Gamma Rays From Blazars

Blazars are high-energy engines providing us natural laboratories to study particle acceleration, relativistic plasma processes, magnetic field dynamics, black hole physics. Key informations are provided by observations at high-energy (in particular by Fermi/LAT) and very-high energy (by Cherenkov telescopes). I give a short account of the current status of the field, with particular emphasis on the theoretical challenges connected to the observed ultra-fast variability events and to the emission of flat spectrum radio quasars in the very high energy band.Comment: 12 pages, invited talk at "Gamma2016", Heidelberg July 11-15, 2016, to be published in a special volume of the AIP Conference Proceeding

Jets from subpc to kpc scale

The Chandra discovery of bright X-ray emission from kpc-scale jets provides us unprecedented insights into the physical state of the plasma in the flow. In particular it is possible to get good constraints on the power and pressure in bright knots. For a group of selected sources with blazar-type cores it is also possible to constrain the physical quantities of the jet at sub-pc scale. We discuss how these results can help us to connect the properties of the jet at different scales.Comment: 4 pages, Proceedings of the conference "The Physics of Relativistic Jets in the CHANDRA and XMM Era", 23-27 September 2002, Bologn

Extragalactic jets: the high energy view

I review the current knowledge of high-energy emission from extragalctic jets. First I discuss gamma-ray emission from blazars, which provides us numerous precious information on the innermost portions of the relativistic jets. I describe the constraints on the dynamics of the jet from the subpc to the pc scale provided by recent VLBI studies of TeV sources, together with the modelling of the emission from the blazar jet. Finally I discuss high energy emission from large scale jets as seen by Chandra and I report on the expected gamma-ray emission from large-scale regions of jets.Comment: 17 pages, 10 figures. Invited review at the Xth Marcel Grossmann Meeting on General Relativity, Rio de Janeiro, Brazil, July 200

Recollimation shocks and radiative losses in extragalactic relativistic jets

We present the results of state-of-the-art simulations of recollimation shocks induced by the interaction of a relativistic jet with an external medium, including the effect of radiative losses of the shocked gas. Our simulations confirm that -- as suggested by earlier semi-analytical models -- the post-shock pressure loss induced by radiative losses may lead to a stationary equilibrium state characterized by a very strong focusing of the flow, with the formation of quite narrow nozzles, with cross-sectional radii as small as $10^{-3}$ times the length scale of the jet. We also study the time-dependent evolution of the jet structure induced of a density perturbation injected at the flow base. The set-up and the results of the simulations are particularly relevant for the interpretation of the observed rapid variability of the $\gamma$-ray emission associated to flat spectrum radio quasars. In particular, the combined effects of jet focusing and Doppler beaming of the observed radiation make it possible to explain the sub-hour flaring events such as that observed in the FSRQ PKS 1222+216 by MAGIC.Comment: 8 pages, 8 figures, Astronomy and Astrophysics accepte

Extreme BL Lacs: probes for cosmology and UHECR candidates

High-energy observations of extreme BL Lac objects, such as 1ES0229+200 or 1ES 0347-121, recently focused interest both for blazar and jet physics and for the implication on the extragalactic background light and intergalactic magnetic field estimate. Moreover, their enigmatic properties have been interpreted in a scenario in which their primary high- energy output is through a beam of high-energy hadrons. However, despite their possible important role in all these topics, the number of these extreme highly peaked BL Lac objects (EHBL) is still rather small. Aiming at increase their number, we selected a group of EHBL candidates considering those undetected (or only barely detected) by the LAT onboard Fermi and characterized by a high X-ray versus radio flux ratio. We assembled the multi-wavelength spectral energy distribution of the resulting 9 sources, using available archival data of Swift, GALEX, and Fermi satellites, confirming their nature. Through a simple one-zone synchrotron self-Compton model we estimate the expected very high energy flux, finding that in the majority of cases it is within the reach of present generation of Cherenkov arrays or of the forthcoming CTA.Comment: 6 pages, to appear in the Proceedings of the Conference "High-Energy Phenomena and Relativistic Outflows V", held in La Plata, 5-8 October 201

Fermi/LAT broad emission line blazars

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.Comment: Accepted for publication in MNRAS. 30 pages, 6 tables, 16 figures. It contains full tables and all SEDs of the source

On the detectability of Lorentz invariance violation through anomalous multi-TeV γ\gamma-ray spectra of blazars

Cosmic opacity for very high-energy gamma rays ($E>10$ TeV) due to the interaction with the extragalactic background light can be strongly reduced because of possible Lorentz-violating terms in the particle dispersion relations expected, e.g., in several versions of quantum gravity theories. We discuss the possibility to use very high energy observations of blazars to detect anomalies of the cosmic opacity induced by LIV, considering in particular the possibility to use -- besides the bright and close-by BL Lac Mkn 501 -- extreme BL Lac objects. We derive the modified expression for the optical depth of $\gamma$ rays considering also the redshift dependence and we apply it to derive the expected high-energy spectrum above 10 TeV of Mkn 501 in high and low state and the extreme BL Lac 1ES 0229+200. We find that, besides the nearby and well studied BL Lac Mkn 501 -- especially in high state --, suitable targets are extreme BL Lac objects, characterized by quite hard TeV intrinsic spectra likely extending at the energies relevant to detect LIV features.Comment: 9 pages, 9 figures, accepted by A&

Gamma-ray emission from AGNs

Blazars, radio-loud active galactic nuclei with the relativistic jet closely aligned with the line of sight, dominate the extragalactic sky observed at gamma-ray energies, above 100 MeV. We discuss some of the emission properties of these sources, focusing in particular on the "blazar sequence" and the interpretative models of the high-energy emission of BL Lac objects.Comment: 8 pages, 4 figures, to appear in the proceedings of the HEPRO II conference, Buenos Aires, October 26-30 200