Observations of very-high-energy (VHE, E > 250 GeV) gamma-ray emission from several blazars at z > 0.1 have placed stringent constraints on the elusive spectrum and intensity of the intergalactic infrared background radiation (IIBR). Correcting their observed VHE spectrum for gamma-gamma absorption even by the lowest plausible level of the IIBR provided evidence for a very hard (photon spectral index Gamma_{ph} < 2) intrinsic source spectrum out to TeV energies. Such a hard VHE gamma-ray spectrum poses a serious challenge to the conventional synchrotron-self-Compton interpretation of the VHE emission of TeV blazars and suggests the emergence of a separate emission component beyond a few hundred GeV. Here we propose that such a very hard, slowly variable VHE emission component in TeV blazars may be produced via Compton upscattering of Cosmic Microwave Background (CMB) photons by shock-accelerated electrons in an extended jet. For the case of 1ES 1101-232, this component could dominate the bolometric luminosity of the extended jet if the magnetic fields are of the order of typical intergalactic magnetic fields B ~ 10 micro-Gauss and electrons are still being accelerated out to TeV energies gamma > 4 X 10^6) on kiloparsec scales along the jet.
The Very High Energy (VHE) gamma-ray emission of the BL Lacertae objects Markarian 501 and Markarian 421 has been observed by the CAT Imaging Atmospheric Cherenkov Telescope in 1997 and 1998. The spectrum extraction method is presented, and the spectral properties of both sources are compared in different activity states. Theoretical implications for jet astrophysics are discussed.
The blazar 1ES 1101-232 was observed with the High Energy Stereoscopic System (H.E.S.S.) of Atmospheric Cherenkov Telescopes (ACT) in 2004 and 2005, for a live time of 43 hours. VHE (E > 10^11 eV) gamma-rays were detected for the first time from this object. VHE observations of blazars are used to investigate the inner parts of the blazar jets, and also to study the extragalactic background light (EBL) in the near-infrared band. Observations in 2005 were conducted in a multiwavelength campaign, together with the RXTE satellite and optical observations. In 2004, simultaneous observations with X…
Recent observations in the TeV band challenge the simplest models developed to describe the overall emission of blazars and radiogalaxies. In particular, the observation of variable TeV emission from M87 and the fast variability shown by PKS 2155-304 challenge the standard framework. We discuss how the existence of a radial structure in the sub-pc scale jet, with faster a component ("spine" or "needles") embedded in a slower layer can explain the basic phenomenology of these sources.
We study the time dependent spectra produced via the bulk Compton process by a cold, relativistic shell of plasma moving (and accelerating) along the jet of a blazar, scattering on external photons emitted by the accretion disc and reprocessed in the broad line region. Bulk Comptonization of disc photons is shown to yield a spectral component contributing in the far UV band, and would then be currently unobservable. On the contrary, the bulk Comptonization of broad line photons may yield a significant feature in the soft X-ray band. Such a feature is time-dependent and transient, and dominates…
The observation of rapidly variable very high energy (VHE) gamma-rays from non-aligned active galactic nuclei (AGNs), as reported from M87, proves challenging for conventional theoretical acceleration and emission models. We re-examine the centrifugal acceleration of particles by rotating jet magnetospheres in the vicinity of accreting supermassive black hole systems and analyze the energy constraints imposed for highly underluminous systems. Applications are presented for conditions expected to be present in the radio galaxy M87, assuming accretion onto the central black hole to occur in an a…
We demonstrate that the spectral differences between Flat Spectrum Radio Quasars (FSRQ) with steep gamma-ray spectra (MeV-blazars) and FSRQ with flat gamma-ray spectra (GeV-blazars) can be explained by assuming that in the MeV-blazars, the production of gamma-rays is dominated by Comptonization of infrared radiation of hot dust, whereas in the GeV-blazars -- by Comptonization of broad emission lines. Additional ingredient, required to reach satisfactory unification, is an assumption that the radiating electrons are accelerated via a two step process, in the lower energy range -- following inst…
We model light curves of flares observed in blazars, assuming that they are produced by relativistic electrons/positrons injected by forward-reverse shocks traveling down the jet with relativistic speeds. We approximate the radiating region as a thin shell enclosed between the two shock fronts. In addition to the conventional radiation processes such as synchrotron radiation and its Comptonization, we take into account Comptonization of light originating in the broad emission line region, and Comptonization of IR radiation from hot dust. We demonstrate that flares produced in the adiabatic reg…
The blazar Markarian 501 (Mrk 501) was observed above 100 GeV with the MAGIC Telescope during May, June and July 2005. The high sensitivity of the instrument made possible the detection of the source with high significance in each of the observing nights. During this observational campaign, the emitted gamma-ray flux from Mkn 501 was found to vary by one order of magnitude, and showed a high correlation with spectral changes. Intra-night flux variability was also observed, with flux-doubling times of ~2 minutes. The data showed a clear evidence of a spectral peak (in the nuFnu representation) …
The energy spectra of TeV gamma-rays from blazars, after being corrected for intergalatic absorption in the Extragalactic Background Light (EBL), appear unusually hard, a fact that poses challenges to the conventional models of particle acceleration in TeV blazars and/or to the EBL models. In this paper we show that the internal absorption of gamma-rays caused by interactions with dense narrow-band radiation fields in the vicinity of compact gamma-ray production regions can lead to the formation of gamma-ray spectra of an almost arbitrary hardness. This allows significant relaxation of the cur…
We study the dependence of nonthermal radiation spectra in OVV quasars on location of the low energy break in the electron/positron injection function. We show that the high energy spectra produced during the outbursts are presumably superposed from two components, one resulting from Comptonization of emission lines, which dominates at MeV-GeV energies, and the other resulting from Comptonization of infrared radiation, which dominates in the X-ray band.
The Very High Energy (VHE) gamma-ray emission of the closest BL Lacertae objects Markarian 501 and Markarian 421 has been observed by the CAT telescope in 1997 and 1998. In 1997 Mrk 501 exhibited a remarkable series of flares, with a VHE emission peaking above 250 GeV. The source showed correlated emissions in the X-ray and VHE gamma-ray bands, together with intensity-spectral hardness correlation in the latter energy range. During small flares in 1998, Mrk 421 became the second extragalactic source detected by CAT. Its spectral properties are compared to those of Mrk 501. Theoretical implicat…
Recent findings by gamma-ray Cherenkov telescopes suggest a higher transparency of the Universe to very-high-energy (VHE) photons than expected from current models of the Extragalactic Background Light. It has been shown that such transparency can be naturally explained by the DARMA scenario, in which the photon mixes with a new, very light, axion-like particle predicted by many extensions of the Standard Model of elementary particles. We discuss the implications of DARMA for the VHE gamma-ray spectra of blazars, and show that it successfully accounts for the observed correlation between spect…
M 87 is the first extragalactic source detected in the Very High Energy (VHE; E > 100 GeV) gamma-ray domain that is not a blazar, its large scale jet not being aligned to the line of sight. Slight modification of standard emission models of TeV blazars allows to account for the gamma-ray spectra obtained with H.E.S.S. We present a multi-blob synchrotron self-Compton model taking explicitly into account large viewing angles and moderate values of the Lorentz factor as inferred from MHD simulations of jet formation. Predictions of the VHE emission for the nearby radiogalaxy Cen A and an interpre…
TeV blazars are known as prominent nonthermal emitters across the entire electromagnetic spectrum with their photon power peaking in the X-ray and TeV bands. If distant, absorption of gamma-ray photons by the extragalactic background light (EBL) alters the intrinsic TeV spectral shape, thereby affecting the overall interpretation. Suzaku observations for two of the more distant TeV blazars known to date, 1ES 1101-232 and 1ES 1553+113, were carried out in 2006 May and July, respectively, including a quasi-simultaneous coverage with the state-of-the-art Cerenkov telescope facilities. We report o…
Photons with very high energy up to TeV (VHE) emitted from active galactic nuclei (AGNs) provide some invaluable information of the origin of $\gam$-ray emission. Although 66 blazars have been detected by {\it EGRET}, only three low redshift X-ray selected BL Lacs (Mrk 421, Mrk 501, and 1ES 2344+514) are conclusive TeV emitters (PKS 2155-304 is a potential TeV emitter) since VHE photons may be absorbed by cosmological background infrared photons ({\it external} absorption). Based on the ``mirror'' effect of clouds in broad line region, we argue that there is an {\it intrinsic} mechanism for th…
Broad-band spectra of the FSRQ (flat-spectrum-radio quasars) detected in the high energy gamma-ray band imply that there may be two types of such objects: those with steep gamma-ray spectra, hereafter called MeV-blazars, and those with flat gamma-ray spectra, GeV-blazars. We demonstrate that this difference can be explained in the context of the ERC (external-radiation-Compton) model using the same electron injection function. A satisfactory unification is reachable, provided that: (a) spectra of GeV-blazars are produced by internal shocks formed at the distances where cooling of relativistic …
The non-thermal spectra of blazars, observed from radio to GeV/TeV gamma-rays, reveal two pronounced components, both produced by radiation by energetic particles. One peaks in the IR - to soft X-ray band, radiating via the synchrotron process; the other, peaking in the high-energy gamma-rays, is produced by the Compton process. These spectra -- and, in particular, the \asca data -- suggest that the origin of the seed photons for Comptonization is diverse. In the High-energy peaked BL Lac objects (HBLs), the dominant seed photons for Comptonization appear to be the synchrotron photons internal…
A prediction for the energy shift of the synchrotron spectrum of flat-spectrum radio quasars (FSRQs) during high-energy flares is presented. If the $γ$-ray emission of FSRQs is produced by Comptonization of external radiation, then the peak of the synchrotron spectrum is predicted to move to lower energies in the flare state. This is opposite to the well-known broadband spectral behavior of high-frequency peaked BL-Lac objects where the external radiation field is believed to be weak and synchrotron-self Compton scattering might be the dominant $γ$-ray radiation mechanism. The synchrotron peak…
The direct measurement of the extragalactic background light (EBL) is difficult at optical to infrared wavelengths because of the strong foreground radiation originating in the Solar System. Very high energy (VHE, E$>$100 GeV) gamma rays interact with EBL photons of these wavelengths through pair production. In this work, the available VHE spectra from six blazars are used to place upper limits on the EBL. These blazars have been detected over a range of redshifts and a steepening of the spectral index is observed with increasing source distance. This can be interpreted as absorption by the EB…
The blazar Mrk421 was observed independently, but contemporaneously, in 2005 at TeV energies by MAGIC, the Whipple 10m telescope, and by a single VERITAS telescope during the construction phase of operations. A comparison of the time averaged spectra, in what was a relatively quiescent state, demonstrates the level of agreement between instruments. In addition, the increased sensitivity of the new generation instruments, and ever decreasing energy thresholds, questions how best to compare new observational data with archival results.
We present the discovery of very high energy (VHE) gamma-ray emission from the high-frequency-peaked BL Lac object 1ES 0806+524 (z=0.138) and the intermediate-frequency-peaked BL Lac object W Comae (z=0.102) with VERITAS. VHE emission was discovered from these objects during the 2007/2008 observing campaign, with a strong outburst from W Comae detected in mid-March, lasting a few days. Quasi-simultaneous spectral energy distributions are presented, incorporating optical (AAVSO), and X-ray (Swift/RXTE) observations. We also present the energy spectrum of the distant BL Lac (z=0.182) 1ES 1218+30…
This paper reviews the topic of sub-parsec - scale jets in quasars, and covers the the following issues: observations of parsec and sub-parsec scale jets; energy dissipation and particle acceleration; radiative processes; magnetic fields, pair content, and energetics; and variability and its relation to the central engine activity. In particular, we describe how internal shocks can explain properties of gamma-ray flares and demonstrate that MeV blazars (those with luminosity peak in the 1-30 MeV range) can be unified with GeV blazars (those with luminosity peak at GeV energies) assuming that i…
The high-frequency peaked BL Lac PKS 2005-489 (z=0.071) was observed in 2003 and 2004 with the H.E.S.S. stereoscopic array of imaging atmospheric-Cherenkov telescopes in Namibia. A signal was detected at the 6.7 sigma level in the 2004 observations (24.2 hrs live time), but not in the 2003 data set (27.3 hrs live time). PKS 2005-489 is the first blazar independently discovered by H.E.S.S. to be an emitter of VHE photons, and only the second such blazar in the Southern Hemisphere. The integral flux above 200 GeV observed in 2004 is (6.9 +/- 1.0 +/- 1.4) x 10$^{-12}$ cm$^{-2}$ s$^{-1}$, correspo…
We present the results from a multiwavelength campaign on the TeV blazar 1ES 1959+650, performed in May, 2006. Data from the optical, UV, soft- and hard-X-ray and very high energy (VHE) gamma-ray (E > 100 GeV) bands were obtained with the SUZAKU and SWIFT satellites, with the MAGIC telescope and other ground based facilities. The source spectral energy distribution (SED), derived from SUZAKU and MAGIC observations at the end of May 2006, shows the usual double hump shape, with the synchrotron peak at a higher flux level than the Compton peak. With respect to historical values, during our campa…
We discuss some phenomenological aspects of $γ$-ray emitting jets. In particular, we present calculations of the $γ$-sphere and $π$-sphere for various target photon fields, and employ them to demonstrate how $γ$-ray observations at very high energies can be used to constraint the Doppler factor of the emitting plasma and the production of VHE neutrinos. We also consider the implications of the rapid TeV variability observed in M87 and the TeV blazars, and propose a model for the very rapid TeV flares observed with HESS and MAGIC in some blazars,that accommodates the relatively small Doppler fa…
All but three (M87, BL Lac and 3C 279) extragalactic sources detected so far at very high energy (VHE) gamma-rays belong to the class of high-frequency peaked BL Lac (HBL) objects. This suggested to us a systematic scan of candidate sources with the MAGIC telescope, based on the compilation of X-ray blazars by Donato et al. (2001). The observations took place from December 2004 to March 2006 and cover sources on the northern sky visible under small zenith distances zd < 30 degrees at culmination. The sensitivity of the search was planned for detecting X-ray bright F(1 keV) > 2 uJy) sources emi…
Aims: We derive limits on the EBL density from the energy spectra of distant sources of very high energetic gamma-rays (VHE gamma-rays). Methods: VHE gamma-rays are attenuated by the photons of the EBL via pair production, which leaves an imprint in the measured spectra from distant sources. In this paper we present a new method to derive constraints on the EBL. Hereby, we use only very basic assumptions about TeV blazar physics and no pre-defined EBL model, but rather a large number of generic shapes constructed from a grid in EBL density vs. wavelength. In our study we utilize spectral data …
M87 is the first extragalactic source detected in the TeV gamma-ray domain that is not a blazar, its large scale jet not being aligned to the line of sight. We present here a multi-blob synchrotron self-Compton model accounting explicitly for large viewing angles and moderate Lorentz factors as inferred from magnetohydrodynamic simulations of jet formation, motivated by the detection of M87 at very high energies (VHE; E > 100GeV). Predictions are presented for the very high-energy emission of active galactic nuclei with extended optical or X-ray jet, which could be misaligned blazars but still…
The double humped SED (Spectral Energy Distribution) of blazars, and their flaring phenomena can be explained by various leptonic and hadronic models. However, accurate modeling of the high frequency component and clear identification of the correct emission mechanism would require simultaneous measurements in both the MeV-GeV band and the TeV band. Due to the differences in the sensitivity and the field of view of the instruments required to do these measurements, it is essential to identify active states of blazars likely to be detected with TeV instruments. Using a reasonable intergalactic …
The rapid variability of the VHE emission reported for some TeV blazars implies Doppler factors well in excess of those inferred from superluminal motions and unification schemes. We propose that those extreme flares may result from radiative deceleration of blobs on scales where local dissipation occurs. The minimum jet power estimated from the resolved synchrotron emission on VLBI scales appears to be consistent with this model. It is shown that if the energy distribution of nonthermal electrons accelerated locally in the blob is reasonably flat, then a background radiation field having a lu…
It is proposed that the spectra of so called 'MeV blazars' can be explained in terms of previously developed models of the external comptonization of accretion disk radiation provided that the structure of the inner and outer parts of the accretion disk is different. The electron acceleration is saturated by the inverse Compton losses in the inner geometrically thick disk and the outer geometrically thin disk at different maximum energies which causes the appearance of two spectral components, one strongly peaked in the MeV energy range and the second one of a power law type extending through …
