INTEGRAL view of AGN Angela Maliziaa,∗, Sergey Sazonovb, Loredana Bassania, Elena Piana, Volker Beckmannc, Manuela Molinaa, Ilya Mereminskiyb, Guillaume Belangerd aOAS-INAF, Via P. Gobetti 101, 40129 Bologna, Italy bSpace Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia cInstitut National de Physique Nuclaire et de Physique des Particules (IN2P3) CNRS, Paris, France dESA/ESAC, Camino Bajo del Castillo, 28692 Villanueva de la Canada, Madrid, Spain Abstract AGN are among the most energetic phenomena in the Universe and in the last two decades INTEGRAL’s contribution in their study has had a significant impact. Thanks to the INTEGRAL extragalactic sky surveys, all classes of soft X-ray detected (in the 2-10 keV band) AGN have been observed at higher energies as well. Up to now, around 450 AGN have been catalogued and a conspicuous part of them are either objects observed at high-energies for the first time or newly discovered AGN. The high-energy domain (20-200 keV) represents an important window for spectral studies of AGN and it is also the most appropriate for AGN population studies, since it is almost unbiased against obscuration and therefore free of the limitations which affect surveys at other frequencies. Over the years, INTEGRAL data have allowed to characterise AGN spectra at high energies, to investigate their absorption properties, to test the AGN unification scheme and to perform population studies. In this review the main results are reported and INTEGRAL’s contribution to AGN science is highlighted for each class of AGN. Finally, new perspectives are provided, connecting INTEGRAL’s science with that at other wavelengths and in particular to the GeV/TeV regime which is still poorly explored. Keywords: INTEGRAL, AGN, Seyferts, Blazars 2010 MSC: 00-01, 99-00 Contents 4.5 Spatial distribution . 14 4.6 Bolometric properties . 15 1 INTRODUCTION 1 5 MainResults: RadioGalaxies 15 2 From first detections to AGN cata- logues 3 6 MainResults:Blazars 17 3 AGN Types and Population studies 6 7 keV to GeV/TeV connection: a case study 20 4 MainResults: Seyfertgalaxies 8 7.1 TeVblazars .............. 22 arXiv:2009.03232v1 [astro-ph.HE] 7 Sep 2020 4.1 Broad Band studies and high energy cutoff ................. 8 8 INTEGRAL heritage and future per- 4.2 Absorption and Compton thick frac- spectives 22 tion .................. 10 4.3 Luminosity Function . 12 1. INTRODUCTION 4.4 Links to the cosmic X-ray background 13 Active Galactic Nuclei (AGN) are one of the most ∗ powerful phenomena in the Universe. After many Corresponding author Email address: [email protected] (Angela decades of observations and studies, our knowledge Malizia) of these objects has made enormous leaps forward. A Preprint submitted to Journal of LTEX Templates September 8, 2020 These galaxies are defined active because they have gion of lower velocity gas (< 900 km/s), the so- in their centre an accreting supermassive black hole called the Narrow Line Region (NLR), is located; 6 (SMBH) of masses higher than ≥10 M⊙ which ra- radiation which passes through this region produces diates across the entire electromagnetic spectrum, permitted, intercombination and forbidden narrow from the radio up to gamma-rays. Accretion, i.e. lines in the optical-UV band. the extraction of gravitational energy from matter Finally, the interaction between the supermassive infalling onto a black hole, is in fact the most effi- black hole’s rotating magnetic field and the accre- cient bulk mass-energy conversion process known. tion disc can also create powerful magnetic jets that The accreting matter orbits around the black hole eject material perpendicular to the disc at relativis- and, having some angular momentum, through dis- tic speeds and extend for hundreds to thousands of sipation of energy, flattens to form a disc where parsecs. magnetic viscosity transfers the angular momentum All these ingredients have contributed over the outward and the mass inward; the accretion disc years to explain a wide variety of AGN classes at together with the central black hole makes up the all wavebands and led to the postulation of the central engine of an AGN. Unified Theory of AGN (Antonucci et al., 1993; It is likely, whether the accretion rate is high or Urry and Padovani, 1995) which in its simplest ver- low, that the gravitational energy liberated by this sion hypothesises that the diversity of AGN can be process is radiated locally with a large fraction in largely explained as a viewing angle effect, although the form of thermal radiation from the surface of also accretion rate and efficiency play an important the disc, peaking in the optical/UV bands. A sig- role. nificant amount of these optical/UV photons are A main ingredient of this orientation-based reprocessed by a) dust located beyond the subli- model is the absorbing material, principally the mation radius and re-emitted in the infrared band optically and geometrically thick torus, which ob- (IR); and b) by a corona of hot electrons close to scures the nuclear regions of an active galaxy (the the accretion disc that up-scatters them via inverse accretion disc and the hot corona as well as the Compton in the soft/hard X-ray bands where AGN BLR). We optically classify an AGN as type 2 or emit a non negligible fraction of their luminosity type 1 depending on whether our line of sight inter- (Maraschi and Haardt, 1997; Zdziarski, 1998). cepts or not this obscuring material. Furthermore, However, disc and corona are just the inner part we classify an AGN as radio loud or radio quiet if of the nucleus of an AGN, and the proof that a its emission at radio frequencies is either strong or complex environment surrounds this region is that weak. Within the radio-loud class, differences in a large fraction of the radiation may be absorbed typologies (BL Lac objects, Flat Spectrum Radio by interstellar gas and dust close to the accretion Quasars and Radio Galaxies) have also been ex- disc and likely re-radiated in other wavelengths. In plained in terms of orientation, i.e. referring to one the classical picture of an AGN, surrounding the ac- or the other type if the jet axis is perfectly aligned cretion disc and the hot corona on 0.01–0.1 parsec with the observer’s line of sight or progressively mis- scale, there is a region of high velocity gas of 1000 aligned. – 5000 km/s, usually referred to as the Broad Line What clearly emerges from this AGN complex Region (BLR), which determines permitted and in- structure is that absorption is a key ingredient to tercombination broad emission lines in the optical understand the physics of these objects. For this spectra and can cause ionised absorption produc- reason, the hard X-ray band is the most appropri- ing characteristic features in the UV and soft X-ray ate for AGN population studies since it is almost bands (the so-called warm absorber). unbiased1 against obscuration and therefore free of At a distance of 1-100 pc from the BH is located the limitations which affect surveys at other fre- an obscuring optically thick torus of gas and dust. quencies, i.e. from optical to soft X-rays. Whether we consider this torus as dusty-compact Furthermore, the hard X-ray band represents an or dusty-cloudy, it is the main structure responsible important window for spectral studies of AGN. The for the absorption of the primary continuum; it can continuum of active galaxies in the X-ray band be so thick that it completely hides the primary emission up to several keV. 1 24 −2 Note that only objects with NH > 10 cm could be Going to distances greater than 100 pc up to kpc missed in these surveys due to their much dimmer flux which scales, a biconical shaped and highly structured re- prevents detections by current hard X-ray telescopes 2 is well explained by the Comptonisation process the CXB at high energy. which is described by a power law of photon in- In the last decades both INTEGRAL/IBIS dex (Γ) in the range 1.5 – 2 showing an exponential (Ubertini et al., 2003) and Swift/BAT cut-off (Ec) at around 100 keV. Reprocessing of X- (Barthelmy et al., 2005), having good sensi- ray photons from the surface of the disc, or from tivity and wide-field sky coverage, were able to more distant material, can produce in addition to make significant progress in the study of the fluorescent emission lines, also a hump at 20-30 keV high-energy domain (20-200 keV). In particular due to Compton reflection. Therefore high-energy they have provided a great improvement in our data are crucial to estimate the slope of the contin- knowledge of the high-energy extragalactic sky by uum emission over a wide energy band but also to detecting more than 1000 (mostly local) AGN at measure the high energy cut-off and the reflection energies above 15 keV. It is worth noting that, due fraction. These are important parameters because to the observational strategy, INTEGRAL plays a they enable us to understand the physical charac- key role in detecting new absorbed objects and in teristics and the geometry of the region around the particular AGN along the Galactic Plane, while central nucleus. In other words, in the framework Swift/BAT is more effective at higher Galactic of the disc-corona system, while the cut-off energy latitudes. This makes the two observatories fully is related essentially to the temperature kTe of the complementary also in the case of extragalactic electrons in the corona, a combination of the tem- studies.