We examine a model for the observed temporal variability of powerful blazars in the $γ$-ray band in which the dynamics is described in terms of a stochastic differential equation, including the contribution of a deterministic drift and a stochastic term. The form of the equation is motivated by the current astrophysical framework, accepting that jets are powered through the extraction of the rotational energy of the central supermassive black hole mediated by magnetic fields supported by a so-called \emph{magnetically arrested} accretion disk. We apply the model to the $γ$-ray light curves of several bright blazars and we infer the parameters suitable to describe them. In particular, we examine the differential distribution of fluxes ($dN/dF_γ$) and we show that the predicted probability density function for the assumed stochastic equation naturally reproduces the observed power law shape at large fluxes $dN/dF_γ \propto F_γ^{-α}$ with $α>2$.
The spectral energy distribution and variability of several blazars (Mkn 501, 1 ES 2344+51.4, PKS 2155-30.4, 1 ES 1218+30.4, 3C 454.3) along the blazar sequence is investigated using a selfconsistent and timedependent lepto-hadronic hybrid emission model. The BL Lac objects in the list are successfully modelled with purely leptonic radiation processes (essentially Synchrotron Self-Compton emission), while the Flat Spectrum Radio Quasar requires highly relativistic hadrons to be present within the jet. Variability is exploited as well with our model to distinguish between Self-Compton and hadro…
Blazars constitute the vast majority of extragalactic $γ$-ray sources. They can also contribute a sizable fraction of the diffuse astrophysical neutrinos detected by IceCube. In the past few years, the real-time alert system of IceCube has led to multiwavelength follow-up of very high-energy neutrino events of plausible astrophysical origin. Spatial and temporal coincidences of these neutrino events with the high-activity state of $γ$-ray blazars can provide a unique opportunity to decipher cosmic-ray interactions in the relativistic jets. Assuming that blazars accelerate cosmic rays up to ult…
We compile 23 Gamma-ray Bursts (GRBs) and 21 blazars with estimated Doppler factors, and the Doppler factors of GRBs are estimated from their Lorentz factors by assuming their jet viewing angles θ->0 degree. Using the conventional assumption that the prompt emission of GRBs is dominated by the synchrotron radiation, we calculate the synchrotron luminosity of GRBs from their total isotropic energy and burst duration. Intriguingly, we discover a uniform correlation between the synchrotron luminosity and Doppler factor, L_syn \propto D^3.1, for GRBs and blazars, which suggests that they may share…
Blazars are often characterized by a spectral break at soft X-rays, whose origin is still debated. While most sources show a flattening, some exhibit a blackbody-like soft excess with temperatures of the order of $\sim$0.1 keV, similar to low-luminosity, non-jetted Seyferts. Here we present the analysis of the simultaneous XMM-Newton and NuSTAR observation of the luminous FSRQ 4C+25.05 ($z=2.368$). The observed 0.3-30 keV spectrum is best described by the sum of a hard X-ray power law ($Γ= 1.38_{-0.03}^{+0.05}$) and a soft component, approximated by a blackbody with $kT_{\rm BB} = 0.66_{-0.04}…
Our present knowledge of the properties of blazars mostly comes from small samples of bright objects, especially regarding studies on their cosmological evolution. Statistically well defined and completely identified samples of faint blazars are very difficult to obtain. We present a new X-ray selected sample of 62 blazars and blazar candidates reaching deep X-ray fluxes. We relied on the availability of large catalogs of astronomical objects combined with on-line services offering simple access to finding charts and magnitude estimates. We built the sample cross-matching X-ray sources in the …
Estimates of magnetic field strength in relativistic jets of active galactic nuclei (AGN), obtained by measuring the frequency-dependent radio core location, imply that the total magnetic fluxes in those jets are consistent with the predictions of the magnetically-arrested disk (MAD) scenario of jet formation. On the other hand, the magnetic field strength determines the luminosity of the synchrotron radiation, which forms the low-energy bump of the observed blazar spectral energy distribution (SED). The SEDs of the most powerful blazars are strongly dominated by the high-energy bump, which is…
Violent multi-wavelength variabilities are observed in gamma-ray-selected blazars. We present an analysis of long-term light curves for eight bright blazars to explore the co-variation pattern in the gamma-ray and radio bands. We extract their gamma-ray light curves and spectra with data observed by the Fermi/LAT since 2008. We find diverse co-variation patterns between the gamma-ray and radio (at 43 GHz) fluxes in these sources. The gamma-ray and radio fluxes of 3C 454.3 and PKS 1633+382 are correlated without any time-lag, suggesting that they are from the same radiation region. Similar corr…
Blazars have been pointed out as promising high-energy (HE) neutrinos sources, although the mechanism is still under debate. The blazars with a hard-TeV spectrum, which leptonic models can hardly explain, can be successfully interpreted in the hadronic scenarios. Recently, Aguilar et al. proposed a lepto-hadronic two-zone model to explain the multi-wavelength observations of the six best-known extreme BL Lacs and showed that the hadronic component could mainly interpret very-high-energy (VHE) emission. In this work, we apply this hadronic model to describe the VHE gamma-ray fluxes of 14 extrem…
The origin of high-energy emission in blazars jets (i.e., leptonic versus hadronic) has been a long-standing matter of debate. Here, we focus on one variant of hadronic models where proton synchrotron radiation accounts for the observed steady $γ$-ray blazar emission. Using analytical methods, we derive the minimum jet power ($P_{j,\min}$) for the largest blazar sample analyzed to date (145 sources), taking into account uncertainties of observables and jet's physical parameters. We compare $P_{j,\min}$ against three characteristic energy estimators for accreting systems, i.e., the Eddington lu…
We consider the largest observed sample including all intermediate-frequency peaked (IBL) and high-frequency peaked (HBL) flaring blazars above 100 GeV up to redshift $z = 0.6$. We show that the best-fit regression line of the emitted spectral indices $Γ_{\rm em} (z)$ is a concave parabola decreasing as $z$ increases, thereby implying a statistical correlation between the $\{Γ_{\rm em} (z) \}$ distribution and $z$. This result contradicts our expectation that such a distribution should be $z$-independent. We argue that the above correlation does not arise from any selection bias. We show that …
Whether a correlation exists between the radio and gamma-ray flux densities of blazars is a long-standing question, and one that is difficult to answer confidently because of various observational biases which may either dilute or apparently enhance any intrinsic correlation between radio and gamma-ray luminosities. We introduce a novel method of data randomization to evaluate quantitatively the effect of these biases and to assess the intrinsic significance of an apparent correlation between radio and gamma-ray flux densities of blazars. The novelty of the method lies in a combination of data…
We compiled the radio, optical, and X-ray data of blazars from the Sloan Digital Sky Survey (SDSS) database, and presented the distribution of luminosities and broad band spectral indices. The distribution of luminosities shows that the averaged luminosity of flat-spectral radio quasars (FSRQs) is larger than that of BL Lacs objects. On the other hand, the broad band spectral energy distribution reveals that FSRQs and low energy peaked BL Lac objects (LBLs) objects have similar spectral properties, but high energy peaked BL Lac objects (HBLs) have a distinct spectral property. This may be due …
IceCube has detected a cumulative flux of PeV neutrinos, which origin is unknown. Blazars, active galactic nuclei with relativistic jets pointing to us, are long and widely expected to be one of the strong candidates of high energy neutrino sources. The neutrino production depends strongly on the cosmic ray power of blazar jets, which is largely unknown. The recent null results in stacking searches of neutrinos for several blazar samples by IceCube put upper limits on the neutrino fluxes from these blazars. Here we compute the cosmic ray power and PeV neutrino flux of Fermi-LAT blazars, and fi…
The variability of the high-frequency peaked BL Lac object PKS 2155-304 is studied using almost 10 years of optical, X-ray and gamma-rays data. Publicly available data have been gathered and analyzed with the aim to characterize the variability and to search for log-normality or periodic behavior. The optical and X-ray range follow a log-normal process; a hint for a periodicity of about 700 days is found in optical and in the high energy (100 MeV <E<300 GeV) range. A one zone, time-dependent, synchrotron self-Compton model is successfully used to reproduce the evolution with energy of the vari…
In the early days of April 2010, the blazar 3C 454.3 (z=0.859) underwent a strong gamma-ray outburst, reaching fluxes (E > 100 MeV) in excess of 10^-5 ph cm^-2 s^-1. The Fermi Gamma ray Space Telescope performed a 200 ks long pointed observation starting from 5 April 2010 19:38 UTC. This allowed us to try probing the variability of the gamma-ray emission on timescales of hours or less. We found the variability on a few hours timescale. On sub-hour timescale we found no evidence of significant variability, although the present statistics is not yet conclusive and further observations are needed…
Axion-Like Particles (ALPs), if exist in nature, are expected to mix with photons in the presence of an external magnetic field. The energy range of photons which undergo strong mixing with ALPs depends on the ALP mass, on its coupling with photons as well as on the external magnetic field and particle density configurations. Recent observations of blazars by the Fermi Gamma-Ray Space Telescope in the 0.1-300 GeV energy range show a break in their spectra in the 1-10 GeV range. We have modeled this spectral feature for the flat-spectrum radio quasar 3C454.3 during its November 2010 outburst, a…
PKS 2155-304 is a blazar located in the Southern Hemisphere, monitored with the High Energy Stereoscopic System (H.E.S.S.) at very high energy (VHE, E>100 GeV) $γ$ rays every year since 2002. Thanks to the large data set collected in the VHE range and simultaneous coverage in optical, ultraviolet (UV), X-ray and high energy $γ$-ray ranges, this object is an excellent laboratory to study spectral and temporal variability in blazars. However, despite many years of dense monitoring, the nature of the variability observed in PKS 2155-304 remains puzzling. In this paper, we discuss the complex spec…
The global contribution of unresolved gamma-ray point sources to the extragalactic gamma-ray background has been recently measured down to gamma-ray fluxes lower than those reached with standard source detection techniques, and by employing the statistical properties of the observed gamma-ray counts. We investigate and exploit the complementarity of the information brought by the one-point statistics of photon counts (using more than 10 years of Fermi-LAT data) and by the recent measurement of the angular power spectrum of the unresolved gamma-ray background (based on 8 years of Fermi-LAT data…
With ten years of operation and an exceptional dataset, the Fermi-Large Area Telescope allows us to unveil the detailed composition of the extragalactic $γ$-ray sky above $\rm 100\,MeV$. In this paper, we derive the intrinsic source-count distribution (logN-logS) of extragalactic sources (i.e., blazars) at $|b|>20^{\circ}$ via the efficiency correction method. With this approach, we are able to measure the distribution down to a photon flux of $\sim10^{-10}$${\rm \, ph \,\, cm^{-2} \, s^{-1}}$ and to an energy flux of $\sim10^{-12}$${\rm \, erg \,\, cm^{-2} \, s^{-1}}$. In both cases the logN-…
We have selected SDSS J222032.50+002537.5 and SDSS J142048.01+120545.9 as best blazar candidates out of a complete sample of extremely radio-loud quasars at z>4, with highly massive black holes. We observed them and a third serendipitous candidate with similar features (PMN J2134-0419) in the X-rays with the Swift/XRT telescope, to confirm their blazar nature. We observed strong and hard X-ray fluxes (i.e. $α_X<0.6$, where $F(ν)\proptoν^{-α_X}$ in the 0.3-10keV observed energy range, ~1-40keV rest frame) in all three cases. This allowed us to classify our candidates as real blazars, being char…
We study the correlation between the gamma-ray flux F_g, averaged over the first 11 months of Fermi survey and integrated above 100 MeV, and the radio flux density (F_r at 20 GHz) of Fermi sources associated with a radio counterpart in the AT20G survey. Considering the blazars detected in both bands, the correlation is highly significant and it is F_g~F_r^0.85+-0.04, similar for BL Lac and FSRQ sources. However, only a small fraction (~1/15) of the AT20G radio sources with flat radio spectrum, are detected by Fermi. To understand if this correlation is real, we examine the selection effects in…
We combined multi-wavelength data for blazars from the Roma-BZCAT catalog and analyzed hundreds of X-ray spectra. We present the fluxes and Spectral Energy Distributions (SEDs), in 12 frequency bands from radio to $γ$-rays, for a final sample of 2214 blazars. Using a model-independent statistical approach, we looked for systematic trends in the SEDs; the most significant trends involved the radio luminosities and X-ray spectral indices of the blazars. We used a Principal Component Analysis (PCA), to determine the basis vectors of the blazar SEDs and, in order to maximize the size of the sample…
Long TeV $γ$-ray campaigns have been carried out to study the spectrum, variability and duty cycle of the BL Lac object Markarian 421. These campaigns have given some evidence of the presence of protons in the jet: i) Its spectral energy distribution which shows two main peaks; one at low energies ($\sim$ 1 keV) and the other at high energies (hundreds of GeV), has been described by using synchrotron proton blazar model. ii) The study of the variability at GeV $γ$-rays and X-rays has indicated no significant correlation. iii) TeV $γ$-ray detections without activity in X-rays, called "orphan fl…
Blazars are highly variable active galactic nuclei which emit radiation at all wavelengths from radio to gamma-rays. Polarized radiation from blazars is one key piece of evidence for synchrotron radiation at low energies and it also varies dramatically. The polarization of blazars is of interest for understanding the origin, confinement, and propagation of jets. However, even though numerous measurements have been performed, the mechanisms behind jet creation, composition and variability are still debated. We performed simultaneous gamma-ray and optical photopolarimetry observations of 45 blaz…
We study constraints on the population of neutrino emitting blazars imposed by the absence of doublets in astrophysical muon neutrino signal and z>0.3 redshift of nearest identified neutrino-emitting blazar (an order of magnitude further away than the nearest gamma-ray emitting blazar). We show that in spite of the absence of correlation of neutrino arrival directions with positions of gamma-ray emitting blazars, cumulative blazar flux could explain most of astrophysical neutrino flux measured in muon neutrino channel. This is possible if the population of neutrino emitting blazars has experie…
We report the first results of timing and spectral studies of \textit{Nuclear Spectroscopic Telescope Array (NuSTAR)} observations of six TeV emitting high-frequency peaked blazars: 1ES 0347$-$121, 1ES 0414$+$009, RGB J0710$+$591, 1ES 1101$-$232, 1ES 1218$+$304 and H 2356-309. Two out of these six TeV blazars, 1ES 1101$-$232 and 1ES 1218$+$304, showed strong evidence of intraday variations in the 3--79 keV energy range during those observations. We also found a hint of an intraday variability timescale of 23.5 ks in the light curve of 1ES 1218$+$304 using an autocorrelation function analysis. …
Quasi-periodic fluctuations in the light curves of blazars can provide insight into the underlying emission process. This type of flux modulation hints at periodic physical processes that result in emission. CTA 102, a flat spectrum radio quasar at $z=1.032$, has displayed significant activity since 2016. The multi-waveband light curve of CTA 102 shows signs of quasi-periodic oscillations during the 2016-2017 flare. Our goal is to quantify the presence of any periodicity in the emitted flux during the mentioned period and to explore the causes that can give rise to it. Techniques such as the L…
We study the gamma-ray variability of 13 blazars observed with the Fermi Large Area Telescope (LAT). These blazars have the most complete light curves collected during the first 4 years of the Fermi sky survey. We model them with the Ornstein-Uhlenbeck (OU) process or a mixture of the OU processes. The OU process has power spectral density (PSD) proportional to 1/f^alpha with alpha changing at a characteristic time scale, tau_0, from 0 (tau>>tau_0) to 2 (tau<<tau_0). The PSD of the mixed OU process has two characteristic time scales and an additional intermediate region with 0<alpha<2. We show…
Various quasi-periods for the long-term variability of the radio emission, optical emission, and structural position angle of the inner part of the parsec-scale jet in the blazar 0716+714 have been detected. The relationships between these quasi-periods are interpreted assuming that the variability arises due to a helical structure of the jet, which is preserved from regions near the jet base to at least 1 milliarcsecond from the core observed in radio interferometric observations. The radiating jet components should display radial motions with Lorentz factors of $\approx3$, and decelerate wit…
Blazars constitute the most enigmatic class of extragalactic gamma-ray sources, and their observational features have been ascribed to a relativistic jet closely aligned to the line of sight. They are generally divided in two main classes: the BL Lac objects (BL Lacs) and the Flat Spectrum Radio Quasars (FSRQs). In the case of BL Lacs the double bumped spectral energy distribution (SED) is generally described by the Synchrotron Self Compton (SSC) emission, while for the FSRQs it is interpreted as due to External Compton (EC) emission. Recently, we showed that in the [3.4]-[4.6]-[12] micron col…
We study the frequently used assumption in multi-messenger astrophysics that the gamma-ray and neutrino fluxes are directly connected because they are assumed to be produced by the same photohadronic production chain. An interesting candidate source for this test is the flat-spectrum radio quasar PKS B1424-418, which recently called attention of a potential correlation between an IceCube PeV-neutrino event and its burst phase. We simulate both the multi-waveband photon and the neutrino emission from this source using a self-consistent radiation model. We demonstrate that a simple hadronic mode…
Long-term gamma-ray variability of a non-blazar Active Galactic Nucleus (AGN) PKS 0521-36 is investigated by using Fermi-LAT pass 8 data covering from 2008 August to 2021 March. The results show that the histogram of the gamma-ray fluxes follows a log-normal distribution. Interestingly, in the analysis of about 5.8-year (from MJD 56317 to 58447) LAT data between two outbursts (occurring during 2012 October and 2019 May respectively), a quasi-periodic oscillation (QPO) with a period of about 1.1 years (about 5 sigma of significance) is found in the Lomb-Scargle Periodogram (LSP), the Weighted W…
Despite the uncovered association of a high-energy neutrino with the apparent flaring state of blazar TXS 0506+056 in 2017, the mechanisms leading to astrophysical particle acceleration and neutrino production are still uncertain. Recent studies found that when transparent to $γ$-rays, $γ$-flaring blazars do not have the opacity for protons to produce neutrinos. Here we present observational evidence for an alternative explanation, in which $γ$-ray emission is suppressed during efficient neutrino production. A large proton and target photon density help produce neutrinos while temporarily supp…
TeV emission from blazars can be used to probe the intergalactic magnetic fields and measure their intensity, coherence length, and helicity. Intergalactic magnetic fields deflect the electron-positron pairs produced by very-high-energy gamma-rays from blazars, resulting in broadened beams of cascade gamma-rays developing along the projected direction of the blazar jet. We present an analysis that uses for the first time the jet orientation of 12 high-synchrotron-peaked (HSP) BL~Lac-type blazars from VLBA radio observations to search for signatures of pair haloes in the Fermi-LAT data. Our sea…
The blazar 1ES 0229+200 is a high frequency peaked BL Lac object with a hard TeV spectrum extending to 10 TeV. Its unusual spectral characteristics make it a frequently used probe for intergalactic radiation and magnetic fields. With new, simultaneous observations in the optical, ultraviolet (UV) and X-rays, the synchrotron emission is probed in great detail. The X-ray emission varies by a factor of ~2 in 2009, while being rather stable in 2010. The X-ray spectrum is very hard (Γ~ 1.8) and it shows an indication of excess absorption above the Galactic value. The X-ray emission is detected up t…
The Fermi-LAT revealed that the census of the gamma-ray sky is dominated by blazars. Looking for a possible connection between radio and gamma-ray emission is a central issue for understanding the blazar physics, and various works were dedicated to this topic. However, while a strong and significant correlation was found between radio and gamma-ray emission in the 0.1-100 GeV energy range, the connection between radio and very high energy (VHE, E>0.1 TeV) emission is still elusive. The main reason is the lack of a homogeneous VHE sky coverage, due to the operational mode of the imaging atmosph…
We evaluate the optical/near-infrared (OIR) color variability of 3C 279 in both gamma-ray flaring and non-flaring states over 7-year timescales using the Small and Medium Aperture Research Telescope System (SMARTS) in Cerro Tololo, Chile and gamma-ray fluxes obtained from the Fermi Gamma-ray Space Telescope. This observing strategy differs from previous blazar color variability studies in two key ways: 1) the reported color variability is assessed across optical through near-infrared wavelengths, and 2) the color variability is assessed over timescales significantly longer than an individual f…
On 22 September 2017, IceCube reported a high-energy neutrino event which was found to be coincident with a flaring blazar, TXS 0506+056. This multi-messenger observation hinted at blazars contributing to the observed high-energy astrophysical neutrinos and raised a need for extensive correlation studies. Recent work shows that the internal absorption of gamma rays, and their interactions intrinsic to the source and with the extragalactic background, will cause a lack of energetic gamma-ray and neutrino correlation while hinting towards a correlation between neutrinos and lower photon energy o…
The ANTARES telescope is well suited to detect neutrinos produced in astrophysical transient sources as it can observe a full hemisphere of the sky at all times with a high duty cycle. The background and point source sensitivity can be drastically reduced by selecting a narrow time window around the assumed neutrino production period. Radio-loud active galactic nuclei with their jets pointing almost directly towards the observer, the so- called blazars, are particularly attractive potential neutrino point sources, since they are among the most likely sources of the observed ultra high energy c…
Optical observations of a sample of 12 $γ$-ray bright blazars from four optical data archives, AAVSO, SMARTS, Catalina, and Steward Observatory, are compiled to create densely sampled light curves spanning more than a decade. As a part of the blazar multi-wavelength studies, several methods of analyses, e. g., flux distribution and RMS-flux relation, are performed on the observations with an aim to compare the results with the similar ones in the \gama-ray band presented in Bhatta & Dhital 2020. It is found that, similar to $γ$-ray band, blazars display significant variability in the optical b…
A new look at the extragalactic Very High Energy sky: searching for TeV-emitting candidates among the X-ray bright, non-Fermi detected blazar population
We present results of a variability study in the optical band of 44 newly identified blazar candidates behind the Magellanic Clouds. Our sample contains 27 flat spectrum radio quasars (FSRQs) and 17 BL Lacertae objects (BL Lacs). However, only nine of them are considered as secure blazar candidates, while the classification of the remaining 35 objects is still uncertain. All studied blazar candidates possess infrequently sampled optical light curves (LCs) in I filter provided by the Optical Gravitational Lensing Experiment group. The LCs were analysed with the Lomb-Scargle periodogram, the Hur…
In this paper we present results of a short-term optical monitoring of 13 blazars. The objects were monitored mostly in the R-band for a total of ~ 160 hours between 2006 and 2011. We study the nature of the short-term variations and show that most of them could be described as slow, smooth, and (almost) linear changes of up to ~ 0.1 mag/hour, but many objects show no short-term variations at all. In fact, we found only ~ 2 per cent chance to observe variability of more than 0.1 mag/hour for the sample we observed. Hints for quasi-periodic oscillations at very low amplitude levels are also fou…
Flat spectrum radio quasars (FSRQs) are a particular class of blazars rich of optical/ultraviolet photons inside the broad line region (BLR), necessarily implying a huge optical depth for $γ$ rays above 20 GeV. As a consequence, photons with energy above such a threshold should not be emitted. However, photons in the energy range 70 - 400 GeV have been observed by MAGIC from the FSRQ PKS 1222+216. Several astrophysical explanations exist in the literature, but they are all ad hoc, namely devised only for that specific purpose. We show that such a surprising discovery can be explained within st…
A broadband study of the high redshift blazar S5 0836+71 (z = 2.172) is presented. Multi-frequency light curves show multiple episodes of X-ray and $γ$-ray flares, while optical-UV fluxes show little variations. During the GeV outburst, the highest $γ$-ray flux measured is (5.22 $\pm$ 1.10) $\times$ 10$^{-6}$ ph cm$^{-2}$ s$^{-1}$ in the range of 0.1-300 GeV, which corresponds to an isotropic $γ$-ray luminosity of (1.62 $\pm$ 0.44) $\times$ 10$^{50}$ erg s$^{-1}$, thereby making this as one of the most luminous $γ$-ray flare ever observed from any blazar. A fast $γ$-ray flux rising time of $\s…
Blazars are a subclass of AGN and flaring in multi-TeV gamma-ray seems to be the major activity in high energy blazars a subgrup of blazars. Flaring is also unpredictable and switches between quiescent and active states involving different time scales and fluxes. While in some high energy blazars a strong temporal correlation between X-ray and multi-TeV gamma-ray has been observed, outbursts in some other have no low energy counterparts and explanation of such extreme activity needs to be addressed through different mechanisms as it is not understood well. The extragalactic background light (E…
The innermost parts of powerful jets in active galactic nuclei are surrounded by dense, high-velocity clouds from the broad-line region, which may penetrate into the jet and lead to the formation of a strong shock. Such jet-cloud interactions are expected to have measurable effects on the $γ$-ray emission from blazars. We characterise the dynamics of a typical cloud-jet interaction scenario, and the evolution of its radiative output in the 0.1-30~GeV energy range, to assess to what extent these interactions can contribute to the $γ$-ray emission in blazars. We use semi-analytical descriptions …
Ultrahigh energy protons and nuclei from extragalactic cosmic ray sources initiate intergalactic electromagnetic cascades, resulting in observable fluxes of $γ$-rays in the GeV-TeV energy domain. The total spectrum of such cascade $γ$-rays of hadronic nature is significantly harder than the one usually expected from blazars. The spectra of some sources known as "extreme TeV blazars" could be well-described by this "intergalactic hadronic cascade model" (IHCM). We calculate the shape of the observable point-like spectrum, as well as the observable angular distibution of $γ$-rays, for the first …
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…
