Optical spectroscopy of Blazars for the Cherenkov Telescope Array
P. Goldoni, S. Pita, C. Boisson, W. Max-Moerbeck, E. Kasai, D. A. Williams, F. D'Ammando, V. Navarro-Aranguiz, M. Backes, U. Barres de Almeida, J. Becerra-Gonza… 展开作者
P. Goldoni, S. Pita, C. Boisson, W. Max-Moerbeck, E. Kasai, D. A. Williams, F. D'Ammando, V. Navarro-Aranguiz, M. Backes, U. Barres de Almeida, J. Becerra-Gonzalez, G. Cotter, O. Hervet, J. -P. Lenain, E. Lindfors, H. Sol, S. Wagner
Context. Blazars are the most numerous class of High Energy (HE; E about 50 MeV - few 100 GeV) and Very High Energy (VHE; E about 100 GeV - 10 TeV) gamma-ray emitters. As of today, a measured spectroscopic redshift is available for only about 50% of gamma-ray BL Lacs, mainly due to the difficulty of measuring reliable redshifts from their nearly featureless, continuum-dominated optical spectra. The knowledge of the redshift is fundamental for understanding the emission from blazars, for population studies and also for indirect studies of the extragalactic background light and searches for Lorentz invariance violation and axion-like particles using blazars. Aims. This paper is the first of a series of papers which aim to measure the redshift of a sample of blazars likely to be detected with the upcoming Cherenkov Telescope Array (CTA), a ground based gamma-ray observatory. Methods. Monte Carlo simulations were performed to select those hard spectrum gamma-ray blazars detected with the Fermi-LAT telescope still lacking redshift measurements but likely to be detected by CTA in 30 hours of observing time or less. Optical observing campaigns involving deep imaging and spectroscopic observations were organised to efficiently constrain their redshifts. We performed deep medium to high resolution spectroscopy of nineteen blazar optical counterparts using the ESI spectrograph at Keck, the RSS spectrograph at the SALT telescope, and the EFOSC2 spectrograph at the ESO NTT. We searched systematically for spectral features and, when possible, we estimated the contribution of the host galaxy to the total flux. Results. We measured eleven firm spectroscopic redshifts with values ranging from 0.1116 to 0.482. one tentative redshift, three redshift lower limits including one at z > 0.449 and another at z > 0.868. There were four objects found to have featureless spectra.
The prospects for future blazar surveys by next-generation very high energy (VHE) gamma-ray telescopes, such as Advanced Gamma-ray Imaging System (AGIS) and Cherenkov Telescope Array (CTA), are investigated using the latest model of blazar luminosity function and its evolution which is in good agreement with the flux and redshift distribution of observed blazars as well as the extragalactic gamma-ray background. We extend and improve the template of spectral energy distributions (SEDs) based on the blazar SED sequence paradigm, to make it reliable also in the VHE bands (above 100 GeV) by compa…
Misaligned blazars have been the subject of some of the most successful radio and gamma-ray multiwavelength campaigns. These campaigns have included many of the major ground and space based gamma-ray telescopes and span decades of energy. Even though misaligned blazars account for only a small number of the total AGN detected at VHE, they provide a unique view on the AGN population. By viewing blazars at larger angles to our line of sight, they become a unique laboratory for the study of AGN jet substructure and the morphology of non-thermal emission processes. This contribution will discuss o…
Blazars are the most numerous type of observed high-energy gamma-ray emitters. However, their emission mechanisms and population properties are still not well-understood. Crucial to this understanding are their cosmological redshifts, which are often not easy to obtain. This presents a great challenge to the next-generation ground-based observatory for very-high-energy gamma rays, the Cherenkov Telescope Array (CTA), which aims to detect a large number of distant blazars to study their intrinsic emission properties and to place tight constraints on the extragalactic background light density, a…
Optical spectroscopy of blazars for the Cherenkov Telescope Array Observatory- IV
Universidad de Chile（智利大学）；Université Paris Cité, CNRS, CEA, Astroparticule et Cosmologie（巴黎西岱大学、法国国家科学研究中心、法国原子能和替代能源委员会、粒子物理与宇宙学研究所）；University of Namibia（纳米比亚大学）；Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz（加州大学圣克鲁兹分校粒子物理研究所及物理系）；Laboratoire Univers et Théories, Observatoire de Paris, Université PSL, Université Paris Cité, CNRS（巴黎天文台宇宙与理论实验室、巴黎文理研究大学、巴黎西岱大学、法国国家科学研究中心）
The sources of IceCube neutrinos are as yet unknown. The multi-messenger observation of their emission in $γ$-rays can be a guide to their identification, as exemplified by the case of TXS 0506+056. We suggest a new method of searching for $γ$-rays with Imaging Air Cherenkov Telescopes from sources in coincidence with possible astrophysical neutrinos. We propose that searches of $γ$-rays are extended, from the current practice of only a few days, to up to one month from a neutrino alert. We test this strategy on simulated sources modeled after the blazar \emph{TXS 0506+056-like}, emitting neut…
Active galactic nuclei (AGNs) make up about 35 per cent of the more than 250 sources detected in very-high-energy (VHE) gamma rays to date with Imaging Atmospheric Cherenkov Telescopes. Apart from four nearby radio galaxies and two AGNs of unknown type, all known VHE AGNs are blazars. Knowledge of the cosmological redshift of gamma-ray blazars is key to enabling the study of their intrinsic emission properties, as the interaction between gamma rays and the extragalactic background light (EBL) results in a spectral softening. Therefore, the redshift determination exercise is crucial to indirect…
For studying variable sources like blazars, it is crucial to achieve unbiased monitoring, either with dedicated telescopes in pointing mode or survey instruments. At TeV energies, the High Altitude Water Cherenkov (HAWC) observatory monitors approximately two thirds of the sky every day. It uses the water Cherenkov technique, which provides an excellent duty cycle independent of weather and season. The First G-APD Cherenkov Telescope (FACT) monitors a small sample of sources with better sensitivity, using the imaging air Cherenkov technique. Thanks to its camera with silicon-based photosensors…
Blazars are active galactic nuclei, and the most numerous High Energy (HE) and Very High Energy (VHE) gamma-ray emitters. Their optical emission is often dominated by non-thermal, and, in the case of BL Lacs, featureless continuum radiation. This renders the determination of their redshift extremely difficult. Indeed, as of today only about 50 % of gamma-ray blazars have a measured spectroscopic redshift. The knowledge of redshift is fundamental because it allows the precise modeling of the VHE emission and also of its interaction with the extragalactic background light (EBL). The beginning of…
Revision of conservative lower bound on the intergalactic magnetic field from Fermi and Cherenkov telescope observations of extreme blazars
École Polytechnique Fédérale de Lausanne（洛桑联邦理工学院）；Université Paris Cité, CNRS, Astroparticule et Cosmologie（巴黎西岱大学，法国国家科学研究中心，亚原子粒子与宇宙学）；Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne（洛桑联邦理工学院天体物理实验室）
The Extragalactic Background Light (EBL) is the integrated light from all the stars that have ever formed, and spans the IR-UV range. The interaction of very-high-energy (VHE: E>100 GeV) gamma-rays, emitted by sources located at cosmological distances, with the intervening EBL results in electron-positron pair production that leads to energy-dependent attenuation of the observed VHE flux. This introduces a fundamental ambiguity in the interpretation of the measured VHE blazar spectra: neither the intrinsic spectra, nor the EBL, are separately known - only their combination is. In this paper we…
Hadron beams are invoked to explain the peculiar properties of a subclass of BL Lac objects, the so-called extreme BL Lacs (EHBLs). This scenario predicts a quite distinctive feature for the high-energy gamma-ray spectrum of these sources, namely a hard energy tail extending up to $\sim100$ TeV. It has been proposed that the detection of this tail can offer an unambiguous way to distinguish between the hadron beam scenario and the standard one, which instead assumes gamma-ray emission from the jet strongly depleted at the highest energies ($E>30$ TeV) because of the interaction with the optica…
Investigating Active Galactic Nuclei variability with the Cherenkov Telescope Array Observatory
Sorbonne Université（索邦大学）；CNRS/IN2P3（法国国家科学研究中心/欧洲核子研究组织）；Université Paris-Saclay（巴黎萨克雷大学）；Université de Paris（巴黎大学）；Institut de Fisica d’Altes Energies (IFAE)（高能物理研究所）；The Barcelona Institute of Science and Technology（巴塞罗那科学技术研究院）；University of Turku（图尔库大学）
Following the great success of the current Imaging Atmospheric Cherenkov Telescopes, the next generation gamma-ray telescope arrays, Cherenkov Telescope Array (CTA), is being prepared. CTA will have an order of magnitude higher sensitivity and wider energy range than current instruments. Large samples of very high energy (VHE; >30 GeV) sources will be obtained with CTA. Here we discuss potential AGN population studies for the CTA era concerning the overall source counts and the statistics of high redshift sources. Based on our latest blazar gamma-ray luminosity function (GLF) model, we find th…
Gamma rays from distant blazars interact with the extragalactic background light, creating electron-positron pairs, and reducing the gamma-ray flux measured by ground-based atmospheric Cherenkov gamma-ray telescopes. These pairs can Compton-scatter the cosmic microwave background, creating a gamma-ray signature observable by the Fermi Large Area Telesope (LAT). The signature is also dependent on the intergalactic magnetic field (IGMF), since it can deflect the pairs from our line of sight, reducing the gamma-ray emission. We present preliminary constraints on the IGMF using Fermi-LAT and Chere…
As a follow-up of the optical spectroscopic campaign aimed at achieving completeness in the Third Catalog of Hard Fermi-LAT Sources (3FHL), we present here the results of a sample of 28 blazars of uncertain type observed using the 4m telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile. Out of these 28 sources, we find that 25 are BL Lacertae objects (BL Lacs) and 3 are Flat Spectrum Radio Quasars (FSRQs). We measure redshifts or lower limits for 16 of these blazars, whereas it is observed that the 12 remaining blazars have featureless optical spectra. These results are part of…
Between the beginning of its full-scale scientific operations in 2007 and 2012, the VERITAS Cherenkov telescope array observed more than 130 blazars; of these, 26 were detected as very-high-energy (VHE; E>100 GeV) γ-ray sources. In this work, we present the analysis results of a sample of 114 undetected objects. The observations constitute a total live-time of ~570 hours. The sample includes several unidentified Fermi-Large Area Telescope (LAT) sources (located at high Galactic latitude) as well as all the sources from the second Fermi-LAT catalog which are contained within the field of view o…
There is mounting evidence that ultra-energetic neutrinos of astrophysical origin may be associated with blazars. Here we investigate a unique sample of 47 blazars, $\sim 20$ of which could be new neutrino sources. In particular, we focus on 17 objects of yet unknown redshift, for which we present optical spectroscopy secured at the Gran Telescopio Canarias and the ESO Very Large Telescope. We find all sources but one (a quasar) to be BL Lac objects. For nine targets we are able to determine the redshift (0.09~$<$~z~$<$~1.6), while for the others we set a lower limit on it, based on either the…
Recent observations by H.E.S.S. and MAGIC strongly suggest that the Universe is more transparent to very-high-energy gamma rays than previously thought. We show that this fact can be reconciled with standard blazar emission models provided that photon oscillations into a very light Axion-Like Particle occur in extragalactic magnetic fields. A quantitative estimate of this effect indeed explains the observed data and in particular the spectrum of blazar 3C279.
The VERITAS array of Cherenkov telescopes was used to observe ~130 blazars from 2007 to 2012. Of these, 25 were detected as very-high-energy (VHE; E > 100 GeV) sources. We present here the results of the analysis of 65 VERITAS non-detected blazars, including upper limits on their VHE flux. Results from a stacked analysis of the entire data set and of smaller sub-sets (defined as a function of the redshift and the blazar class) are presented and discussed.
The VERITAS array of 12-m atmospheric-Cherenkov telescopes is used in an intensive observation program focused on the discovery of VHE (E > 100 GeV) gamma-ray-emitting blazars. Since VERITAS began full-scale operation in 2007, more than 1000 hours of observation, on ~90 targets, have been devoted to the VHE blazar discovery effort and to timely follow-up in multi-wavelength (MWL) observation campaigns. These data have resulted in the discovery of VHE emission from 10 blazars (6 HBLs and 4 IBLs). A summary of these VHE discoveries is presented.
VERITAS is an array of four 12-m imaging Cherenkov telescopes, sensitive to gamma rays in the energy range from 85 GeV to 30 TeV. VERITAS dedicates roughly 40% of its total observing time to blazars. We present recent highlights from the VERITAS blazar discovery and long-term monitoring programs. These observations have led to 25 blazar detections, including 10 VHE discoveries. The VERITAS cameras were recently upgraded, lowering the energy threshold and enhancing the sensitivity. This upgrade helped to establish four new blazar detections with data taken after the completion of the upgrade in…
Probing the high energy emission processes of blazars through their variability relies crucially on long-term monitoring. We present unprecedented light curves from unbiased observations of very high energy fluxes from the blazars Mrk 421 and Mrk 501 based on a joint analysis of data from the First G-APD Cherenkov Telescope (FACT) and the High Altitude Water Cherenkov (HAWC) Observatory. Thanks to an offset of 5.3 hours of the geographic locations, a complementary coverage of up to 12 hours of observation per day allows us to track variability on time scales of hours to days in more detail tha…
Since October 2011, the First G-APD Cherenkov Telescope (FACT) is operated successfully on the Canary Island of La Palma. Apart from the proof of principle for the use of G-APDs in Cherenkov telescopes, the major goal of the project is the dedicated long-term monitoring of a small sample of bright TeV blazars. The unique properties of G-APDs permit stable observations also during strong moon light. Thus a superior sampling density is provided on time scales at which the blazar variability amplitudes are expected to be largest, as exemplified by the spectacular variations of Mrk 501 observed in…
We present multi-epoch optical spectroscopy of seven southern Fermi-monitored blazars from 2008 - 2013 using the Small and Medium Aperture Research Telescope System (SMARTS), with supplemental spectroscopy and polarization data from the Steward Observatory. We find that the emission lines are much less variable than the continuum; 4 of 7 blazars had no detectable emission line variability over the 5 years. This is consistent with photoionization primarily by an accretion disk, allowing us to use the lines as a probe of disk activity. Comparing optical emission line flux with Fermi $γ$-ray flux…
The Cherenkov Telescope Array (CTA) will provide very-high-energy (VHE; >~30 GeV) gamma-ray spectra of unprecedented resolution over a large energy range. It is likely that, at least for bright sources, spectral features will be revealed that cannot be distinguished with the current generation of Cherenkov telescopes. We are investigating the capability of the CTA to detect spectral signatures in TeV bright BL Lac objects that could reveal the emission from a population of relativistic protons in those sources. Such a detection would be crucial for the long-standing question of the origin of e…
The VERITAS array of 12-m atmospheric-Cherenkov telescopes in southern Arizona began full-scale operations in 2007, and it is one of the world's most sensitive detectors of astrophysical VHE (E > 100 GeV) gamma rays. Forty-one blazars are known to emit VHE photons, and observations of blazars are one of the VERITAS Collaboration's Key Science Projects (KSPs). More than 400 hours per year are devoted to this program, and ~100 blazars have already been observed with the array, in most cases with the deepest-ever VHE exposure. These observations have resulted in 20 detections, including 10 new VH…
Early indications by H.E.S.S. and the subsequent detection of blazar 3C279 by MAGIC show that the Universe is more transparent to very-high-energy gamma rays than previously thought. We demonstrate that this circumstance can be reconciled with standard blazar emission models provided that photon oscillations into a very light Axion-Like Particle occur in extragalactic magnetic fields. A quantitative estimate of this effect indeed explains the observed spectrum of 3C279. Our prediction can be tested by the satellite-borne Fermi/LAT detector as well as by the ground-based Imaging Atmospheric Che…
Blazars and in particular the subclass of high synchrotron peaked Active Galactic Nuclei are among the main targets for the present generation of Imaging Atmospheric Cherenkov Telescopes (IACTs) and will remain of great importance for very high-energy $γ$-ray science in the era of the Cherenkov Telescope Array (CTA). Observations by IACTs, which have relatively small fields of view ($\sim$ few degrees), are limited by viewing conditions; therefore, it is important to select the most promising targets in order to increase the number of detections. The aim of this paper is to search for unclassi…
The emission of very-high-energy photons (VHE, E>100 GeV$) in blazars is closely connected to the production of ultra-relativistic particles and the role of these gamma-ray sources as cosmic particle accelerators. This work focuses on a selection of 22 gamma-ray objects from the 2BIGB catalog of high-synchrotron-peaked sources, which are classified as blazar candidates of uncertain type in the 4FGL-DR2 catalog. We study these sources by means of a re-analysis of the first 10 years of gamma-ray data taken with the Fermi Large Area Telescope, including the attenuation by the extragalactic backgr…
Decaying axion-like particles (ALP) with masses in the eV range which might occupy dark matter halos of the Milky Way and other galaxies produce a characteristic "bump" feature in the spectrum of extragalactic background light (EBL). This feature leaves an imprint on the gamma-ray spectra of distant extragalactic sources. We derive constraints on the ALP coupling to photons based on analysis of spectra of very-high-energy gamma-ray loud blazars. We combine gamma-ray spectral measurements by Fermi/LAT and Cherenkov telescopes and fit a model in which the intrinsic source spectrum is modified by…
The VERITAS array of 12-m atmospheric-Cherenkov telescopes in southern Arizona began full-scale operations in 2007, and is one of the world's most-sensitive detectors of astrophysical VHE (E>100 GeV) gamma rays. Approximately 50 blazars are known to emit VHE photons, and observations of blazars are a major focus of the VERITAS Collaboration. Nearly 2000 hours have been devoted to this program and ~130 blazars have already been observed with the array, in most cases with the deepest-ever VHE exposure. These observations have resulted in 21 detections, including 10 VHE discoveries. Recent highli…
We have recently proposed a simplified scenario for blazars in which these sources are classified as flat-spectrum radio quasars or BL Lacs according to the prescriptions of unified schemes, and to a varying combination of Doppler boosted radiation from the jet, emission from the accretion disk, the broad line region, and light from the host galaxy. This scenario has been thoroughly tested through detailed Monte Carlo simulations and reproduces all the main features of existing radio, X-ray, and gamma-ray surveys. In this paper we consider the case of very high energy emission (E > 100 GeV) ex…
We present the results of the optical spectroscopy follow-up of a sample of 28 unclassified blazars from the Third Fermi-LAT Catalog of High-Energy Sources (3FHL). All the spectra were taken with the 4m Mayall telescope at Kitt Peak. With this follow-up program we are able to classify 27 out of 28 objects as BL Lacs, while the remaining one is a flat spectrum radio quasar. We determine a redshift (z) for three of these objects and a lower limit on z for other four sources: the farthest object for which we obtain a redshift has z>0.836. These results are part of a more extended campaign of opti…
Blazars are high-energy engines providing us natural laboratories to study particle acceleration, relativistic plasma processes, magnetic field dynamics, black hole physics. Key informations are provided by observations at high-energy (in particular by Fermi/LAT) and very-high energy (by Cherenkov telescopes). I give a short account of the current status of the field, with particular emphasis on the theoretical challenges connected to the observed ultra-fast variability events and to the emission of flat spectrum radio quasars in the very high energy band.
Over seventy blazars identified as sources of TeV gamma ray emission have been detected, including approximately sixty BL Lac objects and seven flat spectrum radio quasars. The distribution in space of these objects can be described by a luminosity function, which gives their comoving space density as a function of their luminosity. We investigate a source selection method to be used for determining the luminosity function of TeV gamma-ray blazars using observations from VERITAS, a ground-based gamma-ray observatory consisting of an array of four atmospheric Cherenkov telescopes located in sou…
In this paper, we discuss an artificial intelligence based approach to constrain the redshift of blazars using combined $γ$--ray observations from the \emph{Fermi} Large Area Telescope (LAT) and ground based atmospheric Cherenkov telescopes (ACTs) in GeV and \emph{sub} TeV energy regimes respectively. The spectral measurements in GeV and TeV energy bands show a redshift dependent spectral break in the $γ$--ray spectra of blazars. We use this observational feature of blazars to constrain their redshift. The observed spectral information of blazars with known redshifts reported in the \emph{Ferm…
Very high energy (VHE $>$100 GeV) gamma rays coming from blazars can produce pairs when interacting with the Extragalactic Background Light (EBL) and the Cosmic Microwave Background, generating an electromagnetic cascade. Depending on the Intergalactic Magnetic Field (IGMF) intensity, this cascade may result in an extended isotropic emission of photons around the source (halo), or in a broadening of the emission beam. The detection of these effects might lead to important constrains both on the IGMF intensity and the EBL density, quantities of great relevance in cosmological models. Using a Mo…
Misaligned AGN (MAGNs), i.e., radio-loud AGNs with the jet not pointing directly towards us, represent a new class of GeV emitters revealed by the Fermi space telescope. Although they comprise only a small fraction of the high-energy sources, MAGNs are extremely interesting objects offering a different perspective to study high-energy processes with respect to blazars. The aim of this work is to evaluate the impact of the new-generation Cherenkov Telescope Array (CTA) on the MAGN class and propose possible observational strategies to optimize their detection.
The VERITAS array of 12-m atmospheric-Cherenkov telescopes in southern Arizona is currently the world's most-sensitive detector of astrophysical very high energy (VHE; E>100 GeV) gamma rays. Approximately 25 blazars are known to emit VHE photons, and observations of blazars are one of the VERITAS collaboration's key science projects (KSP). More than 50 of these objects have been observed with the array, in most cases with the deepest-ever VHE exposure. This contribution summarizes the VERITAS blazar KSP, with a focus on the upper limits measured in the blazar discovery program. VERITAS blazar …
Recent data from the \emph{Fermi} Large Area Telescope have revealed about a dozen distant hard-spectrum blazars that have very-high-energy (VHE; $\gtrsim 100$ GeV) photons associated with them, but most of them have not yet been detected by imaging atmospheric Cherenkov telescopes. Most of these high-energy gamma-ray spectra, like those of other extreme high-frequency peaked BL Lac objects, can be well explained either by gamma rays emitted at the source or by cascades induced by ultra-high-energy cosmic rays, as we show specifically for KUV 00311$-$1938. We consider the prospects for detecti…
We present results of observations in the UV to near-IR range for eight blazars, three of which have been recently discovered at Very High Energies (VHE) and five appearing as interesting candidates for VHE γ-ray detection. We focus in this paper on the search for their redshifts, which are unknown or considered as uncertain.
We report on HET and Palomar 5 m spectroscopy of recently identified $γ$-ray blazars in the {\it Fermi} LAT Bright Source List. These data provide identifications for 10 newly discovered $γ$-ray flat spectrum radio quasars (FSRQ) and six new BL Lacs plus improved spectroscopy for six additional BL Lacs. We substantially improve the identification completeness of the bright LAT blazars and give new redshifts and $z$ constraints, new estimates of the black hole masses and new measurements of the optical SED.
Extreme high-energy peaked BL Lac objects (EHBLs) are blazars whose synchrotron emission peaks at exceptionally high energies, above few keV, in the hard X-ray regime. So far, only a handful of those objects has been detected at very high energy (VHE, E > 100 GeV) gamma rays by Imaging Atmospheric Cherenkov Telescopes. Very remarkably, VHE observations of some of these blazars (like 1ES 0229+200) have provided evidence of a VHE gamma-ray emission extending to several TeV, which is difficult to explain with standard, one-zone synchrotron self-Compton models usually applied to BL Lac objects. Th…
The VERITAS array of four 12-m diameter imaging atmospheric-Cherenkov telescopes in southern Arizona is used to study very high energy (VHE; E>100 GeV) gamma-ray emission from astrophysical objects. VERITAS is currently the most sensitive VHE gamma-ray observatory in the world and one of the VERITAS collaboration's Key Science Projects (KSP) is the study of blazars. These active galactic nuclei (AGN) are the most numerous class of identified VHE sources, with ~30 known to emit VHE photons. More than 70 AGN, almost all of which are blazars, have been observed with the VERITAS array since 2007, …
The First G-APD Cherenkov Telescope (FACT) was built on the Canary Island of La Palma in October 2011 as a proof of principle for silicon based photosensors in Cherenkov Astronomy. The scientific goal of the project is to study the variability of active galatic nuclei (AGN) at TeV energies. Observing a small sample of TeV blazars whenever possible, an unbiased data sample is collected. This allows to study the variability of the selected objects on timescales from hours to years. Results from the first three years of monitoring will be presented. To provide quick flare alerts to the community …
Very high energy (VHE, E > 30 GeV) gamma-rays are absorbed via interaction with low-energy photons from the extragalactic background light (EBL) if the involved photon energies are above the threshold for electron-positron pair creation. The VHE gamma-ray absorption, which is energy dependent and increases strongly with redshift, distorts the VHE energy spectra observed from distant objects. The observed energy spectra of the AGNs carry therefore an imprint of the EBL. Recent detections of hard spectra of distant blazars (z = 0.11 - 0.54) by H.E.S.S. and MAGIC put strong constraints on the EBL…
We have searched for very high energy (VHE) gamma rays from four blazars using the CANGAROO-III imaging atmospheric Cherenkov telescope. We report the results of the observations of H 2356-309, PKS 2155-304, PKS 0537-441, and 3C 279, performed from 2005 to 2009, applying a new analysis to suppress the effects of the position dependence of Cherenkov images in the field of view. No significant VHE gamma ray emission was detected from any of the four blazars. The GeV gamma-ray spectra of these objects were obtained by analyzing Fermi/LAT archival data. Non-simultaneous wide range (radio to VHE ga…
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 from blazars over 5 decades in energy, from 100 MeV to 10 TeV. In this study, we perform a Fermi-LAT spectral analysis for TeV-detected blazars and combine it with archival TeV data. We examine the observational properties in the gamma-ray band of our sample of TeV-detected blazars and compare the results with X-ray and GeV-selected populations. The spectral energy distributions (SEDs) that result from combining Fermi-LAT and ground-…
The absorption of gamma-rays in the intergalactic medium due to the EBL (Extragalactic Background Light) causes the observed blazar spectrum to be fainter and softer than their intrinsic state. It could thus be expected to see an effective spectral-softening trend with redshift. No such trend is evident in the sample of VHE blazars currently observed. To check which distributions of the properties of the parent blazar population could reproduce the observations, various simulations are done. The resulting subsamples that satisfy a generic detection criterion for the current generation of ACTs …
The Cherenkov Telescope Array (CTA) is about to enter construction phase and one of its main key science projects is to perform an unbiased survey in search of extragalactic sources. We make use of both the latest blazar gamma--ray luminosity function and spectral energy distribution to derive the expected number of detectable sources for both the planned Northern and Southern arrays of the CTA observatory. We find that a shallow, wide survey of about 0.5 hour per field of view would lead to the highest number of blazar detections. Furthermore, we investigate the effect of axion-like particles…
We briefly review contemporary extragalactic γ-ray propagation models. It is shown that the Extragalactic Magnetic Field (EGMF) strength and structure are poorly known. Strict lower limits on the EGMF strength in voids are of order 10^{-17}--10^{-20} G, thus allowing a substantial contribution of a secondary component generated by electromagnetic cascades to the observable spectrum. We show that this "electromagnetic cascade model" is supported by data from imaging Cherenkov telescopes and the Fermi LAT detector.
VERITAS is an array of four imaging atmospheric Cherenkov telescopes that observes gamma-ray sources in the very high energy range (VHE, E>100 GeV). A large fraction of the known gamma-ray sources are of extragalactic origin and belong to the blazar class: active galactic nuclei with relativistic jets pointing along the line of sight. Gamma-ray emission from blazars is typically variable. Part of the VERITAS observation program is devoted to follow-up triggers on flaring blazars, initiated by observations at other wavelengths. We will present those target of opportunity observations based on o…
The VERITAS array of 12-m atmospheric-Cherenkov telescopes in southern Arizona is one of the world's most-sensitive detectors of very-high-energy (VHE, E>100 GeV) gamma rays. More than 50 extragalactic sources are known to emit VHE photons; these include blazars, radio galaxies, and starburst galaxies. Blazar observations are one of the VERITAS Collaboration's Key Science Projects. More than 400 hours per year are devoted to this program and ~100 blazars have already been observed with the array, in most cases with the deepest ever VHE exposure. These observations have resulted in 21 detection…
Blazars are the dominant type of extragalactic sources at microwave and at $γ$-ray energies. In the most energetic part of the electromagnetic spectrum (E>100GeV) a large fraction of high Galactic latitude sources are blazars of the High Synchrotron Peaked (HSP) type, that is BL Lac objects with synchrotron power peaking in the UV or in the X-ray band. HSP blazars are remarkably rare, with only a few hundreds of them expected to be above the sensitivity limits of currently available surveys. To find these very uncommon objects, we have devised a method that combines ALLWISE survey data with mu…
We present the results of an observational photo-polarimetry campaign of the blazar PG 1553+113 at optical wavelengths. The blazar was recently detected at very high energies (> 100 GeV) by the H.E.S.S and MAGIC gamma-ray Cherenkov telescopes. Our high-temporal resolution data show significant variations in the linear polarization percentage and position angle at inter-night time-scales, while at shorter (intra-night) time-scales both parameters varied less significantly, if at all. Changes in the polarization angle seem to be common in gamma-ray emitting blazars. Simultaneous differential pho…
I will review recent advances in the field of blazars, highlighting the contribution of Swift. Together with other operating satellites (most notably Fermi, but also AGILE, WISE, Planck) and ground based facilities such as Cherenkov telescopes, Swift was (and is) crucial for improving our understanding of blazars. The main advances in the blazar field made possible by Swift includes the opening of the time domain investigation, since there are several sources with hundreds of simultaneous optical, UV and X-ray data taken at different times; the possibility to measure the black hole mass in ver…
