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active galactic nuclei ( agn ) are extreme extragalactic objects with an observed luminosity outshining their host galaxy . their non - thermal continuum emission extending from radio band to x rays or @xmath0 rays suggest underlying relativistic emission mechanisms .
agn have been subject to comprehensive studies since their discovery , leading to classification and unification schemes based on multiwavelength observations and polarization measurements  @xcite . in this frame ,
agn are classified as either radio - quiet or radio - loud , and blazars constitute a subclass of the latter , with their jet axis oriented close to the observer s line of sight .
this particular orientation combined with relativistic beaming gives rise to prominent observational features in blazars , such as anisotropic radiation , rapid variability , high polarization and superluminal motion .
blazars are divided into two subclasses , flat spectrum radio quasars ( fsrq ) and bl lacertae objects ( bl lac ) .
fsrqs are characterized by a broad line emission region , depicted with strong lines in their optical spectra , that are not present in bl lacs .
the broadband spectral energy distribution ( sed ) of blazars exhibit a two - component structure , with a low - energy component peaking around ir to uv band and a high - energy one around x - ray to @xmath0-ray energies .
the underlying emission mechanism reponsible for low - energy component is believed to be synchrotron emission from relativistic electrons in the blazar jets . on the other hand , the high - energy component could either have leptonic  @xcite or hadronic  @xcite origin .
instrumental selection effects brought up two distinct classes for bl lacs as radio - selected ( rbl ) or x - ray - selected ( xbl ) .
this instrumental classification was later replaced with a more physical one based on radio to optical and optical to x - ray spectral indices , introducing the terminology of _ high- _ and _ low - frequency - cutoff _ bl lac objects ( hbls and lbls )  @xcite .
another approach based on the peak frequency of the synchrotron component of the sed is also used to define the same classification ( _ high- _ and _ low - frequency - peaked _ bl lacs as hbls and lbls ) .
detection of intermediate objects ( ibls ) between these two observationally distinct groups has made it more plausible that bl lac objects constitute a continuum rather than a discrete sequence  ( e.g.,@xcite ) .
mkn 421 was the first blazar and extragalactic object detected as a very high - energy ( vhe ; @xmath1gev ) @xmath0-ray emitter with the whipple telescope in 1992  @xcite .
since then , different candidate selection methods have been applied to radio , x - ray or high - energy ( he ; @xmath1mev ) @xmath0-ray blazar data in the aim of finding new tev blazars  @xcite . to date , 40 blazars have been detected in the tev sky , with a census consisting of 29 hbls , 4 ibls , 4 lbls and 3 fsrqs .
our blazar sample contains all blazars with a published vhe spectrum before february 2011 , with a total of 26 sources ( see table  [ sample ] ) .
this includes 19 hbls , 3 ibls , 2 lbls and 2 fsrqs .
seven of these blazars were detected with egret and 23 of them are in the _ fermi _ 2-year catalog .
more than half of the sample have been detected multiple times in the vhe band .
these multiple detections extending over several years and obtained mostly with different instruments suggest that spectral variability in the vhe band is a common property for vhe blazars . even though no general pattern has been established for vhe variability , several sources
have been observed to have a flux increase up to a few times their baseline emission  @xcite , occasionally accompanied by a change in spectral index  @xcite and minute - scale flux doubling times  @xcite .
.vhe blazar sample .
no secure redshift is available for 3c 66a , pks 1424 + 240 and pg 1553 + 113 .
the _ fermi _ states `` low '' and `` high '' are as described in the text . in some cases
simultaneous data were available , and in the remaining cases 27-month time - averaged _ fermi _ data were used . [ cols="^,^,^,^",options="header " , ]     the first 27 month of _ fermi _ data and archival vhe spectra published before february 2011 were used to construct combined gev - tev seds .
these include six data sets where vhe data overlaps with the _ fermi _ era ( rgb j0710 + 591 , 1es 1218 + 304 , pks 1424 + 240 , pks 2155 - 304 and two different measurements for 3c 66a ) .
the remainder of the vhe data were taken before the _ fermi _ mission .
all vhe spectra were corrected for the extragalactic background light absorption using  @xcite .
the fact that most of the gev and tev data are not contemporaneous introduces caveats for the interpretation of the combined spectra .
moreover , _ fermi _ data represents an average state over a fairly long period , whereas the vhe spectra consist of day - scale  snapshots " , mostly taken during flares . as a solution to this problem , for bright - enough sources ,
the _ fermi _ data were split into  low " and  high " states as described below .
thus , non - contemporaneous gev and tev measurements were matched in a more realistic way than directly using all the time - averaged _ fermi _ data .
_ diffuse class _ events with energy between 300mev100gev from the first 27-month of _ fermi _ data ( from 4 august 2008 to 4 november 2010 ) were used for the analysis . for blazars that were observed during the _ fermi _ era ,
only the time periods of a few months that cover the corresponding vhe observations were selected .
first , a 27-month light - curve analysis was performed for each source using an aperture photometry technique .
events from a region of @xmath2 radius from the target location were selected and counts were plotted as a function of time , each time bin containing 49 counts . for sources with high statistics , low- and high- flux states were identified and separated using the average count rate as a threshold ( see figure  [ lc_mrk501 ] and table  [ sample ] ) .        next , a spectral analysis was done for each data set .
events from a region of interest of @xmath3 were selected and analyzed with fermi science tools v9r18p6 , using instrument response functions p3_v6_diffuse .
sources from the 1fgl catalog  @xcite , bright spots with test statistics @xmath4 and standard galactic and isotropic diffuse emission background components within the region of interest were included in the source model files .
spectral points were calculated using an unbinned maximum - likelihood analysis technique  @xcite .
figure  [ sed_mrk501 ] shows the combined sed for the hbl mrk501 .
the peak frequency of the inverse - compton ( ic ) component is a salient parameter for describing blazar non - thermal continua and studying population trends .
systematic studies for measuring the ic peak frequency mostly suffer from the lack of statistics and simultaneous data  @xcite . in this work ,
we focus on finding the ic  peak frequency band " rather than the  peak frequency " , using a model independent approach . for each blazar sed , we identify the energy decade in which the largest amount of power is emitted .
figure  [ ic_peak_dist ] shows the distribution of peak frequency bands .
we observe that the low - synchrotron - peaked objects ( lsps ) have the maximum of their emission mostly below 1 gev .
on the other hand , high - synchrotron - peaked objects ( hsps ) tend to peak in the tev range .
a power - law function often fails to describe the combined seds successfully . for most blazars ,
the fits are significantly improved with a broken power law ( see figure  [ sed_mrk501 ] ) or an absorbed power law . particularly , some lbl objects display absorption - like features in the 10 - 100 gev range . to investigate this feature , we considered the following absorption scenarios from the broad line region ( blr )  @xcite for the blazars s5  0716 + 714 ( lbl ) , w comae ( ibl ) and bl lacertae ( lbl ) :    * h i line ( 13.6 ev ) * he ii line ( 54.4 ev ) * h i & he ii combined * full blr spectrum    in all the cases , absorption from the he ii complex seems to be dominant
. a double - absorption scenario does not provide an improved fit over the he ii single - line absorption . on the other hand ,
the full blr absorption underproduces the tev emission ( see figure  [ s5_0716_sed ] ) .
we also modeled a full blr absorption combined with cascade emission from the @xmath0-@xmath0 absorption  @xcite .
no significant contribution from cascading effects in the fermi band is expected ( see figure  [ wcomae_sed ] ) .
in this paper we give some results from our ongoing gev - tev blazar population studies .
we analyzed 27 months of _ fermi _ data and combined it with archival tev data .
we see absorption - like features mostly in lbl spectra , these features are best described as blr absorption from the he ii complex . in addition , we observe that the peak frequency in the gev - tev region follows the synchrotron peak frequency trend .
complete results will be discussed in a forthcoming paper .
9 urry , c. m. , & padovani , p.  1995 , pasp , 107 , 803 maraschi , l. , ghisellini , g. , & celotti , a.  1992 , apj , 397 , l5 dermer , c.  d. & schlickeiser , r.  1993 , apj , 416 , 458 aharonian , f.  2000 , newa , 5 , 377 mcke , a. , et al .
2003 , aph , 18 , 593 padovani , p. & giommi , p.  1995 , apj , 444 , 567 nieppola , e. , valtaoja , e. , tornikoski , m. , hovatta , t. , kotiranta , m.  2008 , a&a , 488 , 867 punch , m. , et al .
1992 , nature , 358 , 477 costamante , l. , & ghisellini , g.  2002 , a&a , 384 , 56 de la calle prez , i. , et al .
2003 , apj , 599 , 909 behera , b. , & wagner , s.  i.  2009 , arxiv0908.4574b acciari , v. a. , et al .
2009 , apj , 707 , 612 albert , j. , et al .
2007 , apj , 669 , 862 aharonian , f. , et al .
2007 , apj , 664 , l71 domnguez , a. et al .
2011 , mnras , 410 , 2556 abdo , a.  a. , et al .
2010 , apj , 715 , 429 cash , w.  1979 , apj , 228 , 939 mattox , j.  r. , et al .
1996 , apj , 461 , 396 anderhub , h. et al .
2009 , apj , 705 , 1624 zhang , j. , et al .
2011 , arxiv:1108.0607v1 [ astro-ph.he ] poutanen , j. et al .  2010 ,
apj , 711 , 118 roustazadeh , p. et al .
2010 , apj 717 , 468 anderhub , h. et al .
2009 , apjl , 704 , 129 acciari , v. a. et al .
2008 , apjl , 684 , 73
the synergy between the _ fermi_-lat and ground - based cherenkov telescope arrays gives us the opportunity for the first time to characterize the high energy emission ( 100 mev  10 tev ) from more than 30 blazars . in this study we performed a _ 
 fermi_-lat spectral analysis for all tev - detected blazars and combined it with archival tev spectra . 
 our results for low synchrotron - peaked bl lacs ( lbl ) show hints of absorption features in the gev band that could be interpreted as internal opacity at the source . 
 we note that simple or broken power laws can not describe all the observed gev - tev spectra and more complex spectral shapes seem required .