ABSTRACT
Extreme high-energy peaked BL Lac objects (EHBLs) are an emerging class of blazars with exceptional spectral properties. The non-thermal emission of the relativistic jet peaks in the ...spectral energy distribution (SED) plot with the synchrotron emission in X-rays and with the gamma-ray emission in the TeV range or above. These high photon energies may represent a challenge for the standard modelling of these sources. They are important for the implications on the indirect measurements of the extragalactic background light, the intergalactic magnetic field estimate, and the possible origin of extragalactic high-energy neutrinos. In this paper, we perform a comparative study of the multiwavelength spectra of 32 EHBL objects detected by the Swift-BAT telescope in the hard X-ray band and by the Fermi-LAT telescope in the high-energy gamma-ray band. The source sample presents uniform spectral properties in the broad-band SEDs, except for the TeV gamma-ray band where an interesting bimodality seems to emerge. This suggests that the EHBL class is not homogeneous, and a possible subclassification of the EHBLs may be unveiled. Furthermore, in order to increase the number of EHBLs and settle their statistics, we discuss the potential detectability of the 14 currently TeV gamma-ray undetected sources in our sample by the Cherenkov telescopes.
The intergalactic magnetic field (IGMF) present in the voids of large-scale structures is considered to be the weakest magnetic field in the Universe. Gamma-ray observations of blazars in the GeV–TeV ...domain have led to lower limits on the IGMF strength based on the search for delayed or extended emission. Nevertheless, these results have been obtained with strong assumptions placed on the unknown source properties. The recent discovery of TeV radiation from gamma-ray bursts (GRBs) has paved the way for IGMF studies with these bright transients. Among the current TeV-detected GRBs, GRB 190114C, located at a redshift of z = 0.42, is the best sampled. Therefore, it can be considered to be representative of the properties of GRBs in the VHE domain. In addition, GRB 221009A ( z = 0.151) is the brightest event ever detected. We present a phenomenological model based on the intrinsic properties of GRB 190114C and GRB 221009A to predict the delayed emission component (pair-echo) in the GeV–TeV band. We investigate the detectability of this component from low-redshift ( z ≤ 1) GRBs for three values of IGMF strength (10 −19 G, 10 −18 G, and 10 −17 G), different observational times (3 h, 6 h, and 9 h) and source intrinsic properties. We find that for current and future generation γ -ray instruments, extending the observation for at least 3 h after the GRB detection is a viable strategy for probing the IGMF. We also confirm that GeV–TeV observations of GRBs can probe IGMF strengths on the order of 10 −17 − 10 −19 G, representing a competitive alternative to the current studies performed with active galactic nuclei (AGNs).
Context. Measurements of the extragalactic background light (EBL) are a fundamental source of information on the collective emission of cosmic sources. Aims. At infrared wavelengths, however, these ...measurements are precluded by the overwhelming dominance from interplanetary dust emission and the Galactic infrared foreground. Only at λ > 300 μm, where the foregrounds are minimal, has the infrared EBL (IR EBL) been inferred from analysis of the COBE maps. The present paper aims to assess the possibility of evaluating the IR EBL from a few micrometers up to the peak of the emission at > 100 μm using an indirect method that avoids the foreground problem. Methods. To this purpose we exploit the effect of pair-production from gamma-gamma interaction by considering the highest-energy photons emitted by extragalactic sources and their interaction with the IR EBL photons. We simulate observations of a variety of low-redshift emitters with the forthcoming Imaging Atmospheric Cherenkov Telescope (IACT) arrays (CTA in particular) and water Cherenkov observatories (LHAASO, HAWC, SWGO) to assess their suitability to constrain the EBL at such long wavelengths. Results. We find that even under the most extremely favorable conditions of huge emission flares, extremely high-energy emitting blazars are not very useful for our purpose because they are much too distant (> 100 Mpc the nearest ones, MKN 501 and MKN 421). Observations of more local AGNs displaying very high-energy emission, like low-redshift radio galaxies (M 87, IC 310, Centaurus A), are better suited and will potentially allow us to constrain the EBL up to λ ≃ 100 μm.
Context. Blazars are radio-loud active galactic nuclei (AGN) with a jet pointing at small angles towards the observer. The overall emitted spectrum is typically non-thermal, and in some cases the ...emission and/or absorption lines are so faint as to prevent the determination of the redshift based on optical spectroscopy methods. PKS 0447−439 is a bright blazar recently detected at very high energy. The redshift of the source is still disputed: a recent spectral analysis reports only a lower limit of z > 1.246, which contradicts the previous measure of z = 0.205 reported in the literature. Aims. We aim to give a redshift estimate of the blazar PKS 0447−439 based on combined GeV (Fermi/LAT) and TeV (HESS) observations. Methods. Taking into account the absorption of TeV photons by the interaction with the extragalactic background light (EBL), we applied the method we developed in a previous work to derive the redshift of PKS 0447−439. Moreover, we compiled the overall spectral energy distribution (SED) using optical-UV, soft X-ray, and γ-ray data, nearly simultaneous to the HESS observations at TeV energies. Finally we modelled the spectral energy distribution (SED) within the framework of a homogeneous, leptonic synchrotron self-Compton (SSC) model. Results. Using the recent TeV spectrum measured by HESS we obtain for PKS 0447−439 a redshift of zrec = 0.20 ± 0.05, which is our estimate on the source distance. This value agrees very well with the value reported in the literature and confirms that our method can be successfully used to constrain blazars distances. Assuming this distance, the SED can be well fitted with the above mentioned model. The physical parameters that we find suggest a strongly matter-dominated jet. Conclusions. Our analysis confirms that the redshift of PKS 0447−439 is likely 0.2, and supports the result present in the literature.
We discuss a method to constrain the distance of blazars with unknown redshift using combined observations in the GeV and TeV regimes. We assume that the Very High Energies (VHE) spectrum corrected ...for the absorption through the interaction with the extragalactic background light cannot be harder than the spectrum in the Fermi/Large Area Telescope (LAT) band. Starting from the observed VHE spectral data we derive the EBL-corrected spectra as a function of the redshift z and fit them with power laws to be compared with power-law fits to the LAT data. We apply the method to all TeV blazars detected by LAT with known distance and derive an empirical law describing the relation between the upper limits and the true redshifts that can be used to estimate the distance of unknown redshift blazars. Using different EBL models leads to systematic changes in the derived upper limits. Finally, we use this relation to infer the distance of the unknown redshift blazar PKS 1424+240.
ABSTRACT
Gamma-ray Bursts (GRB) were discovered by satellite-based detectors as powerful sources of transient γ-ray emission. The Fermi satellite detected an increasing number of these events with ...its dedicated Gamma-ray Burst Monitor (GBM), some of which were associated with high energy photons ($E \gt 10$ GeV), by the Large Area Telescope (LAT). More recently, follow-up observations by Cherenkov telescopes detected very high energy emission ($E \gt 100$ GeV) from GRBs, opening up a new observational window with implications on the interpretation of their central engines and on the propagation of very energetic photons across the Universe. Here, we use the data published in the 2nd Fermi-LAT Gamma Ray Burst Catalogue to characterize the duration, luminosity, redshift, and light curve of the high energy GRB emission. We extrapolate these properties to the very high energy domain, comparing the results with available observations and with the potential of future instruments. We use observed and simulated GRB populations to estimate the chances of detection with wide-field ground-based γ-ray instruments. Our analysis aims to evaluate the opportunities of the Southern Wide-field-of-view Gamma-ray Observatory (SWGO), to be installed in the Southern Hemisphere, to complement CTA. We show that a low-energy observing threshold ($E_{low} \lt 200$ GeV), with good point source sensitivity ($F_{lim} \approx 10^{-11} \,\mathrm{erg\, cm^{-2}\, s^{-1}}$ in $1$ yr), are optimal requirements to work as a GRB trigger facility and to probe the burst spectral properties down to time-scales as short as $10$ s, accessing a time domain that will not be available to Imaging Atmospheric Cherenkov Telescopes instruments.
Imaging Atmospheric Cherenkov Telescopes (IACT) are designed to detect γ-ray photons in the very-high energy (VHE) band from 50 GeV to more than 10 TeV. The few degree-scale field of view and the ...good angular resolution < 0.1 deg, favor morphological studies on extended sources and on diffuse emission. The description of the PSF (Point Spread Function) of IACT is a prerequisite to assess, for example, the possibility to reveal diffuse emission around point like sources. We study the PSF on γ-ray photons of IACTs using Crab Nebula data collected with the MAGIC stereoscopic system. Through a careful description of the background and response of the instrument we can build the γ-ray PSF corrected for the angular acceptance. We find a good analytical model describing the γ-ray PSF for IACTs and assess its stability.
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