VERY HIGH-ENERGY gamma-ray ASTRONOMY

Much of our information about quasars is derived from their emission-line spectra. The analysis of such spectra has become an intricate subject which differs considerably from traditional, low-density nebular astrophysics. This review is intended to explain our present understanding of the situation, including some aspects of galactic nuclei whose luminosities are more modest than quasars. Quasars' line-emitting regions are probably photoionized (even if supplementary heating processes also occur). So far, models have been constructed which include ionization and thermal equilibria, the transfer of resonance-line and related photons, and the likely effects of absorption and scattering by dust grains. From comparisons between emission-line intensities produced in these models and observed quasars' spectra, it appears that certain densities and pressures and size scales occur in or around quasars. The relative abundances of elements are not very far from solar values, although it is suspected that heavy elements---carbon, nitrogen, and oxygen in particular---are moderately "overabundant" in quasars. The emission-line intensities also provide indirect information about quasars' ultraviolet and soft-x-ray continua; there are hints that photons with energies between 20 and 300 eV---which are not directly observable---may even represent the peak of the luminous output of a typical quasar. Finally, some gas-dynamical questions, while extremely important, are very difficult to answer, because of a lack of observables.

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The emission lines of quasars and similar objects

We study the broad emission line blazars detected in the gamma-ray band by the Large Area Telescope onboard the Fermi satellite and with the optical spectrum studied by Shaw et al. (2012, 2013). The observed broad line strength provides a measure of the ionizing luminosity of the accretion disk, while the gamma-luminosity is a proxy for the bolometric non-thermal beamed jet emission. The resulting sample, composed by 217 blazars, is the best suited to study the connection between accretion and jet properties. We compare the broad emission line properties of these blazars with those of radio-quiet and radio-loud quasars present in the Sloan Digital Sky Survey, to asses differences and similarities of the disc luminosity and the virial black hole mass. For most sources, we could derive the black hole mass by reproducing the IR-optical-UV data with a standard accretion disc spectrum, and we compared the black hole masses derived with the two methods. The distributions of the masses estimated in the two ways agree satisfactorily. We then apply a simple, one-zone, leptonic model to all the 217 objects of our sample. The knowledge of the black hole mass and disc luminosity helps to constrain the jet parameters. On average they are similar to what found by previous studies of smaller samples of sources.

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Fermi/LAT broad emission line blazars

This article reviews the recent observational results regarding γ-ray emission from active galaxies. The most numerous discrete extragalactic γ-ray sources are AGNs dominated by relativistic jets pointing in our direction (commonly known as blazars), and they are the main subject of the review. They are detected in all observable energy bands and are highly variable. The advent of the sensitive γ-ray observations, afforded by the launch and continuing operation of the Fermi Gamma-ray Space Telescope and the AGILE Gamma-ray Imaging Detector, as well as by the deployment of current-generation Air Cerenkov Telescope arrays such as VERITAS, MAGIC, and HESS-II, continually provides sensitive γ-ray data over the energy range of ∼100 MeV to multi-TeV. Importantly, it has motivated simultaneous, monitoring observations in other bands, resulting in unprecedented time-resolved broadband spectral coverage. After an introduction, in Sections 3, 4, and 5, we cover the current status and highlights of γ-ray observation...

Gamma-Ray Observations of Active Galactic Nuclei

Spallation processes and nuclear interaction products of cosmic rays

THE discovery of a high-energy γ-ray source at α(1950) = 12h 29 min±6 min, δ(1950)= +3°±15° is reported here Arguments are given for the identification with 3C273 If this identification is correct, the γ-ray luminosity of 3C273 in the energy range 50–500 MeV is 2×1046 erg s−1 for H0 = 60 km s−1 Mpc−1

P. A. Caraveo

Papers

COS B observation of high-energy γ radiation from 3C273

New large-scale properties of the galactic gamma radiation observed by COS-B in the context of the diffuse emission within the galactic disc