[en] The magnetic field in intergalactic space gives important information about magnetogenesis in the early universe. The properties of this field can be probed by searching for radiation of secondary e ⁺ e ⁻ pairs created by TeV photons that produce GeV range radiation by Compton-scattering cosmic microwave background photons. The arrival times of the GeV “echo” photons depend strongly on the magnetic field strength and coherence length. A Monte Carlo code that accurately treats pair creation is developed to simulate the spectrum and time-dependence of the echo radiation. The extrapolation of the spectrum of powerful gamma-ray bursts (GRBs) like GRB 130427A to TeV energies is used to demonstrate how the intergalactic magnetic field can be constrained if it falls in the 10⁻²¹–10⁻¹⁷ G range for a 1 Mpc coherence length.

[en] Auger results on clustering of ≥ 60 EeV UHECR ions and the interpretation of the γ-ray spectra of TeV blazars are connected by effects from the extragalactic background light (EBL). The EBL acts as an obscuring medium for γ rays and a reprocessing medium for ultra-high energy cosmic ray (UHECR) ions and protons, causing the GZK cutoff. The study of the physics underlying the coincidence between the GZK energy and the clustering energy of UHECR ions favors a composition of ≥ 60 EeV UHECRs in CNO (up to Mg) group nucleons. This has interesting implications for the sources of UHECRs. We also comment on the Auger analysis

[en] We investigate the spectral and timing signatures of the internal-shock model for blazars. For this purpose, we develop a semi-analytical model for the time-dependent radiative output from internal shocks arising from colliding relativistic shells in a blazar jet. The emission through synchrotron and synchrotron-self Compton radiation as well as Comptonization of an isotropic external radiation field are taken into account. We evaluate the discrete correlation function (DCF) of the model light curves in order to evaluate features of photon-energy-dependent time lags and the quality of the correlation, represented by the peak value of the DCF. The almost completely analytic nature of our approach allows us to study in detail the influence of various model parameters on the resulting spectral and timing features. This paper focuses on a range of parameters in which the γ-ray production is dominated by Comptonization of external radiation, most likely appropriate for γ-ray bright flat-spectrum radio quasars (FSRQs) or low-frequency peaked BL Lac objects (LBLs). In most cases relevant for FSRQs and LBLs, the variability of the optical emission is highly correlated with the X-ray and high-energy (HE: > 100 MeV) γ-ray emission. Our baseline model predicts a lead of the optical variability with respect to the higher-energy bands by 1-2 hr and of the HE γ-rays before the X-rays by about 1 hr. We show that variations of certain parameters may lead to changing signs of inter-band time lags, potentially explaining the lack of persistent trends of time lags in most blazars.

[en] The Auger Collaboration reports (Auger Collaboration 2007 Science 318 939, The Pierre Auger Collaboration 2008 Astropart. Phys. 29 188) that the arrival directions of ∼>60 EeV ultra-high-energy cosmic rays (UHECRs) cluster along the supergalactic plane and correlate with active galactic nuclei (AGN) within ∼ 100 Mpc. The association of several events with the nearby radio galaxy Centaurus A supports the paradigm that UHECRs are powered by supermassive black-hole engines and accelerated to ultra-high energies in the shocks formed by variable plasma winds in the inner jets of radio galaxies. The GZK horizon length of 75 EeV UHECR protons is ∼100 Mpc, so that the Auger results are consistent with an assumed proton composition of the UHECRs. In this scenario, the sources of UHECRs are FR II radio galaxies and FR I galaxies like Cen A with scattered radiation fields that enhance UHECR neutral-beam production. Radio galaxies with jets pointed away from us can still be observed as UHECR sources due to deflection of UHECRs by magnetic fields in the radio lobes of these galaxies. A broadband ∼1 MeV-10 EeV radiation component in the spectra of blazar AGN is formed by UHECR-induced cascade radiation in the extragalactic background light. This emission is too faint to be seen from Cen A, but could be detected from more luminous blazars.

[en] The flat-spectrum radio quasar 3C454.3 underwent an extraordinary 5-day γ-ray outburst in November 2010 when the daily flux measured with the Fermi Large Area Telescope (LAT) at photon energies E > 100MeV reached (66±2)x 10^-6ph cm^-2s^-1. This is a factor of 3 higher than its previous maximum flux recorded in December 2009 and > 5 times brighter than the Vela pulsar, which is normally the brightest source in the γ-ray sky. The 3-hr peak flux was (85±5)x10^-6ph cm^-2s^-1, corresponding to an apparent isotropic luminosity of (2.1±0.2)-10⁵⁰ erg s^-1, the highest ever recorded for a blazar. In this talk, we investigate the features of this exceptional event in the γ-ray band of the Fermi-LAT.

[en] Fermi has provided the largest sample of γ-ray-selected blazars to date. In this work we use a complete sample of flat spectrum radio quasars (FSRQs) detected during the first year of operation to determine the luminosity function (LF) and its evolution with cosmic time. The number density of FSRQs grows dramatically up to redshift ∼0.5-2.0 and declines thereafter. The redshift of the peak in the density is luminosity dependent, with more luminous sources peaking at earlier times; thus the LF of γ-ray FSRQs follows a luminosity-dependent density evolution similar to that of radio-quiet active galactic nuclei. Also, using data from the Swift Burst Alert Telescope we derive the average spectral energy distribution (SED) of FSRQs in the 10 keV-300 GeV band and show that there is no correlation between the luminosity at the peak of the γ-ray emission component and its peak frequency. Using this luminosity-independent SED with the derived LF allows us to predict that the contribution of FSRQs to the Fermi isotropic γ-ray background is 9.3^+1.6_–1.0% (±3% systematic uncertainty) in the 0.1-100 GeV band. Finally we determine the LF of unbeamed FSRQs, finding that FSRQs have an average Lorentz factor of γ = 11.7^+3.3_–2.2, that most are seen within 5° of the jet axis, and that they represent only ∼0.1% of the parent population.

[en] Thanks to its sensitivity, the GLAST LAT should detect many more (upwards of a thousand) gamma-ray blazars than currently known. This large blazar sample will enable detailed population studies to be carried out. Moreover, the LAT large field-of-view combined with the scanning mode will provide a very uniform exposure over the sky, allowing a constant monitoring of all detected blazars and flare alerts to be issued. Major specific issues regarding the blazar phenomenon that the LAT data should shed light on thanks to these capabilities are discussed, as well as the different approaches foreseen to address them. The associated data required in other bands, to be collected in contemporaneous/simultaneous multiwavelength campaigns are mentioned as well

[en] We report the Fermi Large Area Telescope (LAT) detection of two very-high-energy (VHE, E > 100 GeV) γ-ray photons from the directional vicinity of the distant (redshift, z = 1.1) blazar PKS 0426–380. The null hypothesis that both the 134 and 122 GeV photons originate from unrelated sources can be rejected at the 5.5σ confidence level. We therefore claim that at least one of the two VHE photons is securely associated with PKS 0426–380, making it the most distant VHE emitter known to date. The results are in agreement with recent Fermi-LAT constraints on the extragalactic background light (EBL) intensity, which imply a z ≅ 1 horizon for ≅ 100 GeV photons. The LAT detection of the two VHE γ-rays coincided roughly with flaring states of the source, although we did not find an exact correspondence between the VHE photon arrival times and the flux maxima at lower γ-ray energies. Modeling the γ-ray continuum of PKS 0426–380 with daily bins revealed a significant spectral hardening around the time of the first VHE event detection (LAT photon index Γ ≅ 1.4) but on the other hand no pronounced spectral changes near the detection time of the second one. This combination implies a rather complex variability pattern of the source in γ-rays during the flaring epochs. An additional flat component is possibly present above several tens of GeV in the EBL-corrected Fermi-LAT spectrum accumulated over the ∼8 month high state

[en] We report on our study of high-energy properties of two peculiar TeV emitters: the 'extreme blazar' 1ES 0347–121 and the 'extreme blazar candidate' HESS J1943+213 located near the Galactic plane. Both objects are characterized by quiescent synchrotron emission with flat spectra extending up to the hard X-ray range, and both were reported to be missing GeV counterparts in the Fermi Large Area Telescope (LAT) two-year Source Catalog. We analyze a 4.5 yr accumulation of the Fermi-LAT data, resulting in the detection of 1ES 0347–121 in the GeV band, as well as in improved upper limits for HESS J1943+213. We also present the analysis results of newly acquired Suzaku data for HESS J1943+213. The X-ray spectrum is well represented by a single power law extending up to 25 keV with photon index 2.00 ± 0.02 and a moderate absorption in excess of the Galactic value, which is in agreement with previous X-ray observations. No short-term X-ray variability was found over the 80 ks duration of the Suzaku exposure. Under the blazar hypothesis, we modeled the spectral energy distributions of 1ES 0347–121 and HESS J1943+213, and we derived constraints on the intergalactic magnetic field strength and source energetics. We conclude that although the classification of HESS J1943+213 has not yet been determined, the blazar hypothesis remains the most plausible option since, in particular, the broadband spectra of the two analyzed sources along with the source model parameters closely resemble each other, and the newly available Wide-field Infrared Survey Explorer and UKIRT Infrared Deep Sky Survey data for HESS J1943+213 are consistent with the presence of an elliptical host at the distance of approximately ∼600 Mpc.

[en] The Large Area Telescope on board the Fermi satellite observed a gamma-ray flare in the Crab Nebula lasting for approximately nine days in April of 2011. The source, which at optical wavelengths has a size of ≈11 lt-yr across, doubled its gamma-ray flux within eight hours. The peak photon flux was (186 ± 6) × 10^–7 cm^–2 s^–1 above 100 MeV, which corresponds to a 30-fold increase compared to the average value. During the flare, a new component emerged in the spectral energy distribution, which peaked at an energy of (375 ± 26) MeV at flare maximum. The observations imply that the emission region was likely relativistically beamed toward us and that variations in its motion are responsible for the observed spectral variability.