Implications for High Energy Blazar Spectra from Intergalactic Absorption Calculations
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Given a knowledge of the density spectra intergalactic low energy photons as a function of redshift, one can derive the intrinsic gamma-ray spectra and luminosities of blazars over a range of redshifts and look for possible trends in blazar evolution. Stecker, Baring & Summerlin have found some evidence hinting that TeV blazars with harder spectra have higher intrinsic TeV gamma-ray luminosities and indicating that there may be a correlation of spectral hardness and luminosity with redshift. Further work along these lines, treating recent observations of the blazers lES02291+200 and 3C279 in the TeV and sub-TeV energy ranges, has recently been explored by Stecker & Scully. GLAST will observe and investigate many blazars in the GeV energy range and will be sensitive to blazers at higher redshifts. I examine the implications high redshift gamma-ray absorption for both theoretical and observational blazer studies.Keywords:
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Recent observations of the TeV γ-ray spectra of the two closest active galactic nuclei (AGNs), Markarian 501 (Mrk 501) and Markarian 421 (Mrk 421), by the Whipple and HEGRA collaborations have stimulated efforts to estimate or limit the spectral energy density (SED) of extragalactic background light (EBL) which causes attenuation of TeV photons via pair-production when they travel cosmological distances. In spite of the lack of any distinct cutoff-like feature in the spectra of Mrk 501 and Mrk 421 (in the interval 0.26–10 TeV) which could clearly indicate the presence of such a photon absorption mechanism, we demonstrate that strong EBL attenuation signal (survival probability of 10 TeV photon ∼10−2) may still be present in the spectra of these AGNs. This attenuation could escape detection due to a special form of SED of EBL and unknown intrinsic spectra of these blazars. Here we show how the proposed and existing experiments, VERITAS, HESS, MAGIC, STACEE and CELESTE may be able to detect or severely limit the EBL SED by extension of spectral measurements into the critical 100–300 GeV regime.
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Abstract We study the quasi-simultaneous near-IR, optical, ultraviolet and X-ray photometry of 11 γ-ray selected blazars for which redshift estimates larger than 1.2 have been recently provided. Four of these objects turn out to be high-power blazars with the peak of their synchrotron emission between ∼3 × 1015 and ∼1016 Hz, and therefore of a kind predicted to exist but never seen before. This discovery has important implications for our understanding of physical processes in blazars, including the so-called ‘blazar sequence’, and might also help in constraining the extragalactic background light through γ-ray absorption since two sources are strongly detected even in the 10–100 GeV Fermi-LAT band. Based on our previous work and their high powers, these sources are very likely high-redshift flat-spectrum radio quasars, with their emission lines swamped by the non-thermal continuum.
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The recent High-Energy Gamma-Ray Array (HEGRA) observations of the blazar Mrk 501 show strong curvature in the very high energy γ-ray spectrum. Applying the γ-ray opacity derived from an empirically based model of the intergalactic infrared background radiation field to these observations, we find that the intrinsic spectrum of this source is consistent with a power law: dN/dE∝E, with α=2.00±0.03 over the range 500 GeV-20 TeV. Within current synchrotron self-Compton scenarios, the fact that the TeV spectral energy distribution of Mrk 501 does not vary with luminosity, combined with the correlated, spectrally variable emission in X-rays as observed by the BeppoSAX and Rossi X-Ray Timing Explorer instruments, also independently implies that the intrinsic spectrum must be close to α=2. Thus, the observed curvature in the spectrum is most easily understood as resulting from intergalactic absorption.
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Measurements of the very high energy (VHE) gamma-ray flux from high-redshift blazars are useful for understanding the cosmological evolution of galaxies, because gamma-ray emission from distant blazars is affected by extragalactic background light (EBL).We searched for detectable VHE gamma-ray blazars with known redshifts using the Fermi archive data and selected three high-redshift sources, PKS 0118-272, PKS0454-234, and PKS 1244-255.In addition, we investigated the possibility of detecting VHE gamma-ray emission from two new flaring HE gamma-ray sources that were not included in the Second Fermi LAT Catalog (2FGL).Compared with the sensitivity of H.E.S.S., MAGIC, and CTA, the VHE gamma-ray flux of these sources is difficult to detect using existing ground-based Cherenkov telescopes (H.E.S.S. and MAGIC), but PKS 0118-272 could be detected constantly with a next-generation ground-based telescope (CTA), and PKS 0454-234 could be detected by CTA when the target object is in the bright period.Unfortunately, PKS 1244-255 is difficult to detect by CTA.Two new flaring HE gamma-ray sources J1418+3542 and J1628+7707 are judged to be detectable with MAGIC and CTA during the active period of the HE gamma rays if the redshifts are z < 0.5 and z < 1.0, respectively.
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AGNs are believed to be the source of very energetic cosmic rays which, in turn, produce gamma rays throughout the interactions with matter. Therefore, the study of their very-high-energy (VHE) gamma-ray spectra is fundamental to understand the cosmic-ray acceleration mechanisms in these objects. Blazars are the most common type of AGNs populating gamma-ray catalogs, among these sources BL-Lacs present featureless or very weak emission/absorption lines spectra. Given that the extragalactic VHE gamma-ray catalog is poorly populated (about 70 blazars), having a redshift estimation of every single source is important for the study of cosmic rays origin. In this work we use EBL absorption as a tool to estimate KUV 00311-1938 redshift by comparing HE gamma-ray observations with modeled EBL absorbed flux points. KUV 00311-1938 is a very interesting BL-Lac; a priori it can be one of the most distant blazars since it has an estimated redshift in the range 0.5 to 1.54. Assuming that there is no spectral cutoff, we consider the intrinsic VHE spectrum to be the extrapolation of the Fermi-LAT spectrum and model the intergalactic absorption by using the main current EBL models. Comparing these results with the VHE measured point by H.E.S.S., we give an estimate for the redshift of the blazar within observation errors.
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Photons with very high energy (VHE) up to TeV emitted from active galactic nuclei (AGNs) provide some invaluable information on the origin of gamma-ray emission. Although 66 blazars have been detected by EGRET, only three low-redshift X-ray-selected BL Lac objects (Mrk 421, Mrk 501, and 1ES 2344+514) are conclusive TeV emitters (PKS 2155-304 is a potential TeV emitter), since VHE photons may be absorbed by cosmological background infrared photons (external absorption). Based on the mirror effect of clouds in the broad-line region, we argue that there is an intrinsic mechanism for the deficiency of TeV emission in blazars. Employing the observable quantities, we derive the pair production optical depth tau(gamma gamma)(epsilon(obs)) due to the interaction of VHE photons with the reflected synchrotron photons by electron Thomson scattering in the broad-line region. This sets stronger constraints on very high energy emission, and provides a sensitive upper limit on the Doppler factor of the relativistic bulk motion. It has been suggested that the intrinsic absorption be distinguished from the external by the observation on variation of the multiwavelength continuum.
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High-redshift ($z>2$) blazars are the most powerful members of the blazar family. Yet, only a handful of them have both X-ray and $\gamma$-ray detection, thereby making it difficult to characterize the energetics of the most luminous jets. Here, we report, for the first time, the Fermi-Large Area Telescope detection of the significant $\gamma$-ray emission from the high-redshift blazar DA 193 ($z=2.363$). Its time-averaged $\gamma$-ray spectrum is soft ($\gamma$-ray photon index = $2.9\pm0.1$) and together with a relatively flat hard X-ray spectrum (14$-$195 keV photon index = $1.5\pm0.4$), DA 193 presents a case to study a typical high-redshift blazar with inverse Compton peak being located at MeV energies. An intense GeV flare was observed from this object in the first week of 2018 January, a phenomenon rarely observed from high-redshift sources. What makes this event a rare one is the observation of an extremely hard $\gamma$-ray spectrum (photon index = $1.7\pm0.2$), which is somewhat unexpected since high-redshift blazars typically exhibit a steep falling spectrum at GeV energies. The results of our multi-frequency campaign, including both space- (Fermi, NuSTAR, and Swift) and ground-based (Steward and Nordic Optical Telescope) observatories, are presented and this peculiar $\gamma$-ray flare is studied within the framework of a single-zone leptonic emission scenario.
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The standard model for propagation of gamma rays from very high energy (VHE, E≥100 GeV) blazars through the universe requires an understanding of the diffuse extragalactic background light (EBL). Due to photon-photon collision and pair production from gamma ray/infrared photon interaction in the intergalactic medium, we expect a redshift horizon beyond which gamma rays from these sources are significantly attenuated. Two TeV-bright blazars examined in this project pose a problem to the horizon hypothesis due to their substantial redshifts. Given that an unusually low density of foreground galaxies could account for the unexpected TeV observations, this project establishes estimates of foreground galactic populations by using HI (Lyman α) absorbers seen in HST/COS far-UV spectra of PG1424+240 and 3C66A as proxies for counting individual galaxies along the lines of sight. Comparisons to global averages of both number density dN/dz of absorbers and the luminosity function space density constant φ* result in a conclusion of higher-than-average foreground galactic populations for both PG1424+240 and 3C66A, requiring reevaluation of either the gamma-ray propagation model or the EBL model.
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The uncharted territory of the high-redshift ($z\gtrsim3$) Universe holds the key to understand the evolution of quasars. In an attempt to identify the most extreme members of the quasar population, i.e., blazars, we have carried out a multi-wavelength study of a large sample of radio-loud quasars beyond $z=3$. Our sample consists of 9 $\gamma$-ray detected blazars and 133 candidate blazars selected based on the flatness of their soft X-ray spectra (0.3$-$10 keV photon index $\leq1.75$), including 15 with NuSTAR observations. The application of the likelihood profile stacking technique reveals that the high-redshift blazars are faint $\gamma$-ray emitters with steep spectra. The high-redshift blazars host massive black holes ($\langle \log~M_{\rm BH,~M{\odot}} \rangle>9$) and luminous accretion disks ($\langle L_{\rm disk} \rangle>10^{46}$ erg s$^{-1}$). Their broadband spectral energy distributions are found to be dominated by high-energy radiation indicating their jets to be among the most luminous ones. Focusing on the sources exhibiting resolved X-ray jets (as observed with the Chandra satellite), we find the bulk Lorentz factor to be larger with respect to other $z>3$ blazars, indicating faster moving jets. We conclude that the presented list of the high-redshift blazars may act as a reservoir for follow-up observations, e.g., with NuSTAR, to understand the evolution of relativistic jets at the dawn of the Universe.
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