We investigate the prospects for discovering blazars at very high-redshifts (z>3-6) with the Fermi Gamma-Ray Space Telescope (Fermi), employing a model for the evolving gamma-ray luminosity function (GLF) of the blazar population. Our previous GLF model is used as a basis, which features luminosity-dependent density evolution implied from X-ray data on active galactic nuclei, as well as the blazar sequence paradigm for their spectral energy distribution, and which is consistent with EGRET and current Fermi observations of blazars.Here we augment the high-redshift evolution of this model by utilizing the luminosity function of quasars from the Sloan Digital Sky Survey (SDSS), which is well-constrained up to z~5. We find that Fermi may discover a few blazars up to z~6 in the entire sky during its 5-year survey. We further discuss how such high-redshift blazar candidates may be efficiently selected in future Fermi data.
Blazars are expected to produce both gamma rays and cosmic rays. Therefore, observed high-energy gamma rays from distant blazars may contain a significant contribution from secondary gamma rays produced along the line of sight by the interactions of cosmic-ray protons with background photons. Unlike the standard models of blazars that consider only the primary photons emitted at the source, models which include the cosmic-ray contribution predict that even ~10 TeV photons should be detectable from distant objects with redshifts as high as z> 0.1. Secondary photons contribute to signals of poin…
Based on its broad-band spectral energy distribution, and the X-ray spectrum in particular, the radio-loud active galactic nucleus (AGN) SDSS J102623.61+254259.5 (J1026+2542) has recently been classified as a blazar. The extremely high redshift of the source, z=5.3, makes it one of the most distant and most luminous radio-loud AGN known to date. From published 5-GHz very long baseline interferometry (VLBI) imaging data obtained in 2006, the source has a typical blazar appearance on mas scales, with a prominent one-sided jet extending to ~20 mas. We estimate the brightness temperature of J1026+…
The radio-loud quasar SDSS J102623.61+254259.5, at a redshift z=5.3, is one of the most distant radio-loud objects. Since its radio flux exceeds 100 mJy at a few GHz, it is also one of the most powerful radio-loud sources. We propose that this source is a blazar, i.e. we are seeing its jet at a small viewing angle. This claim is based on the spectral energy distribution of this source, and especially on its strong and hard X-ray spectrum, as seen by Swift, very typical of powerful blazars. Observations by the Gamma-Ray Burst Optical/Near-Infrared Detector (GROND) and by theWide-field Infrared …
We report the detection of a transient $γ$-ray source in the direction of B3 1428+422 ($z=4.72$) by analyzing the 110-month {\it Fermi}-LAT Pass 8 data. The new transient $γ-$ray source is far away from the Galactic plane and has a rather soft spectrum, in agreement with being a high redshift blazar. We suggest that the newly discovered transient is the $γ$-ray counterpart of B3 1428+422, which could be the {\it most distant} GeV source detected so far. The detection of a group of such distant $γ-$ray blazars will be helpful to reconstruct the evolution of the luminosity function and to study …
Context. Very high-energy (VHE) $γ$-ray measurements of distant TeV blazars can be nicely explained by TeV spectra induced by ultra high-energy cosmic rays. Aims. We develop a model for a plausible origin of hard spectra in distant TeV blazars. Methods. In the model, the TeV emission in distant TeV blazars is dominated by two mixed components. The first is the internal component with the photon energy around 1 TeV produced by inverse Compton scattering of the relativistic electrons on the synchrotron photons (SSC) with a correction for extragalactic background light absorbtion and the other is…
Blazar variability appears to be stochastic in nature. However, a possibility of low-dimensional chaos was considered in the past, but with no unambiguous detection so far. If present, it would constrain the emission mechanism by suggesting an underlying dynamical system. We rigorously searched for signatures of chaos in Fermi-Large Area Telescope light curves of 11 blazars. The data were comprehensively investigated using the methods of nonlinear time series analysis: phase-space reconstruction, fractal dimension, maximal Lyapunov exponent (mLE). We tested several possible parameters affectin…
Secondary photons and neutrinos produced in the interactions of cosmic ray protons emitted by distant Active Galactic Nuclei (AGN) with the photon background along the line of sight can reveal a wealth of new information about the intergalactic magnetic fields (IGMF), extragalactic background light (EBL), and the acceleration mechanisms of cosmic rays. The secondary photons may have already been observed by gamma-ray telescopes. We show that the secondary neutrinos improve the prospects of discovering distant blazars by IceCube, and we discuss the ramifications for the cosmic backgrounds, magn…
We calculate the gamma-ray spectrum of a distant blazar, PKS 1424+240, located at redshift z > 0.6035, where the optical depth to primary gamma rays of the highest observed energies is τ> 5. The spectral shape agrees well with what is expected from secondary gamma rays produced in line-of-sight interactions of cosmic rays with background photons. In particular, we find an acceptable agreement with the source redshifts in the range 0.6 < z < 1.3. We discuss the implications of existing and future data for models of extragalactic background light (EBL) and for the redshift of the source. In the …
The VERITAS collaboration has approved long-term observations on several distant, hard-spectrum blazars. We present first results from VERITAS long-term observations of 1ES1218+304, 1ES0229+200, and 1ES0414+009. Gamma-ray observations of distant, hard-spectrum blazars has emerged as an effective, indirect probe of the universe's diffuse extragalactic background light (EBL), due to the extinction of gamma-rays via the pair production interaction $γ_{TeV}γ_{EBL}\rightarrow e^{+}e^{-}$, which is expected to produce absorption features in distant gamma-ray sources. The sources presented here emit …
Radio-loud sources with blazar-like properties, but having a jet that does not directly point in the direction of the observer are among the most interesting classes of gamma-ray emitters. These sources are known as Misaligned Active Galactic Nuclei (MAGN). Understanding MAGN properties is useful to improve the knowledge of blazar energetics. We searched for new MAGN candidates among the remaining blazars of uncertain type detected by the Fermi Large Area Telescope (LAT) using a methodology based on characterizing their radio morphology. We identified seven new candidates associated with gamma…
We report on OVRO, WISE, Swift and Fermi-LAT observations of the high redshift blazar CGRaBS J0733+0456, from which significant flux variations in radio, infrared (IR) as well as $γ$-ray domains are detected. Particularly, the amplitude of long-term IR variation is over one order of magnitude and the IR variability timescale can be constrained as short as a few hours in the source frame. The IR and $γ$-ray light curves are found to be rather similar, and the strong quasi-simultaneous infrared and $γ$-ray flares are proved to be unique among the nearby $γ$-ray sources. This is the first time to…
The radio-loud quasar SDSS J114657.79+403708.6 at a redshift z=5.0 is one of the most distant radio-loud objects. The IR-optical luminosity and spectrum suggest that its black hole has a very large mass: M=(5+-1)x 1e9 Msun. The radio-loudness (ratio of the radio to optical flux) of the source is large (around 100), suggesting that the source is viewed at small angles from the jet axis, and could be a blazar. The X-ray observations fully confirm this hypothesis, due to the high level and hardness of the flux. This makes SDSS J114657.79+403708.6 the third most distant blazar known, after Q0906+6…
QSO B0218+357 is a blazar located at a cosmological redshift of z=0.944. It is gravitationally lensed by a spiral galaxy at a redshift of z=0.68. The blazar and its lens are well studied in the radio through X-ray bands, and several blazar outbursts were detected by Fermi-LAT at energies above 100 MeV. Strong gravitational lensing was invoked to explain the two components appar- ent in the radio and GeV light curves, separated by 10-12 days. In July 2014 another outburst was observed by Fermi-LAT, triggering follow-up observations with the MAGIC telescopes at energies above 100 GeV. The observ…
The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope discovered a rapid (about 5 days duration), high-energy (E >100 MeV) gamma-ray outburst from a source identified with the blazar PKS 1502+106 (OR 103, S3 1502+10, z=1.839) starting on August 05, 2008 and followed by bright and variable flux over the next few months. Results on the gamma-ray localization and identification, as well as spectral and temporal behavior during the first months of the Fermi all-sky survey are reported here in conjunction with a multi-waveband characterization as a result of one of the first F…
Line-of-sight interactions of cosmic rays provide a natural explanation of the hard gamma-ray spectra of distant blazars, which are believed to be capable of producing both gamma rays and cosmic rays. For sources with redshifts z> 0.1, secondary gamma rays produced in cosmic-ray interactions with background photons close to an observer can dominate over primary gamma rays originating at the source. The transition from one component to another is accompanied by a change in the spectral index depending on the source redshift. We present theoretical predictions and show that they agree with the d…
Our goal is to limit the redshifts of three blazars PG 1553+113, 3C 66A and PKS 1424+240, through the investigation of their Fermi and VHE (very high energy) $γ$-ray observations. We assume that the intrinsic spectra of PG 1553+113, 3C 66A, and PKS 1424+240 have not any cutoff across the Fermi and VHE $γ$-ray energy ranges. The intrinsic spectra of VHE $γ$-rays are obtained through the extrapolation of Fermi spectra. Comparing the measured and intrinsic VHE spectra due to extragalactic background light (EBL) absorption, we give the redshift upper limits of three blazars assuming a specific EBL…
We searched for gamma-ray blazar candidates among the 382 unidentified hard X-ray sources of the 3rd Palermo BAT Catalog (3PBC) obtained from the analysis of 66 months of SWIFT-BAT survey data and listing 1586 sources. We adopted a recently developed association method based on the peculiar infrared colors which characterize the gamma-ray blazars included in the second catalog of active galactic nuclei detected by the Fermi Large Area Telescope (2LAC). We used this method exploiting the data of the all-sky survey performed by the Wide-Field Infrared Survey Explorer (WISE) to establish correspo…
We propose a scenario for self-consistent interpretation of GeV - TeV spectrum of a distant blazar PKS 1424+240. In this scenario, ultra-high energy (UHE) protons are assumed to exist in the blazar jet and produce gamma rays through synchrotron emission emitted by relativistic protons and pair cascades (resulted from $pγ$ interaction); meanwhile, some of these UHE protons may escape from the jet and are injected into intergalactic space. Therefore, we assume that UHE cosmic rays (CR) originate from relativistic protons in the jet and use energy-independent escape timescale to obtain UHECR inje…
The observation in the GeV band of distant blazars has been recently used to put constraints on the Extragalactic Background Light (EBL) and Extragalactic Magnetic Fields (EGMF). To support such claims one has to assume that the leptonic component of the electromagnetic cascade initiated by blazar gamma-rays is deflected away by strong enough EGMF, suppressing the signal in the Fermi window. Apart from magnetic fields, the development of such a cascade might be affected by plasma instabilities due to interactions with the ionized component of the Intergalactic Medium (IGM). In this paper we mo…
We investigate the formation of TeV spectrum of a distant blazar PKS 1424+240 residing at a redshift $z\geq0.6$ in two scenarios in the frame of a lepto-hadronic jet model taking both the uncertainties of the extragalactic background light (EBL) and its redshift into account. In the first scenario, TeV emission is attributed to the synchrotron emission of pair cascades resulting from $pγ$ interaction; in the second scenario, TeV emission is attributed to the proton-synchrotron emission, and an internal absorption due to interaction with the photons around the jet is included. The results show …
Active galactic nuclei (AGN) can produce both gamma rays and cosmic rays. The observed high-energy gamma-ray signals from distant blazars may be dominated by secondary gamma rays produced along the line of sight by the interactions of cosmic-ray protons with background photons. This explains the surprisingly low attenuation observed for distant blazars, because the production of secondary gamma rays occurs, on average, much closer to Earth than the distance to the source. Thus the observed spectrum in the TeV range does not depend on the intrinsic gamma-ray spectrum, while it depends on the ou…
The identification of low-energy counterparts for gamma-ray sources is one of the biggest challenge in modern gamma-ray astronomy. Recently, we developed and successfully applied a new association method to recognize gamma-ray blazar candidates that could be possible counterparts for the unidentified gamma-ray sources above 100 MeV in the second Fermi Large Area Telescope (LAT) catalog (2FGL). This method is based on the Infrared (IR) colors of the recent Wide-Field Infrared Survey Explorer (WISE) all-sky survey. In this letter we applied our new association method to the case of unidentified …
Variability Study and Searching for QPOs with day-like periods in the blazar S5 0716+714 with TESS
Aryabhatta Research Institute of Observational Sciences (ARIES)（阿亚巴塔观测科学研究所）；Indian Institute of Astrophysics (IIA)（印度天体物理研究所）；The College of New Jersey（新泽西学院）
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 develop a model for a possible origin of hard very high energy spectra from a distant blazar. In the model, both the primary photons produced in the source and secondary photons produced outside the source contribute to the observed high energy $γ$-rays emission. That is, the primary photons are produced in the source through the synchrotron self-Compton (SSC) process, and the secondary photons are produced outside the source through high energy protons interaction with the background photons along the line of sight. We apply the model to a characteristic case was the very high energy (VHE)…
We present the redshift lower limit of z>0.6035 for the very-high-energy (VHE; E>100 GeV) emitting blazar PKS 1424+240 (PG 1424+240). This limit is inferred from Lyman beta and gamma absorption observed in the far-ultraviolet spectra from the Hubble Space Telescope/Cosmic Origins Spectrograph. No VHE-detected blazar has shown solid spectroscopic evidence of being more distant. At this distance, VHE observations by VERITAS are shown to sample historically large gamma-ray opacity values at 500 GeV, extending beyond tau=4 for low-level models of the extragalactic background light (EBL) and beyond…
In 2017, IceCube detected a high energy neutrino in coincidence with the blazar TXS 0506+056. In a follow up analysis of archival data, evidence for previous neutrino emission from the blazar during 2014-15 was found. In this analysis, we investigate whether other blazars might have had episodes of neutrino emission. We use a 5-year sample of muon-neutrino data and the same timedependent algorithm that was used for the archival analysis of TXS 0506+056, applying it to all of the objects in the northern sky listed in the Fermi 3LAC catalog. We fit for the most significant neutrino flare from ea…
The expected opacity of the intergalactic space limits the mean free path of TeV gamma rays to dozens of Megaparsecs. However, TeV photons from numerous more distant sources have been detected. This might be interpreted, in each particular case, in terms of hardening of the emitted spectrum caused by presently unknown mechanisms at work in the sources. Here we show that this interpretation is not supported by the analysis of the ensemble of all observed sources. In the frameworks of an infrared-background model with the lowest opacity, we reconstruct the emitted spectra of distant blazars and …
We report results of a multiband monitoring campaign of the flat spectrum radio quasar TXS 0536+145 at redshift 2.69. This source was detected during a very high gamma-ray activity state in 2012 March by the Large Area Telescope on board Fermi, becoming the gamma-ray flaring blazar at the highest redshift detected so far. At the peak of the flare the source reached an apparent isotropic gamma-ray luminosity of 6.6 x 10^49 erg/s which is comparable to the values achieved by the most luminous blazars. This activity triggered radio-to-X-rays monitoring observations by Swift, Very Long Baseline Ar…
GB 1508+5714 is a high-redshift blazar ($z$ = 4.3) and a spectrally soft $γ$-ray source has been detected in its direction. By analyzing 11.4-yr {\it Fermi}-LAT data, significant long-term variability of the $γ$-ray source is confirmed. More importantly, a $γ$-ray emission flare appeared in an epoch of several tens of days in year 2018, when the flux is about four times of the value from the global fit. Meanwhile, optical flares displayed in both $r$- and $i$-bands from the {\it Zwicky} Transient Facility light curves. Detections of the simultaneous $γ$-ray and optical brightening provide a de…
The extragalactic background light (EBL) from the infrared to the ultraviolet is difficult to measure directly, but can be constrained with a variety of methods. EBL photons absorb gamma-rays from distant blazars, allowing one to use blazar spectra from atmospheric Cherenkov telescopes (ACTs) to put upper limits on the EBL by assuming a blazar source spectrum. Here we apply a simple technique, similar to the one developed by Schroedter (2005), to the most recent very-high energy (VHE) gamma-ray observations of blazars to put upper limits on the EBL energy density. This technique is independent…
We explore the axion-like particle (ALP)-photon oscillation effect in the $γ$-ray spectra of the blazars Markarian 421 (Mrk 421) and PG 1553+113, which are measured by the Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) and Fermi Large Area Telescope (Fermi-LAT) with high precision. We investigate the constraints on the ALP parameter space using the Mrk 421 and PG 1553+113 observations of 15 and 5 phases, respectively. We find that the combined analysis with all the 15 phases improves the limits from the Mrk 421 observations. The combined limit set by the Mrk 421 observations has …
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…
The very hard $γ$-ray spectrum from distant blazars challenges the traditional synchrotron self-Compton (SSC) model, which may indicate that there is the contribution of an additional high-energy component beyond the SSC emission. In this paper, we study the possible origin of the hard $γ$-ray spectrum from distant blazars. We develop a model to explain the hard $γ$-ray spectrum from blazar 1ES 1101-232. In the model, the optical and X-ray radiation would come from the synchrotron radiation of primary electrons and secondary pairs, the GeV emission would be produced by the SSC process, however…
A direct search of $γ$-ray emission centered on multi-frequency selected candidates is a valuable complementary approach to the standard search adopted in current $γ$-ray Fermi-LAT catalogs. Our candidates are part of the 2WHSP sample, that was assembled with the aim of providing targets for Imaging Atmospheric Cherenkov Telescopes (IACT), and is currently the largest set of high synchrotron peaked (HSP) blazars. We perform a likelihood analysis with the Fermi Science Tools using positions from 400 2WHSP blazars as seeds of tentative $γ$-ray sources. This enabled us to detect 150 $γ$-ray exces…
The recent detection of blazar 3C279 by MAGIC has confirmed previous indications by H.E.S.S. 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 photon oscillations into a veri light Axion-Like Particle occur in extragalactic magnetic fields. A quantitative estimate of this effect explains the observed spectrum of 3C279. Our prediction can be tested in the near future by the satellite-borne GLAST detector as well as by the ground-based Imaging Atmospheric Cherenkov T…
In 2008 AGILE and Fermi detected gamma-ray flaring activity from the unidentified EGRET source 3EG J1236+0457, recently associated with a flat spectrum radio quasar GB6 J1239+0443 at z=1.762. The optical counterpart of the gamma-ray source underwent a flux enhancement of a factor 15-30 in 6 years, and of ~10 in six months. We interpret this flare-up in terms of a transition from an accretion-disk dominated emission to a synchrotron-jet dominated one. We analysed a Sloan Digital Sky Survey (SDSS) archival optical spectrum taken during a period of low radio and optical activity of the source. We…
The object 4C 71.07 is a high-redshift blazar whose spectral energy distribution shows a prominent big blue bump and a strong Compton dominance. We present the results of a two-year multiwavelength campaign led by the Whole Earth Blazar Telescope (WEBT) to study both the quasar core and the beamed jet of this source. The WEBT data are complemented by ultraviolet and X-ray data from Swift, and by gamma-ray data by Fermi. The big blue bump is modelled by using optical and near-infrared mean spectra obtained during the campaign, together with optical and ultraviolet quasar templates. We give pres…
The IceCube neutrino observatory, a cubic-kilometer particle detector at the South Pole, first announced the discovery of an astrophysical flux of high-energy neutrinos in the TeV-PeV range in 2013, followed in 2017 by the detection of a high-energy neutrino event in temporal and directional correlation with the flaring gamma-ray blazar TXS 0506+056. This observation, combined with archival neutrino detections in 2014-2015, has provided compelling evidence for the detection of the first high-energy astrophysical neutrino source. A promising way of detecting additional sources is to correlate n…
The detection of high-redshift ($z>$3) blazars enables the study of the evolution of the most luminous relativistic jets over cosmic time. More importantly, high-redshift blazars tend to host massive black holes and can be used to constrain the space density of heavy black holes in the early Universe. Here, we report the first detection with the \fermi-Large Area Telescope of five \gm-ray emitting blazars beyond $z=3.1$, more distant than any blazars previously detected in $γ$-rays. Among these five objects, NVSS J151002+570243 is now the most distant known \gm-ray emitting blazar at $z=4.31$.…
We re-examine possible dependencies on redshift of the spectral parameters of blazars observed at very-high energies (VHEs) with Imaging Atmospheric Cherenkov telescopes (IACTs). This is relevant to assess potential effects with the source distance of the photon to axion-like particle (ALP) mixing, that would deeply affect the propagation of VHE photons across the Universe. We focus our spectral analysis on 38 BL Lac objects (32 high-peaked and 6 intermediate-peaked) up to redshift $z\simeq 0.5$, and a small sample of 5 Flat Spectrum Radio Quasars up to $z=1$ treated independently to increase …
Axion-like particles (ALPs) are a broad class of pseudo-scalar bosons that generically arise from broken symmetries in extensions of the standard model. In many scenarios, ALPs can mix with photons in regions with high magnetic fields. Photons from distant sources can mix with ALPs, which then travel unattenuated through the Universe, before they mix back to photons in the Milky Way galactic magnetic field. Thus, photons can traverse regions where their signals would normally be blocked or attenuated. In this paper, we study TeV $γ$-ray observations from distant blazars, utilizing the signific…
Very high energy (VHE) $γ$-rays above 10$'$s of GeV energy, emitted from distant blazars, are attenuated by photons from the extragalactic background light (EBL). Unfortunately, neither the EBL nor the intrinsic blazar spectrum is accurately known to derive one quantity from the other. In this work we use a homogeneous one zone model involving synchrotron, synchrotron self Compton (SSC) and external Compton (EC) emission mechanisms to estimate the intrinsic VHE spectra of blazars. The model is applied on three VHE blazars, namely PKS2155-304, RGB J0710+591 and 3C279, for which simultaneous mul…
At TeV energies and above gamma rays can induce electromagnetic cascades, whose charged component is sensitive to intervening intergalactic magnetic fields (IGMFs). When interpreting gamma-ray measurements in the energy range between a few GeV and hundreds of TeV, one has to carefully account for effects due to IGMFs, which depend on their strength and power spectrum. Therefore, gamma-ray-induced electromagnetic cascades can be used as probes of cosmic magnetism, since their arrival distribution as well as spectral and temporal properties can provide unique information about IGMFs, whose origi…
We present an analysis of the optical spectra available in the Sloan Digital Sky survey data release nine (SDSS DR9) for the blazars listed in the ROMA-BZCAT and for the gamma-ray blazar candidates selected according to their IR colors. First, we adopt a statistical approach based on MonteCarlo simulations to find the optical counterparts of the blazarslisted in the ROMA-BZCAT catalog. Then we crossmatched the SDSS spectroscopic catalog with our selected samples of blazars and gamma-ray blazar candidates searching for those with optical spectra available to classify our blazar-like sources and…
1ES 1218+304 is among a group of several TeV blazars that exhibit surprisingly hard energy spectra given their redshifts. The VERITAS collaboration has carried out an intensive observing campaign of 1ES 1218+304 in early 2009 to characterize the emission properties of one of these hard-spectra distant blazars in more detail. While hard-spectra TeV blazars are most promising for unveiling putative signatures from EBL absorption, they may also have implications on shock acceleration scenarios in relativistic jets. Our observations of 1ES 1218+304 demonstrate significant flux variability. Further…
Leading models of galaxy formation require large-scale energetic outflows to regulate the growth of distant galaxies and their central black holes. However, current observational support for this hypothesis at high redshift is mostly limited to rare z>2 radio galaxies. Here we present Gemini-North NIFS Intregral Field Unit (IFU) observations of the [OIII] emission from a z~2 ultraluminous infrared galaxy (L_IR>10^12 solar luminosities) with an optically identified Active Galactic Nucleus (AGN). The spatial extent (~4-8 kpc) of the high velocity and broad [OIII] emission are consistent with tha…
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…
The recent discovery of a diffuse neutrino flux up to PeV energies raises the question of which populations of astrophysical sources contribute to this diffuse signal. One extragalactic candidate source population to produce high-energy neutrinos are Blazars. We present results from a likelihood analysis searching for cumulative neutrino emission from Blazar populations selected with the 2nd Fermi-LAT AGN catalog (2LAC) using an IceCube data set that has been optimized for the detection of individual sources. In contrast to previous searches with IceCube, the investigated populations contain u…
The unexpectedly hard very-high-energy (VHE; $E > 100$ GeV) $γ$-ray spectra of a few distant blazars have been interpreted as evidence for a reduction of the $γγ$ opacity of the Universe due to the interaction of VHE $γ$-rays with the extragalactic background light (EBL) compared to the expectation from our current knowledge of the density and cosmological evolution of the EBL. One of the suggested solutions to this problem consisted of the inhomogeneity of the EBL. In this paper, we study the effects of such inhomogeneities on the energy density of the EBL (which then also becomes anisotropic…
The high energy peaked BL Lac (HBL) object 1ES 0414+009 (z=0.287) is a distant very high-energy (VHE, E > 100 GeV) blazars with well-determined redshift. This source was detected with the High Energy Stereoscopic System (H.E.S.S.) between October 2005 and September 2009. It was also detected with the Fermi Large Area Telescope (LAT) in 21 months of data. The combined high energy (HE) and VHE spectra, once corrected for gamma-gamma absorption on the extragalactic background light (EBL), indicate a Compton peak located above few TeV, among the highest in the BL Lac class.
VERITAS, an array of imaging atmospheric-Cherenkov telescopes, studies blazars in the energy range between ~100 GeV and ~30 TeV. With its excellent sensitivity at these energies, and ever-deepening source exposures, VERITAS is in a position to probe distant blazars for clear absorption signatures in their very-high-energy gamma-ray spectra due to interactions with the extragalactic background light (EBL). We discuss results from recent VERITAS observations of PG 1553+113 (z > 0.4) which have resulted in the most significant very-high-energy detection ever obtained for this source. The most rec…
