
She was a clever, independent and desirable woman once. Not someone's second best with a broken spirit. And as Marjorie looks at the red letter A - Amy? Angela? - she realises that it's time to rediscover herself and take back control of her life.


You can learn just about anything through trial and error. It may take a while, but eventually you'll figure out what to do—and what not to do. The problem is, this method of learning is not very effective.
Don't waste your time learning how not to photograph jewelry. Learn from our experience instead.
Jewelry product photography comes with its own unique set of challenges. Products are small, reflective, and can be difficult to stage for shooting. However, these challenges can be easily overcome with a little bit of planning.
Here are ten common mistakes to avoid when photographing jewelry.
Clean jewelry after touching it. Consider using gloves.
It sounds obvious, right? Of course, your jewelry should be clean and polished. The challenge comes from the level of detail captured by a DSLR camera and proper lighting setup. The final image will often be blown up several times the jewelry's real life size. Details invisible to your naked eye will be revealed when photographed.
Wipe down your jewelry every time you touch it. Wearing cotton gloves may save you time and reshoots.
Even slight changes can have jarring consequences.
Develop a set of guidelines for your jewelry photography and follow them. Consistency is key in product photography, and variations from product to product or shot to shot will distract your customer. Document everything: camera settings, lighting, background, and equipment position. Make sure you can resume shooting on a different day without any noticeable changes.
Some inexperienced retailers look at a white background and think "boring," or "missed opportunity for branding." They try to differentiate themselves with colorful, active backgrounds. That's a mistake.
Don't add distracting elements into the background.
There's a reason Amazon, eBay, Rakuten, and most marketplaces require or recommend a white or neutral background. A plain white or light gray background keeps the focus where it should be: on your product. Black is also popular in jewelry photography, but be aware you may have difficulty submitting images at some marketplaces.
White backdrops are cheap and easy to create. You can use seamless white paper, create a lightbox, and even photograph white products on white backgrounds.
This mistake is based on the same "keep your focus" principle we just discussed. Props are a distraction you don't need. You may think your jewelry will look best when staged elaborately, or maybe you just want to show it in a lifelike manner using a mannequin.
Left: The mannequin obscures part of the necklace and dominates the shot.
Right: This is a branding shot. Editorial photography differs greatly from product photography.
The reality is that most props draw attention away from your product. There are times where it is appropriate to use stylish props, models, and interesting locations in shots that include your product. Those are editorial shoots, for branding purposes. Editorials belong on magazine covers, as hero images on websites, and banners in emails. They do not belong on product category pages.
Use minimal props intended to showcase jewelry that can be easily removed in post-production processing.
Jewelry photography kits are relatively inexpensive. Remember, you want your jewelry photography to be consistent and focused on your product. Most props create inconsistency and distraction.
You want your customers to be looking at your jewelry without trying to puzzle out if that's the photographer's reflection or a flaw in the stone.
Reflections can ruin a product image.
The presence of gemstones and metal creates a technical challenge for jewelry product photography. It's difficult to shoot highly reflective objects without capturing distracting reflections. Follow this guide to controlling reflections, and try a double overhead light for shooting metallic jewelry.
A double overhead lighting setup can minimize reflections.
Place your jewelry on a sturdy surface, like a table or a block, and position one large studio light on either side. Diffuse both lights with umbrellas. Hang and sweep a roll of seamless white paper behind and underneath your product, and attach the paper to the bottom of your camera lens. This will block off reflections from the foreground and reflect more light back onto the subject.
The right setup prevents ugly bright spots on your product images.
Position your camera on its tripod so that you are shooting slightly down at the product. Position your lights (with diffusing umbrellas) above your product to either side, angle each down at the product, and set them to the same power. This setup should evenly fill the frame with light without creating ugly bright spots on your product.
To get professional looking results without a full photo studio, check out the Halo and Foldio: Inexpensive mini photo studios you can easily set up at home or your office to take product photos.
It doesn't matter what lighting setup you use if you try to handhold your camera. Handholding your camera or smartphone will either result in camera shake, or you will have to use something less than full focus. Neither is a desirable situation.
Always use a tripod. Tripods are cheap, effective, and improve both quality and consistency. When your camera is held steady by a tripod, you can use optimal aperture and ISO settings. Mark a spot on the floor for your tripod and it will be easy to replicate your setup even when shooting on different days.
Don't take artsy low aperture shots of your jewelry that focus on only one part of the product. High aperture, full focus photography will create a sharp image that your customers can trust.
Your customer wants to see every detail. Give it to them with aperture settings of f/11 or greater, and set your ISO to as low as possible—preferably ISO 100.
If your white balance is off, a gold image can look blue or vice versa. Remember that meme about the gold/blue dress? Vision is ambiguous.
The human eye is easily deceived. This is the same dress.
Ensure your white balance is set accurately so that you capture your jewelry with the colors you intended. Either set it manually or use your camera's automatic mode and double check the results.
Different backgrounds and white balance make the same necklace look different colors.
It's incredibly disappointing to click on a product on a category page and then find there are no more images. Maybe the customer liked an overhead shot of a necklace but wanted a detail shot to clarify some engraving on the side. Give it to them.
You've gone to all the trouble of prepping your product, lighting, and camera. Don't stop now at one or two images. Build trust by showcasing every aspect of your jewelry. More images lead to more sales, so capture as many angles as possible of your product.
Few things kill a sale as quickly as a bad Photoshop job. Poorly retouched photos feel fake and destroy a customer's trust in the product image, and therefore in the product. Either invest the time to become skilled in software like Adobe Lightroom and Photoshop, or hire a professional.
In addition to retouching, you should create templates that consistently crop, set margins, and align your product images. Remember: consistency is key.
If you don't have the time or skillset to brush up your own images, consider hiring a post-production processing company.
Now that you know what not to do, get shooting! Avoid these ten common mistakes and you'll be well on your way to creating beautiful product photography.


Depart Victoria Harbour by seaplane for an unforgettable trip over the Gulf Islands and directly into downtown Vancouver's bustling Coal Harbour. While in Vancouver, enjoy a four-hour fully narrated tour of the city of Vancouver. While in Vancouver, visit the art gallery, take in the lush West Coast forest of Stanley Park or explore Granville Island.


text:
Q: Root cannot be null I am trying to open a new window. Specifying the controller programmatically.
OverviewController overviewController = new OverviewController();

final FXMLLoader loader = new FXMLLoader(getClass().getClassLoader().getResource("overview.fxml"));
loader.setController(overviewController);

Parent root = loader.getRoot(); // Gives me a "Root cannot be null" error...

Stage stage = new Stage();
Scene scene = new Scene(root);

stage.setTitle("FXML Welcome");
stage.setScene(scene);
stage.setResizable(false);
stage.show();

So it goes wrong after loading the root.
The following code does work, but i want to specify my own controller in the code.
Parent root = null;

try {
    root = FXMLLoader.load(getClass().getClassLoader().getResource("overview.fxml"));

    Stage stage = new Stage();
    Scene scene = new Scene(root);

    stage.setTitle("FXML Welcome");
    stage.setScene(scene);
    stage.setResizable(false);
    stage.show();
} catch (IOException e) {
    e.printStackTrace();
}

A: The reason getRoot() is returning null is that you haven't actually loaded the FXML file. You need:
loader.load();
Parent root = loader.getRoot();

or, more simply,
Parent root = loader.load();

A: You have to loader.setRoot() in case you use a dynamic root, <fx:root type="..."> …
And as @James_D pointed out, you have not loaded the fxml file…

redpajama_set_name: RedPajamaStackExchange


Anonymous, 21 Nov 2016You should buy it. I've been using my S5 for over 2 years, no problems at all.Probably this coming week. been hearing a lot of good feedbacks about this phone. Thanks for the feedback.
Anonymous, 21 Nov 2016Planning to buy S5. any suggestions?You should buy it. I've been using my S5 for over 2 years, no problems at all.
Planning to buy S5. any suggestions?
AnonD-611521, 18 Nov 2016This is still one of the Samsung's greatest gadget together with the S3 (I suppose)...Get it a... morethank you for your reply, and Yes i got it ordered ! excited to get my hands on it !
do its lag playing high graphics games such as nova 3,fifa 16 and gangster Vegas???battery life is good or not for 2016???
AnonD-611521, 18 Nov 2016This is still one of the Samsung's greatest gadget together with the S3 (I suppose)...Get it a... moreAgreed this is truly an amazing one hell of a phone and literally a beast..a;; phones have bad points and good but weigh these points and the good completely outweigh the bad. In fact nothing to complain about. Only the back cover should have been more premium looking but that's ok it still looks good. Other than that no complains. Fast, steady and smooth, reliable, good colors, good camera and good battery life. Charging is fast too...great phone A TOTAL PACKAGE!!!!!!
Anonymous, 17 Nov 2016Hello everyone, I was going to get myself an S5, i wanted to know, is S5 still good in 2016 ? ... moreThis is still one of the Samsung's greatest gadget together with the S3 (I suppose)...Get it and you wont regret!!!
I have been with S5 for two good years now and it still gives all the utilities I want...a very good gadget that I am always with!!!


We would like to correct a few previous reports made here on Vulture: The upcoming remake of A Star Is Born will not star Lady Gaga. Lady Gaga will not sing in the film. Lady Gaga will not appear on any promotional posters. The film will, however, star Stefani Germanotta. According to People, which has some greasy first-look images of the Bradley Cooper–directed film, the pop star will be credited by her birth name when the movie arrives in theaters on May 18. This is serious cinema, after all, and Bradley, Stefani, and their characters' matching haircuts demand to be treated as the artists they are!


text:
Download for free : CMMS specifications
Maintenance professionals
Mobility Work CMMS
TRY FOR FREE Improve your maintenance management with the first community-based CMMS software. CMMS features
Showcase your expertise to the first community of maintenance professionals.Hub features
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The CMMS blog
Discover the Mobility Work CMMS maintenance blog for industry professionals.See the latest articles
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CMMS,
Industrial maintenance: Towards an uberisation of the industry
Célia Sagnier
Duration: 8 mn
The industrial maintenance sector is progressively changing: how can we take advantage of these changes to develop a new model?
For about ten years the development of technology has been punctuating our daily life and transforming our world, just like Google, Facebook, Uber, and others did. These tools progressively have revolutionized the use we make of technology and have a strong influence on us.
Industrial maintenance, though, still has to transform. Many gaps still need to be filled and we should, therefore, analyze the situation in order to be able to address the issues correctly. This sector suffers from a bad reputation even though small changes can already be observed. Mobility Work wants to prove that far-reaching changes are possible: it will be slow but our application is proof that another maintenance is possible. We should ask ourselves: what could a more advanced next-gen maintenance look like?
How to deal with industrial maintenance?
Once again, one should admit that industrial maintenance suffers from a bad reputation. We often tend to picture ourselves hard-working technicians in dusty factories, changing motors and spare parts, fixing up greasy equipment… This perception is completely false because of the prejudice we have in mind. As a consequence, people who could have an interest in the field get afraid. Figures don't lie: in some high schools, the number of students in industrial maintenance has dropped by 50%. This training is not as attractive as it used to be in the past for dynamic young people, who tend to be more interested in conception and R&D.
Improve your maintenance management with the first community-based CMMS
Maintenance hasn't been brought out very well in recent years, on the contrary: as a consequence, the level of this Advanced Technician's Certificate training has drastically dropped. Teachers are categorical: in order to appeal to new students, administrations planned on lowering the level of this very technical field so that the students who had a vocational baccalaureate could opt for this training.
The future of engineering schools doesn't seem to be much brighter. Very few students choose to study maintenance, which leads to fewer maintenance classes being taught: only twenty hours during the three required years. It is obvious that, if it goes on, future managers won't have the required skills and won't be able to make the right decisions about their equipment's maintenance, even though it guarantees the production's flow in a company.
Observable consequences within companies
In return, gaps have progressively grown and companies can nowadays tell how difficult it is to hire. Again, maintenance managers are unanimous and regret this lack of qualified technicians. "We offer two positions while there are only two candidates, it is really hard to hire in the maintenance field!", explains one of them. While it is true that the students' level is worse than ever, recruitment agencies don't hesitate to pay a high price to appeal to them as soon as they pass their diploma.
Because of these major difficulties, differences and gaps can also be witnessed between employees as they don't have a clear and identical perception of their work. They indeed all have learned different techniques and have different knowledge. One could argue that they all are complementary but these differences can, in fact, lead to a lack of understanding between them and to dysfunctions. That is why the whole maintenance sector has to evolve.
Create your equipment sheets in the maintenance management app Mobility Work in order to trigger tasks and activities
A strong need for consistency to achieve change
If technicians sure want their field to change and to modernize, it remains important to truly take into account all the difficulties that pave to way to technology. It is not an easy task to revolutionize such a huge sector and everything has to be done little by little.
Today's production plants face a lot of challenges. Some of them are equipped with very modern production lines while they still use bottleneck equipment that was manufactured in the 80's, 70's or even 60's. We obviously can't expect from a technician to have enough knowledge to know how to maintain all of this equipment as the functionalities vary drastically from one device to another.
There also is a generation gap: technicians' profiles can be very different. Some still work on paper while others are far more reactive and can easily learn how to master IoT (Internet of Things), industry 4.0's technologies, augmented reality, etc. To conclude, there's a strong pending need for consistency because the modern solutions technicians and managers could benefit from are just tools, their expertise and know-how will always be needed.
The solution: technology
It is now clear for everyone that industrial maintenance needs a profound rejuvenation in order to attract new profiles: areas of improvement in terms of technology are numerous. Robotics, automation, tablets, sensors, applications, and others are so many arguments that we need to put the emphasis on in order to attract the young generations towards this 4.0 maintenance. Future devices will, of course, be always more connected, automated and smart, and technicians and managers will have to have more precise and technical knowledge.
Eventually, we can say that Mobility Work is completely opposed to conventional maintenance management tools. It is not rare to meet, in the companies, technicians, and managers who still think that they entirely control their maintenance using Excel files. This method is completely outdated and can even be damaging because you lose information, you can make mistakes more easily, it slows down the production process… Unfortunately, today's CMMS solutions are mostly way too expensive and inadequate. The user interfaces are not intuitive at all, the features are hard to understand, the design is very basic and no one would ever want to try to deal with this software on a daily basis. Instead of making the work of technicians easier, these tools make it even harder.
Technology should really be at the service of industrial maintenance so that all fields can change. This is exactly Mobility Work's philosophy: a technician can manage his tasks thanks to the application, he can see what task his colleagues are performing, follow his own work, exchange with his team as well as the community's members, check the state of a piece of equipment… It is a very simple and intuitive tool that even allows beginners to have a whole new vision of maintenance and that attracts the future potential maintenance experts.
From employee to freelancer?
But it goes even further: the conventional concept of work in itself is challenged. Thanks to the development of applications such as Uber, the entire world has quickly understood that more and more people wanted to become their own bosses. The idea of working for yourself and to benefit from certain independence is really in the spirit of the times so we should consider these opportunities. What would tomorrow's industrial maintenance world look like?
Maintenance technicians wouldn't be part of a company anymore but rather of a community, where independence and mutual aid would be the main keywords. Everybody could choose the service providers and customers they want to work with thanks to real-time localization, work the amount of time per day that suits them, decide the type of equipment they want to work on… The most hard-working and skilled professionals could benefit from a rating system to increase their workload and to extend their network. The maintenance management platform Mobility Work already features all these characteristics and can help you transition toward industry 4.0 very easily.
This all makes you understand how 4.0 industrial maintenance can be challenging: we have the possibility to develop a more free and open model where everyone would have an important role to play within this huge community. Given these observations, Mobility Work is the perfect community-based tool because it is gradually becoming the first maintenance social network. Communicating and interacting with each other, two current major values, are at the core of our application.
Mobility Work CMMS is based on a community of users who exchange and follow their plant's activity thanks to the newsfeed
Mobility Work follows from these conclusions because it offers innovative features, allowing you to take a fresh look at the next-gen industrial maintenance. If you want to learn more about our application, don't hesitate to download our mobile app to have a preview of it thanks to the free trial!
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Mobility Work raises 2 million euros from strategic partners to accelerate its commercial development and bring new features to its product.
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redpajama_set_name: RedPajamaCommonCrawl


I have to finish this by 7/30, medications since other obligations will prevent me from working on it much more until the week before Burning Man.
So I've spent the past week solid (nearly) chopping through task after task. At right are the steel-conduit outriggers I installed at the top of the framework – this will give me something to hang light-bars and speakers from.
Here are the lightbars, viagra sale cut from 1x4s and painted with a dull-silver hammertone paint.
David (our architect) spent much of last night using a cutting wheel (courtesy of the Disco Kremlin crew – thanks!) to cut out cloud shapes from the sheets of perforated steel that is usually used in plastering walls.
These will be screwed to the crown rail around the top of the pianos once we get to the playa.
Here's another shot of the work, with the camera flash turned off. Much better.
Earlier, my son had silver-painted all the 1×2 pine furring strips we had, which I tacked up as proof-of-concept for a design layer on the pianos' top edge. We'll be getting more of these and painting them the week before the burn so that we can batten down the top and bottom edges of JANUS' skin.
We then gave JANUS a healthy bath of Flame-X – a fire retardant chemical to protect the otherwise pretty-flammable cotton from following the path of combustion.
… and started the teardown. We basically un-built JANUS – stripping off the canvas skins, unscrewing all the framing, unbolting the foundational lumber, and then removing the hinge-pinned profiles. It took about two hours.
David peeled off for bed at around 1 a.m. this morning, and I finished up, stacking everything as neatly as I could on top of the van. I got done around 4 a.m., came home to catch a few hours sleep, and returned with a new set of come-along straps from Pep Boys to secure the load.
But for now, JANUS is complete.
– David Hoffman, who took my half-assed architectural ideas, and made my mutant vehicle design concept into a portable, reusable, customizable and easily-assembled reality.
– My kids, Biomass and Hitgirl, for lending a hand when it was needed most, and finally my long-suffering wife, Kristina, whose encouragement, support and patronage made it happen.
We built an art car in 30 days or less. Now all we have to do is get it to the playa.
This entry was posted on Thursday, July 28th, 2011 at 8:04 pmand is filed under . You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.


Click here to read press release, "Proof: Cosmic Rays Come from Exploding Stars"
SCIENTISTS HAVE KNOWN ABOUT COSMIC RAYS FOR A CENTURY. But these high-energy subatomic particles, which stream through space at nearly the speed of light and crash into the Earth's upper atmosphere, have been mostly a mystery. The primary reason: researchers have been unable to tell where they come from, or how they're born.
Now, a research team led by the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University and SLAC has discovered evidence that cosmic rays are born in the shock waves of distant supernovae.
Using data from NASA's Fermi Gamma-ray Space Telescope, the research team led by Dr. Stefan Funk of KIPAC was able to track gamma rays – the most energetic form of electromagnetic radiation, or light – back to the remnants of supernovae explosions where they were born as decay products of collisions between cosmic rays and lower energy particles. The finding offers the first astrophysical evidence for how cosmic rays are produced, as well as where they are generated: in the shock waves that emanate from an exploded star.
Dr. Funk and his team confirmed a chain of events that researchers have theorized for years: The shock waves from supernovae accelerate subatomic particles called protons to near light speeds, turning them into cosmic rays. These cosmic rays collide with low-energy protons residing in gas and dust gathered by the shock waves, and the collisions give rise to other subatomic particles called pions. Pions rapidly decay into gamma rays with a very specific distribution of energies. Gamma rays below a certain energy related to the rest mass of the pion are completely absent from the signal. The detection of gamma rays with that characteristic distribution of energies, in the direction of two known supernovae remnants, provided evidence for the entire process – and a smoking gun for how cosmic rays are produced.
While other objects in the universe generate cosmic rays, most probably active galactic nuclei located far beyond our own Milky Way galaxy, supernovae in our own galactic neighborhood are thought to produce a large fraction of the cosmic rays that impact Earth.
The study by Dr. Funk and his team of researchers appears in the Feb. 15 issue of Science. Dr. Funk spoke this week with The Kavli Foundation about the discovery, what it means, and what the next steps are in his team's work.
THE KAVLI FOUNDATION: Dr. Funk, can you begin by telling us – what are cosmic rays?
STEFAN FUNK: Cosmic rays are made up of high-energy particles that reach the upper atmosphere from outer space. They consist mostly of protons, in other words hydrogen nuclei, but they also can consist of nuclei of Helium or heavier elements, of electrons and other subatomic particles. They were discovered in 1912 by an Austrian named Victor Hess. He demonstrated in a series of balloon flights that the radiation increased at higher altitudes, and this was observational proof that the source of radiation was coming from outer space.
Cosmic rays arrive here going at near the speed of light. Their energies can be enormous - much higher than what we can produce in our terrestrial accelerators, such as the Large Hadron Collider (LHC) in Geneva.
TKF: For anyone unfamiliar with cosmic rays, are they dangerous?
FUNK: They could be, but the good thing about the Earth is that it has a shield. When these high-energy protons – i.e. cosmic rays – enter the atmosphere, they interact with particles in the atmosphere and as a consequence lose most of their energy. So, very few of the cosmic rays that arrive in the upper atmosphere actually make it to the ground. Only the very highest energy protons, the so-called ultra-high energy cosmic rays, reach the ground. Therefore here on the ground, most of the radiation we get is from the Earth itself and cosmic rays account for only a few percent of the total. This changes as you go higher in the atmosphere. During trans-Atlantic flights, for example, you're much more exposed to cosmic rays. If you leave the atmosphere, you're totally exposed to all the cosmic rays, and this is a significant hazard for long-duration spaceflight.
TKF: And why is it so hard to tell where cosmic rays originate?
FUNK: In the past 100 years we've studied them in great detail as they arrive here on Earth, but it's been much more difficult to pinpoint where they come from. That's because protons are deflected by magnetic fields that permeate our galaxy – in other words, they're deflected from their source on their way to us so they don't point back in that direction. And that means we cannot do astrophysics with these protons because it's so hard to find where they come from.
In order to understand the origin and acceleration of cosmic ray protons, researchers used data from the Fermi Gamma-ray Space Telescope and targeted W44 and IC 443, two supernova remnants thousands of light years away. Both turned out to be strong sources of gamma rays, but not at energies below what neutral pion decay would produce - the observational proof scientists had been looking for. (Credit: NASA/DOE/Fermi LAT Collaboration).
Click here to see NASA Supernova Remnant video showing specific remnants and their appearance at different wavelengths of electromagnetic radiation.
TKF: So how did you track them back to their source?
FUNK: In our study we turned to "neutral messengers" to tell us where at least some cosmic rays originate. We studied gamma rays, the highest energy photons in the electromagnetic spectrum. Because they are photons, they're electrically neutral and so they're not deflected by magnetic fields in the galaxy; they travel in a straight line. Theory predicts that the gamma rays coming directly from supernovae remnants will have a particular energy distribution if they're generated by the decay of pions, which are only produced by the collision of high-energy protons – cosmic rays – with lower energy protons. So finding gamma rays coming directly from supernovae remnants, with that signature energy distribution – the complete absence of gamma rays with energies below the pion restmass, was our smoking gun. In this case, the cosmic rays interact with interstellar material in the immediate vicinity of the supernova shock wave and hence don't make it as cosmic rays here to Earth, but the gamma rays produced in these proton-proton collisions do, and those gamma rays are what we measured.
TKF: Was finding the gamma rays difficult?
FUNK: To observe these gamma rays in space, you need a special telescope and you need a detector on the telescope that is sensitive to gamma rays with the energy predicted for this process. The Fermi Gamma-ray Space Telescope was sensitive enough to detect the kind of gamma-ray signature we were looking for. We have detected several additional supernovae remnants but detecting this smoking gun feature so far was only possible in the two brightest ones – at those low energies the analysis is extremely complex and it is very much an effort from the whole LAT team that allowed us to reach those low energies. So it was the combination of having a sensitive-enough instrument and very bright supernova remnants that allowed us to study this.
TKF: Scientists have long believed that supernovae remnants are one major source of cosmic rays, correct?
FUNK: Yes. The idea that the remnants of exploded stars should be the dominant source goes back to the 1960s. There are very few astrophysical objects in our galaxy that could sustain the constant stream of cosmic rays that reaches Earth. Back in the 1960s, researchers calculated that if supernova remnants are somehow able to put 10 percent of their explosion energy into the acceleration of protons, they can sustain the amount of cosmic rays that we see impacting Earth. In our study, we've shown that the remnants of the two supernova explosions we targeted in fact convert about the right level of the explosion energy into the production of cosmic rays. But of course, we haven't confirmed that for all supernovae remnants in the Milky Way – although all of them, collectively, are thought to contribute to the cosmic rays that impact Earth.
TKF: What is the estimated number of supernova remnants in the Milky Way Galaxy?
FUNK: Supernovae in our galaxy occur at a rate of two to three per century. We think that the shock waves from those explosions last about 50,000 years, and they accelerate protons continuously during that time. That means at any given moment you expect to have about a thousand objects actively accelerating cosmic rays.
TKF: The press release states, "A new study confirms what scientists have long suspected." Was there anything surprising about your findings?
FUNK: Making this discovery was one of the goals of the Fermi Large Area Telescope, but it's surprising that the instrument has worked so well. Our discovery was made after the telescope had been operating for four years, and our instrument team worked hard to improve the instrument's sensitivity. It was very much a team effort that allowed us to make this measurement.
TKF: The next step in your research is to study in more detail how the shock waves from supernovae can accelerate protons to near the speed of light. Why is this your next step, and how will you do this?
These shock waves get slower and slower over time as they sweep up material, and we would like to understand in which of their evolutionary stages they are best able to accelerate particles. In which of their evolutionary stages over this lifetime of 50,000 years do they accelerate a particle to which energy? To understand this, we need to study a large number of supernova remnants. We can then separate them into evolutionary stages, see some commonalities between stages, and then try to learn something more about how the particles are accelerated.
By studying these stages, we can learn more about how much energy is needed to generate cosmic rays, and how long after an initial supernova they are generated. These insights will help us better understand the ocean of high-energy particles that permeates space and showers the Earth with radiation.


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Make Real and local charity Stay Up Late / Gig Buddies has continued with the Google Daydream Impact project, utilising high-end 360° camera equipment to create a series of VR films designed to empower members with learning disabilities to have the social confidence to attend events at local venues in the real world.
Following on from a successful trial at The Old Market Theatre, the team have been to C:\Side_Quest gaming cafe on the seafront by the Brighton Palace Pier, to film the venue over a succession of events to create a more personal experience for this particular venue.
Other venues due for the treatment include The Komedia comedy club and Duke's At Komedia, the Brighton Dome and the Brighton Centre venues. Details of these films will be released in the near future but you can watch the latest film here: https://www.youtube.com/watch?v=wvswaE-t0o4 or take a Google Poly Tour with photos here: https://poly.google.com/u/1/view/aloUEKZs3CI - if you have a Google Cardboard or other VR headset that supports 360° YouTube content, you can also view these in VR!


As compared to the previous decade, India's internet scenery has entirely transformed over the past couple of years. Not only fast internet became cheaper nowadays but also smartphones and laptops. Mostly everybody is connected to each other via the Internet. But at the same time, the internet has become a risky place because of data hacking and virus threats.
Lots of devices connected to internet putting large numbers of data online, which is troublesome for the internet users. Nowadays apps and games on smartphone devices demand sensitive information to proceed further.These vulnerabilities permit hackers to steal your passwords and other sensitive information. By taking these problems into consideration, it is vital to take protective steps to stay safe online.
Mostly, people try to keep passwords which are easy to remember. As per the latest report of 2017, the most generally used passwords were qwerty, 123456, 11111111, password and many more. To have a strong security, you should use a strong password. Perfectly, a mix of upper and lower cases alphabets, with numbers and special characters use for a strong password. As an example, this could be like "$aMeY_201" which has upper and lower cases alphabets, numbers and special characters and length should be approximately above 8 characters.
Avoid using your vehicle number and pets name or date of birth which are easy to track down.
Most of the people tend to use the same password for many years and that something needs to be changed. It is good practice, changing passwords of your account every 45-60 days. Most people have a hard time to remember the passwords, you can write them down or use services such as LastPass or Dashlane which helps you control all your passwords. Also, it is significant to have unique passwords for each website than having same on all websites.
Many of social and e-commerce websites allow users to sign-ups for login notifications. Whenever you log in to your account, you will get a notification email about the activity. If you log in from unfamiliar locations or devices, you will get an instant message related to account security. This is a good account security practice you can do to secure your accounts across the internet.
2FA is very useful for banking websites. This adds one more layer of security when logging into a website.
With 2FA enabled, websites send text messages such as OTP or one-time password which you need to enter to verify your identity.
Avoid using internet from public Wi-Fi The best thing about public Wi-Fi at the location such as railway stations, airports, malls and cafes is that it is free to access and open. But it is venerable to attacks from hackers and they can steal sensitive information from your device. Use public Wi-Fi only if it is necessary.
You are familiar with emails in your Inbox which are from unknown sources or brands such as "Get a home loan on 0% interest" or "You won a Jackpot". These are kind of click baits or phishing attacks.
When you click on links in these emails, hackers get a backdoor entry to your sensitive information. Then access lots of things from your computer and use it for evil purposes. nIt is better not to open emails or links from unknown sources.
A virtual private network (VPN) creates a secure pathway between the device or smartphone and website you are visiting. It manages to keep your online activities private and secure. Your ISP or government cannot track you or websites you are visiting. There numerous free and paid VPN services available in the market.
Do you ever notice, how many times you have searched for t-shirts or shoes or trips? And then, how many times you see ads which are similar to websites you have searched for?
Do you think you are followed? Hmm, maybe "Yes".
Whenever you visit any website, a third party tracker gets stored on your computer. These trackers such as cookies, pixel tags, flash cookies etc.
With the use of trackers, companies come to know what you're searching and most visiting. This can be useful because you can find a better deal via ad network but at the same time, it is threatening to your online privacy.
If you don't want to get followed, you can use browser extensions such as Adblock or many more. These extensions help you browse freely on the internet without getting disturbed by ad networks.
Even if your network is safe and sound, it is always better to use a firewall. Firewall is kind of internet barrier that blocks unauthorized access to your computer. Firewall secures devices connected via the internet to your computer.
Using good antivirus software is also an important step in securing your laptop or desktop from virus and other attacks.
In the end, if you have installed antivirus software on your computer then don't forget to update it.
Install latest security patches each time the new version comes up. So you can keep your laptop or computer for safety and security.
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