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Comptes Rendus Physique
Volume 17, n° 6
pages 594-616 (juin 2016)
Doi : 10.1016/j.crhy.2016.04.004
Active galactic nuclei at gamma-ray energies
Noyaux actifs de galaxie dans le domaine des rayons gamma

Charles Dennison Dermer a, , Berrie Giebels b
a Code 7653, Naval Research Laboratory, Washington, DC, USA 
b LLR, École polytechnique, CNRS/IN2P3, Université Paris-Saclay, 91128 Palaiseau cedex, France 

Corresponding author.

Active Galactic Nuclei can be copious extragalactic emitters of MeV–GeV–TeV γ rays, a phenomenon linked to the presence of relativistic jets powered by a super-massive black hole in the center of the host galaxy. Most of γ -ray emitting active galactic nuclei, with more than 1500 known at GeV energies, and more than 60 at TeV energies, are called “blazars”. The standard blazar paradigm features a jet of relativistic magnetized plasma ejected from the neighborhood of a spinning and accreting super-massive black hole, close to the observer direction. Two classes of blazars are distinguished from observations: the flat-spectrum radio-quasar class (FSRQ) is characterized by strong external radiation fields, emission of broad optical lines, and dust tori. The BL Lac class (from the name of one of its members, BL Lacertae) corresponds to weaker advection-dominated flows with γ -ray spectra dominated by the inverse Compton effect on synchrotron photons. This paradigm has been very successful for modeling the broadband spectral energy distributions of blazars. However, many fundamental issues remain, including the role of hadronic processes and the rapid variability of a few FSRQs and several BL Lac objects whose synchrotron spectrum peaks at UV or X-ray frequencies. A class of γ -ray-emitting radio galaxies, which are thought to be the misaligned counterparts of blazars, has emerged from the results of the Fermi-Large Area Telescope and of ground-based Cherenkov telescopes. Soft γ -ray emission has been detected from a few nearby Seyfert galaxies, though it is not clear whether those γ rays originate from the nucleus. Blazars and their misaligned counterparts make up most of the ≳100 MeV extragalactic γ -ray background (EGB), and are suspected of being the sources of ultra-high energy cosmic rays. The future “Cherenkov Telescope Array”, in synergy with the Fermi-Large Area Telescope and a wide range of telescopes in space and on the ground, will write the next chapter of blazar physics.

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Les noyaux actifs de galaxie peuvent être de puissants émetteurs dans tout le domaine γ , du MeV au TeV, un phénomène dû à la présence de jets relativistes, en liaison avec un trou noir super-massif au centre de la galaxie hôte. La classe d'émetteurs de rayons γ la plus abondante parmi les noyaux actifs de galaxie, avec plus de 1500 sources établies aux énergies du GeV, et plus de 60 aux énergies du TeV, sont les « blazars ». Le paradigme actuel du blazar met en jeu un jet de plasma magnétisé, orienté à faible angle de la ligne de visée, et éjecté depuis le voisinage d'un trou noir accrétant et super-massif en rotation. Les observations permettent de distinguer deux types de blazars : les quasars radio à spectre plat (ou FSRQ) comprennent des champs de rayonnement externes puissants, des zones avec des raies d'émission optiques larges, et des tores de poussières. La classe des BL Lac (du nom d'un de ses membres, BL Lacertae) possède des flots d'accrétion plus faibles, dominés par l'advection, et dans lequel l'émission des rayons γ vient essentiellement de l'effet Compton inverse sur les photons synchrotron. Ce paradigme permet de modéliser l'émission des blazars sur tout le spectre électromagnétique. Cependant, beaucoup de problèmes fondamentaux restent sans réponse, notamment le rôle des processus hadroniques, et la variabilité très rapide de l'émission de certains objets BL Lac, ceux dont le spectre synchrotron émet le maximum de puissance dans les domaines UV et X. Les observations du satellite Fermi-LAT et celles des observatoires Tcherenkov au sol ont également mis en évidence une nouvelle classe de radio-galaxies émettrices de rayons γ , considérées comme les contreparties non alignées des blazars. On a aussi détecté l'émission de rayons γ de basse énergie provenant de galaxies de type Seyfert, mais il n'est pas encore sûr que cette émission vienne du noyau. Les blazars avec leurs contreparties non alignées sont à l'origine de la plus grande partie de l'émission gamma extragalactique diffuse au-dessus de 100 MeV, et sont soupçonnés d'être les sources des rayons cosmiques d'ultra-haute énergie. Le futur réseau Cherenkov Telescope Array (CTA), en synergie avec le télescope spatial Fermi et une grande variété de télescopes dans l'espace et au sol, écriront le prochain chapitre de la physique des blazars.

The full text of this article is available in PDF format.

Keywords : Active galactic nuclei, Gamma rays, Supermassive black holes

Mots-clés : Noyaux actifs de galaxie, Rayons gamma, Trous noirs supermassifs

1  The spin parameter is the ratio of the angular momentum a of the black hole to its mass M .
2  The altitude is the distance to the central nucleus projected along the rotation axis of the accretion-disk/torus system.
3  Similarly, the latitude is defined as the angle with respect to the accretion disk considered as an equatorial plane.
4  According to the Fanaroff–Riley classification [[5]], FR1 have radio jets that are brighter in the center, while FR2 object jets are fainter towards the center but feature brighter radio spots towards the end of their jets.
5  When the radiative zone is moving at relativistic velocity along a direction close to the line of sight, its apparent velocity as measured on the basis of the observer's proper time may be greater than c .
6  For the definition and the interest of the SED, see [[8]]; ν is the frequency and   is the power received per unit area and frequency.
7  According to TeVCat; see
8  VLBI: Very Long Baseline Interferometry.
9  The importance of γ rays in extragalactic jet astronomy otherwise received little attention at that time [[26]].
10  H.E.S.S.: High Energy Stereoscopic System, in Namibia; MAGIC: Major Atmospheric Imaging Cherenkov, in the Canary Islands; VERITAS: Very Energetic Radiation Imaging Telescope Array System, in Arizona.
11  As of 2015 May, no GRB has been detected with a ground-based VHE instrument.
12  The luminosity distance   is conventionally defined in such a way that the ratio of the absolute to the apparent luminosity (assuming isotropic emission) be equal to  .
13  When redshift is unknown, the measured   peak synchrotron frequency is an uncertain factor   smaller than the rest frame   peak synchrotron frequency.
14  If all sources with the same intrinsic isotropic luminosity were distributed uniformly in Euclidean space, the number N of sources with an apparent luminosity greater than S would vary like  ; for blazars, the slope of the cumulative distribution of   versus   does not have an index of −1.5.
15  When the angle θ between the velocity βc of the radiative zone and the line of sight n is small enough, the frequency of the jet emission, which relativistic aberration confines within a half-angle of   radians from β (with the Lorentz factor  ), is seen by the observer multiplied by a Doppler beaming factor  . This enhances significantly the observed flux by a factor   compared to what is expected from isotropic   flux-dependency.
16  Since c.g.s. units are used in most AGN SED's in the literature,   is here expressed in cm.
17  δ is the Doppler beaming factor mentioned above.
18  Astro-Rivelatore Gamma a Immagini L'Eggero, an Italian Space Agency project launched in April 2007.
19  The model assumes equipartition between non-thermal electron and magnetic-field energy densities.
20  Expressing that the size of the accelerator region should not be smaller than the accelerated particle's Larmor radius.

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