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 cannot describe all the observed GeV-TeV spectra and more complex spectral shapes seem required.
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 γ 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 [1]. 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 γ-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 [2,3] or hadronic [4,5] 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-frequencycutoff BL Lac objects (HBLs and LBLs) [6]. 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-frequencypeaked 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., [7]).
Mkn 421 was the first blazar and extragalactic object detected as a very high-energy (VHE; E > 100 GeV) γ-ray emitter with the Whipple telescope in 1992 [8]. Since then, different candidate selection methods have been applied to radio, X-ray or highenergy (HE; E > 100 MeV) γ-ray blazar data in the aim of finding new TeV blazars [9][10][11]. To date, 40 blazars have been detected in the TeV sky [25], 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 I). 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 eConf C110509
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 [12][13][14], occasionally accompanied by a change in spectral index [13] and minute-scale flux doubling times [13,14]. 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 [15].
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 300 MeV-100 GeV 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
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