aa r X i v : . [ a s t r o - ph . H E ] S e p INTEGRAL view of AGN
Angela Malizia a, ∗ , Sergey Sazonov b , Loredana Bassani a , Elena Pian a , Volker Beckmann c , ManuelaMolina a , Ilya Mereminskiy b , Guillaume Belanger d a OAS-INAF, Via P. Gobetti 101, 40129 Bologna, Italy b Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia c Institut National de Physique Nuclaire et de Physique des Particules (IN2P3) CNRS, Paris, France d ESA/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 ’scontribution in their study has had a significant impact. Thanks to the
INTEGRAL extragalactic skysurveys, all classes of soft X-ray detected (in the 2-10 keV band) AGN have been observed at higher energiesas well. Up to now, around 450 AGN have been catalogued and a conspicuous part of them are either objectsobserved 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 AGNpopulation studies, since it is almost unbiased against obscuration and therefore free of the limitations whichaffect surveys at other frequencies. Over the years,
INTEGRAL data have allowed to characterise AGNspectra at high energies, to investigate their absorption properties, to test the AGN unification scheme andto perform population studies. In this review the main results are reported and
INTEGRAL ’s contributionto 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 stillpoorly explored.
Keywords: INTEGRAL, AGN, Seyferts, Blazars2010 MSC: 00-01, 99-00
Contents1 INTRODUCTION 12 From first detections to AGN cata-logues 33 AGN Types and Population studies 64 Main Results: Seyfert galaxies 8 ∗ Corresponding author
Email address: [email protected] (AngelaMalizia)
Active Galactic Nuclei (AGN) are one of the mostpowerful phenomena in the Universe. After manydecades of observations and studies, our knowledgeof these objects has made enormous leaps forward.
Preprint submitted to Journal of L A TEX Templates September 8, 2020 hese galaxies are defined active because they havein their centre an accreting supermassive black hole(SMBH) of masses higher than ≥ M ⊙ which ra-diates across the entire electromagnetic spectrum,from the radio up to gamma-rays. Accretion, i.e.the extraction of gravitational energy from matterinfalling onto a black hole, is in fact the most effi-cient bulk mass-energy conversion process known.The accreting matter orbits around the black holeand, having some angular momentum, through dis-sipation of energy, flattens to form a disc wheremagnetic viscosity transfers the angular momentumoutward and the mass inward; the accretion disctogether with the central black hole makes up thecentral engine of an AGN.It is likely, whether the accretion rate is high orlow, that the gravitational energy liberated by thisprocess is radiated locally with a large fraction inthe form of thermal radiation from the surface ofthe disc, peaking in the optical/UV bands. A sig-nificant amount of these optical/UV photons arereprocessed by a) dust located beyond the subli-mation radius and re-emitted in the infrared band(IR); and b) by a corona of hot electrons close tothe accretion disc that up-scatters them via inverseCompton in the soft/hard X-ray bands where AGNemit a non negligible fraction of their luminosity(Maraschi and Haardt, 1997; Zdziarski, 1998).However, disc and corona are just the inner partof the nucleus of an AGN, and the proof that acomplex environment surrounds this region is thata large fraction of the radiation may be absorbedby interstellar gas and dust close to the accretiondisc and likely re-radiated in other wavelengths. Inthe classical picture of an AGN, surrounding the ac-cretion disc and the hot corona on 0.01–0.1 parsecscale, there is a region of high velocity gas of 1000– 5000 km/s, usually referred to as the Broad LineRegion (BLR), which determines permitted and in-tercombination broad emission lines in the opticalspectra and can cause ionised absorption produc-ing characteristic features in the UV and soft X-raybands (the so-called warm absorber).At a distance of 1-100 pc from the BH is locatedan obscuring optically thick torus of gas and dust.Whether we consider this torus as dusty-compactor dusty-cloudy, it is the main structure responsiblefor the absorption of the primary continuum; it canbe so thick that it completely hides the primaryemission up to several keV.Going to distances greater than 100 pc up to kpcscales, a biconical shaped and highly structured re- gion of lower velocity gas ( <
900 km/s), the so-called the Narrow Line Region (NLR), is located;radiation which passes through this region producespermitted, intercombination and forbidden narrowlines in the optical-UV band.Finally, the interaction between the supermassiveblack hole’s rotating magnetic field and the accre-tion disc can also create powerful magnetic jets thateject material perpendicular to the disc at relativis-tic speeds and extend for hundreds to thousands ofparsecs.All these ingredients have contributed over theyears to explain a wide variety of AGN classes atall wavebands and led to the postulation of theUnified Theory of AGN (Antonucci et al., 1993;Urry and Padovani, 1995) which in its simplest ver-sion hypothesises that the diversity of AGN can belargely explained as a viewing angle effect, althoughalso accretion rate and efficiency play an importantrole.A main ingredient of this orientation-basedmodel is the absorbing material, principally theoptically and geometrically thick torus, which ob-scures the nuclear regions of an active galaxy (theaccretion disc and the hot corona as well as theBLR). We optically classify an AGN as type 2 ortype 1 depending on whether our line of sight inter-cepts or not this obscuring material. Furthermore,we classify an AGN as radio loud or radio quiet ifits emission at radio frequencies is either strong orweak. Within the radio-loud class, differences intypologies (BL Lac objects, Flat Spectrum RadioQuasars and Radio Galaxies) have also been ex-plained in terms of orientation, i.e. referring to oneor the other type if the jet axis is perfectly alignedwith the observer’s line of sight or progressively mis-aligned.What clearly emerges from this AGN complexstructure is that absorption is a key ingredient tounderstand the physics of these objects. For thisreason, the hard X-ray band is the most appropri-ate for AGN population studies since it is almostunbiased against obscuration and therefore free ofthe limitations which affect surveys at other fre-quencies, i.e. from optical to soft X-rays.Furthermore, the hard X-ray band represents animportant window for spectral studies of AGN. Thecontinuum of active galaxies in the X-ray band Note that only objects with N H > cm − could bemissed in these surveys due to their much dimmer flux whichprevents detections by current hard X-ray telescopes
2s well explained by the Comptonisation processwhich is described by a power law of photon in-dex (Γ) in the range 1.5 – 2 showing an exponentialcut-off (E c ) at around 100 keV. Reprocessing of X-ray photons from the surface of the disc, or frommore distant material, can produce in addition tofluorescent emission lines, also a hump at 20-30 keVdue to Compton reflection. Therefore high-energydata are crucial to estimate the slope of the contin-uum emission over a wide energy band but also tomeasure the high energy cut-off and the reflectionfraction. These are important parameters becausethey enable us to understand the physical charac-teristics and the geometry of the region around thecentral nucleus. In other words, in the frameworkof the disc-corona system, while the cut-off energyis related essentially to the temperature kT e of theelectrons in the corona, a combination of the tem-perature and optical depth, τ , of the scattering elec-trons determines the spectral slope. Thus simulta-neous measurements of Γ and E c allow us to under-stand the physical parameters of the Comptonisingregion. Therefore, the more accurate are the mea-surements of these parameters, the better we candetermine the geometry and the physical proper-ties of the inner region of AGN.Finally, high-energy data provide also important in-formation about the AGN contribution to the cos-mic X-ray background (CXB). While the fractionof the CXB resolved into discrete sources is ∼ ∼
90% (Luo et al., 2017), ∼ ∼
35% (Harrison et al.,2016) in the 0.5–2, 2–7, 5–10 and 8–24 keV energybands, respectively, it becomes much lower (see sec-tion 4.4) at higher energies, near the peak of theCXB spectral intensity at around 30 keV. In or-der to reproduce the shape of the CXB, synthesismodels (e.g., Comastri et al. (2006)) need to useseveral parameters, such as the fraction of heav-ily obscured sources (the so-called Compton thickAGN characterised by N H ≥ cm − ), the cov-erage and the geometry of the cold gas distributedaround the black hole responsible for the reflectionhump, the photon index and high-energy cut-off ofthe primary continuum emission as well as the lu-minosity function in the energy range of interest.Therefore, the determination of such parameters,in particular photon indices and cut-off energies,their mean values, and their distributions over awide sample of sources, covering a broad range ofenergies (above 100 keV), is essential to obtain amuch firmer estimate of the AGN contribution to the CXB at high energy.In the last decades both INTEGRAL /IBIS(Ubertini et al., 2003) and
Swift /BAT(Barthelmy et al., 2005), having good sensi-tivity and wide-field sky coverage, were able tomake significant progress in the study of thehigh-energy domain (20-200 keV). In particularthey have provided a great improvement in ourknowledge of the high-energy extragalactic sky bydetecting more than 1000 (mostly local) AGN atenergies above 15 keV. It is worth noting that, dueto the observational strategy,
INTEGRAL plays akey role in detecting new absorbed objects and inparticular AGN along the Galactic Plane, while
Swift /BAT is more effective at higher Galacticlatitudes. This makes the two observatories fullycomplementary also in the case of extragalacticstudies.In this work we will review the contribution of
INTEGRAL and in particular of the imager IBISto AGN science, highlighting the most importantresults reached in its 17 years in orbit.
2. From first detections to AGN catalogues
As mentioned before, the most important appli-cation of
INTEGRAL has been for finding hardX-ray emitting sources along the Galactic Plane.The so-called ”Zone of Avoidance” refers to thearea comprised between ± ◦ , above and be-low the Galactic plane. Gas and dust obscurestarlight within this region and screen nearly allbackground extragalactic objects from traditionaloptical-wavelength surveys; in the optical, as muchas 20% of the extragalactic sky is obscured by theGalaxy. As a consequence, historically the GalacticPlane has not been a focus for extragalactic astron-omy. Hard X-rays ( ≥
10 keV) are however able topenetrate this zone providing a window that is vir-tually free of obscuration relative to optical wave-lengths and partly also to soft X-rays.The hard X-ray band has been poorly exploredbefore the
INTEGRAL and
Swift satellites and theonly previous truly all-sky survey conducted, datesback to the eighties. This pioneering work, madewith the
HEAO1 -A4 instrument (Levine et al.,1984), yielded a catalogue of about 70 sources downto flux level of typically 1/75 of the Crab (or 2-3 × − erg cm − s − ) in the 13-80 keV band.Only 7 extragalactic objects were reported in thissurvey: none of these objects is within 10 ◦ of theGalactic Plane and only two (Centaurus A and the3 igure 1: Evolution of source type and number throughthe five INTEGRAL
IBIS/ISGRI catalogs produced to date(courtesy of A. J. Bird)
Perseus cluster) are located below 20 ◦ in Galacticlatitude. Pointed observations by BeppoSAX /PDS(Frontera et al., 1997) have unveiled more sourcesbut observations were sometime limited by the nonimaging capability of the high energy instrument(PDS) which is particularly crucial in the GalacticPlane region.A decisive step forward in the exploitation ofthe entire hard X-ray sky has been possible thanksto the imager IBIS on board
INTEGRAL , whichsince the beginnings of the mission, allowed thedetection of AGN with a sensitivity up to a fewmCrab in the most exposed regions (i.e. the Galac-tic centre) with an angular resolution of 12 arcminand a point source location accuracy of 2-3 arcmin(Ubertini et al., 2003).The capabilities of
INTEGRAL /IBIS in study-ing extragalactic sources, were revealed soon af-ter the launch of the satellite and in particularduring the Core Programme which lasted for thefirst five years of the mission and consisted of 45- 35% of the total observing time. The Core Pro-gram (CP) was dedicated to key investigations anddevoted to regular surveys of the Galactic Planeand selected deep sky fields at high galactic lati-tudes. By observing around 9000 square degreesof the sky, the CP allowed the detection of adozen AGN as well as a quite large number ofnew unidentified objects firstly detected at high en-ergies. Most of these firstly detected AGN werebright Seyfert 2 systems, i.e. absorbed objects, 3 ofwhich were Compton thick, all located in the Galac-tic Plane (Bassani et al., 2004a; Soldi et al., 2006;Bassani et al., 2004b). Many of these AGN havebeen previously studied at energies above 20 keV and for them
INTEGRAL largely confirmed pre-vious findings. Among these, there is also the firstblazar detected by
INTEGRAL : PKS 1830-211 at arelatively high redshift ( z = 2.507) (Bassani et al.,2004b).Furthermore, IBIS/ISGRI data of the bright-est AGN have been used alone or in conjunc-tion with soft 2–10 keV data to perform dedi-cated studies (Sazonov et al., 2004b; Soldi et al.,2005; Beckmann et al., 2005) exploring their spec-tral characterisation.It became soon evident that the population ofAGN emitting above 20 keV was growing thanks tothe discovery that many of the new detections wereindeed active galaxies.The first IBIS survey (Bird et al., 2004), basedon the first year of INTEGRAL observations,counted only 5 AGN which became 33 (almost20% of the entire catalogue) in the second survey(Bird et al., 2006). A couple of dedicated AGN sur-veys were published by Beckmann et al. (2006) andBassani et al. (2006) which listed 42 and 66 sourcesrespectively. Both surveys highlighted the capabil-ity of
INTEGRAL to probe the extragalactic highenergy sky and most of all to find new and/or ab-sorbed active galaxies. Since 2004, a sequence ofIBIS all-sky survey catalogues (Bird et al., 2004,2006, 2007, 2009, 2016) based on data from the IS-GRI detector have been published at regular inter-vals, making use of an ever-increasing data set asnew observations become publicly available. Thelast edition of the IBIS all sky survey (Bird et al.,2016) lists 939 sources: is clearly visible from Fig-ure 1, the majority of these sources are AGN andnewly discovered (i.e. those with an IGR designa-tion) sources which, in large part, are expected tobe AGN after proper follow up. On the other hand,also deep extragalactic surveys have been producedover the years thanks to long pointings at spe-cific sky areas such as the Large Magellanic Cloud(Grebenev et al., 2012; Mereminskiy et al., 2016),the 3C 273/Coma and M81 regions (Paltani et al.,2008; Mereminskiy et al., 2016). Figure 2 takenfrom Mereminskiy et al. (2016), shows these threefields (M81 (exposure of 9.7 Ms), Large MagellanicCloud (6.8 Ms) and 3C 273/Coma (9.3 Ms)) in the17-60 keV band as seen by
INTEGRAL /IBIS after12 years (2003-2015) of observations.As the total number of known or newly dis-covered AGN grew, it was possible to assemblethem for population studies as done over the yearsby Malizia et al. (2012) and Malizia et al. (2016).4 igure 2: Hard X-ray maps of the M81, LMC and 3C 273/Coma fields as in Mereminskiy et al. (2016)
So far the number of AGN listed in this datasetamounts to more than 400 objects.It is also worth mentioning here the IBIS highenergy catalogues i.e. those collecting sourcesdetected above 100 keV. There were 10 AGNlisted in Bazzano et al. (2006) and 28 AGN inKrivonos et al. (2015), 7 of which detected also inthe 150–300 keV band. Furthermore, a catalogueproduced in 2008 by Bouchet et al. (2008) basedon SPI (the other primary wide field instrument on
INTEGRAL ) data, reported 34 AGN, 10 of whichdetected up to 200 keV and 4 in the 200–600 keVband.As mentioned above
INTEGRAL gave a fun-damental contribution in finding new high energyemitters, their number increasing more and morealong surveys. The
INTEGRAL community madea huge effort in the identification and classifica-tion of these new IGR sources. This led to the discovery of new AGN as well as of new classesof objects emitting at high-energies. However,the classification of a new, high-energy detectedsource is by no means trivial. First of all, onehas to reduce the positional uncertainty associ-ated with the high energy detection, which in somecases can be as high as 4–5 arcminutes. To dothis, 2-10 keV data are important because theyprovide a unique tool to associate the high en-ergy source with a single/multiple X-ray counter-part/s. When archival observations were not avail-able, follow-up campaigns have been performedwith all X-ray instruments such as
XMM , Chan-dra and
Swift /XRT which allowed also the spec-tral characterisation of the sources (Sazonov et al.,2005; Malizia et al., 2007; Sazonov et al., 2008b;Landi et al., 2010; Landi et al., 2017).Association of the X-ray source with an objectdetected at other wavelength is a fundamental step5n the analysis as it allows to pinpoint the cor-rect counterpart, locate it with arcsec accuracy andtherefore provide a way to study the source at otherwavelengths. Optical, IR or radio catalogues arethen searched in order to find the appropriate clas-sification of the object. If this search does not yieldany result, follow-up observations, above all in theoptical, are then planned and carried out.A parallel effort regarding the spectroscopic iden-tification of newly-detected or poorly studied hardX-ray sources has been performed by several groupsworldwide soon after the publication of the 1st IBISsurvey.To this date, this task has allowed the determina-tion of the nature of around 150 AGN, with nearly60% of them classified as broad emission line nuclei.The bulk ( ∼ INTE-GRAL and
Swift /BAT (e.g. Oh et al. (2018)) cat-alogues. Therefore, several identifications of
Swift hard X-ray emitters may be also accommodated inthe
INTEGRAL surveys (e.g., Rojas et al. (2017)and references therein; Karasev et al. (2018)).Thus, the total number of identifications reportedabove should actually be considered as a strict lowerlimit.This association/classification method has alsobeen used to construct
INTEGRAL
AGN cata-logues and so guarantees that all objects in thesecatalogues are fully characterised in terms of opticalidentification/classification as well as fully studiedin terms of X-ray spectral properties.
3. AGN Types and Population studies
The total number of AGN so far detected by
IN-TEGRAL , including recent additions, amounts to440 objects. In Figure 3 their 20–100 keV observedluminosity is plotted against redshift, differentiat-ing objects in three main optical classes: broad lineAGN (red filled squares), narrow line AGN (goldfilled circles) and blazars (blue stars). The lumi-nosities have been calculated for all sources assum-ing H =69.6 km s − Mpc − and q = 0. Figure 3: Observed hard X-ray (20-100 keV) luminosity ver-sus redshift for the whole
INTEGRAL
AGN sample. Goldfilled circles are narrow line AGN, red filled squares are broadline AGN and blue stars are balzars.
We find that the source redshifts span a rangefrom 0.00084 to 3.7 with a median of z =0.035,while the Log of 20-100 keV luminosities in ergs s − (assuming isotropic emission) ranges from 40.23 to ∼
48 with a mean at around 44. M81 (a Seyfert1.8/LINER) is the closest and least luminous AGNseen by
INTEGRAL , while IGR J22517+2218 (abroad line QSO) is the farthest and most luminousobject so far detected; the former hosts a black holeof mass M = 7 × M ⊙ while the latter housesa more massive one (M = 10 M ⊙ , Lanzuisi et al.(2012)). INTEGRAL also detected NGC 4395, aSeyfert 2 galaxy which hosts a black hole of about10 M ⊙ ; this mass has recently been estimatedthrough reverberation mapping of the broad lineregion and resulted to be among the smallest cen-tral black hole masses ever reported for an AGN(Woo et al., 2019). In conclusion, the INTEGRAL sample spans a large range in source parametersand is therefore representative of the population of6GN selected in the hard X-ray band.After 17 years of
INTEGRAL surveying the ex-tragalactic sky, we can now say that all classes ofAGN that are seen in the 2-10 keV band, are alsodetected at higher energies. In the pie chart of Fig-ure 4 the main classes seen by
INTEGRAL are high-lighted: it is evident from the figure that a largefraction is made up of Seyfert galaxies, equally di-vided in type 1 and 2; the second most numerousclass is that of blazars, followed by a small num-ber of objects of other classifications which are alsointeresting to study. The large database accumu-lated, allowed over the years to probe new AGNclasses, to investigate the absorption properties ofactive galaxies, to test the AGN unification schemeand to perform population studies.For example
INTEGRAL has detected for thefirst time at high energies Narrow Line Seyfert1 galaxies (NLS1) (Malizia et al., 2008). Theseare interesting targets as they are characterisedby unique properties when compared to theirbroad line analogues, both in the optical (see e.g.Osterbrock and Pogge (1985)) and in the X-rays,where they show stronger variability (both in fluxand spectral shape) and steeper power law spectra.The most widely accepted explanation for thesedifferences is that NLS1 have smaller black holemasses than normal Seyfert 1s; however their lumi-nosities are comparable (Pounds et al., 1995), sug-gesting that they must be emitting at higher frac-tions of their Eddington luminosity and thereforeshould also have higher fractional accretion rates. Aplausible scenario suggests that black holes in NLS1have not yet been fed enough to become massiveand are in a rapidly growing phase (Mathur, 2000);if NLS1 are indeed in an early phase of black holeevolution, then they are key targets for the study ofAGN formation and evolution. Within the INTE-GRAL AGN sample only 15 objects are classifiedas NLS1 (or 3% of the sample). Most of these ob-jects have been studied in detail by Panessa et al.(2011) who found that hard X-ray selected NLS1show variability over a broad range of X-ray fre-quencies, lack a strong soft excess, and often displayfully or partially covering absorption. As expected,NLS1 detected at high energies by
INTEGRAL arealso associated to small black hole masses and oc-cupy the lower tail of the Eddington ratios distri-bution with respect to classical NLS1.Another interesting type of AGN first detectedin hard X-rays by
INTEGRAL are the XBONGs,i.e. X-ray Bright Optically Normal Galaxies
Figure 4: Pie chart of the main classes of AGN in the
IN-TEGRAL sample. (Comastri et al., 2002) which are bright in X-rays(X-ray luminosity of 10 -10 erg s − ) but are op-tically dull, i.e. they are hosted by normal galax-ies whose optical spectra show no emission lines.Over the years, XBONG have shown to be a mixedbag of objects primarily including normal ellipticalgalaxies and AGN whose optical nuclear spectrumis probably diluted by the strong stellar continuum. INTEGRAL
XBONG (6 detected so far) are allheavily absorbed in X-rays (Log N H from 23 tomore than 24 cm − ) and are quite bright above 20keV (L − keV in the range 10 – 10 erg s − ).These are luminosities typical of AGN which im-plies that INTEGRAL can detect heavily absorbedobjects whose hosting galaxy outshines the activenucleus; these AGN would normally be missed inoptical surveys.Equally absorbed and bright are the majorityof LINERs (Low-Ionization Nuclear Emission Re-gions) seen by
INTEGRAL and in fact many ofthem are classified as LINERs of type 2 i.e. showingonly narrow lines in their optical spectra. There are20 LINERs in the
INTEGRAL
AGN sample andonly 2 are of type 1, i.e. unabsorbed. Also LIN-ERs have been detected at high energies for thefirst time by
INTEGRAL (first identifications byMasetti et al. (2008b)), clarifying the controversialorigin (AGN versus starburst) of the ultimate powersource of these objects. Given their high 20-100keV luminosities, it is almost certain that all
IN-TEGRAL
LINERs are powered by an AGN, evenif their mean luminosity ( ∼ × erg s ) isslightly lower than that of classical Seyfert galax-ies ( ∼ × erg s ). The result that emergedby the INTEGRAL studies (Malizia et al., 2012),is that LINERs are numerous as a class and like7eyfert galaxies, come in two flavours: unabsorbedtype 1 and absorbed type 2, with
INTEGRAL hav-ing the capability of detecting the second type inlarge numbers.Regarding population studies, Beckmann et al.(2009) first used a sample of
INTEGRAL detectedAGN to study source parameters on a large scale.An interesting result that emerged from this studyis the significant correlation found between the hardX-ray and optical luminosity and the mass of thecentral black hole in the sense that more luminousobjects appear also to be more massive. This find-ing allowed to construct a black hole fundamentalplane similar to the one found using radio data (L V ∝ L . X M . BH ).The accurate study, at optical and X-rays fre-quencies of all INTEGRAL
AGN has also allowedthe study of the correlation between optical classi-fication and X-ray absorption thus allowing to per-form a strong test on the AGN unification model.Although the presence of a correlation is expectedand indeed found, i.e. type 1 AGN are typicallyunabsorbed while type 2 AGN are often absorbed,the strength of the correlation is never 100%. Usinga large sample of
INTEGRAL
AGN, Malizia et al.(2012) have found that only a small percentage ofsources (12.5%) does not fulfil the expectation ofthe Unified Theory and looking in depth at theseoutliers concluded that the standard-based AGNunification scheme is followed by the majority ofbright AGN. The only outliers are absorbed type1 AGN characterised by complex X-ray absorptionlikely due to ionised gas located in an accretion discwind or in the biconical structure associated to thecentral nucleus and so unrelated to the torus. Theother outliers are type 2 AGN which do not showX-ray absorption but this could be either due tovariability (meaning that their different optical/X-ray classifications can be explained in terms of statetransitions and/or non simultaneous X-ray and op-tical observations) or to a high dust-to-gas ratio.Finally, Panessa et al. (2015) studied the radioproperties of a complete sample of
INTEGRAL de-tected AGN, found a significant correlation betweenthe radio flux at 0.8/1.4 GHz and the 20-100 keVflux, with a slope between these two parametersconsistent with that expected for radiatively effi-cient accreting systems. This indicates that thehigh-energy emission coming from the inner accre-tion regions correlates with the radio emission av-eraged over hundreds of pc scales (i.e., thousandsof years).
Figure 5: Combined
INTEGRAL and (non-simultaneous)
Fermi /LAT unfolded spectrum of Cen A as inBeckmann et al. (2011). The data are modelled by adouble broken power-law with individual normalisation forthe data of different epochs. The highest energy bins forIBIS/ISGRI and SPI are only upper limits.
4. Main Results: Seyfert galaxies
The study of the X/hard-X ray emission of AGN,i.e. from 2 keV to ≥
100 keV, is a fundamental toolin order to have a direct probe of their innermostregions. As said before, the primary continuumover this broad band can be roughly representedby a cut-off power law produced by the Comptoni-sation mechanism which is believed to arise in thecorona, close to the central super-massive blackhole. Clearly to have an overview of the physicsand structure of the corona we need to study thebroad-band spectra of a large sample of AGNin order to account for all spectral components,remove the degeneracy between parameters andtherefore being able to obtain a precise estimateof the photon index and high-energy cut-off for alarge number of objects. These two parametersare, as said, strictly linked to the temperature andthe optical depth of the corona (Petrucci et al.,2001).Before
INTEGRAL and
Swift /BAT, broad bandspectra were available only for a limited numberof bright AGN, basically due to the scarcity ofmeasurements above 10-20 keV, with most ofthe information coming from broad-band spectraprovided by the
BeppoSAX satellite which had abroad energy coverage (0.1-100 keV) but no imag-ing capability above 10 keV. Analysis of
BeppoSAX data of types 1 and 2 AGN (Perola et al., 2002;8adina, 2007; Malizia et al., 2003) gave evidencefor a wide range of values for the cut-off energy,ranging from 30 to 300 keV and higher, and anapparent different distribution of photon index andreflection parameter in different classes of AGN,e.g. Malizia et al. (2003).Spectral parameters of a few individual local brightAGN have been measured using
INTEGRAL datawith the addition of very high energy data as in thecase of Cen A (Beckmann et al., 2011), see Figure5. More sources have been studied using
INTE-GRAL data in conjunction with those of otherX-ray satellites, mainly
Swift /XRT and
XMM-Newton , e.g. NGC 4151 (Lubi´nski et al., 2010),NGC 4388 (Beckmann et al., 2004), NGC 2110(Beckmann and Do Cao, 2010), NGC 4945(Fedorova and Zhdanov, 2016) and many others.Spectral studies have also been performed forthe first time for AGN located in the Galac-tic Plane such as for GRS 1734-292 and other6 objects as in Molina et al. (2006). Further-more, thanks to the good statistical qualityof the
INTEGRAL data, for these local AGN,flux variability and spectral changes have beenstudied for the first time for many of them(e.g. Soldi et al. (2008); Fedorova and Zhdanov(2016); Molina et al. (2013)). Although snap-shot observations in the soft X-rays (typicallyfrom
XMM-Newton or from
Swift /XRT) and time-averaged (on timescales of years) measurementsat high energies are not contemporaneous andacquired differently, the match between the twois generally good, with the cross-calibration con-stant between the two instruments being typicallyaround 1 (Panessa et al., 2008; de Rosa et al., 2012;Molina et al., 2013; Fedorova et al., 2011). Broad-band spectral analysis of the brightest Setyfert 1galaxies as in Panessa et al. (2008) and Seyfert 2 asin de Rosa et al. (2012), have also been performeddefining the characteristics of the two classes onquite large samples of objects.The broad-band spectral analysis of 41 Seyfert1 galaxies belonging to the
INTEGRAL completesample (Malizia et al., 2009) performed by fittingtogether
XMM , Swift /BAT, and
INTEGRAL /IBISdata in the 0.3–100 keV energy band allowedMalizia et al. (2014) to confirm the distribution ofphoton indices (Γ= 1.73 with standard deviation of0.17) and, for the first time, to provide the high-energy cut-off distribution for a large sample of ob-jects. Malizia et al. (2014) found that the meanhigh energy cut-off was 128 keV with a spread of 46
Figure 6: High-energy cut-off distribution of the entire sam-ple from Malizia et al. (2009). The diagonally hatched his-togram represents sources for which only lower limits of E c are available. keV (see Figure 6), clearly indicating that the pri-mary continuum typically decays at much lower en-ergies than previously thought. This value is morein line with the synthesis models of the cosmic dif-fuse background, which often assume an upper limitof ∼
200 keV for the cut-off (Gilli et al. (2007), seealso section 4.4).In Malizia et al. (2014) the main parameters ofthe primary continuum have been estimated by em-ploying a baseline phenomenological model (
PEXRAV model in
XSPEC ) composed of an exponentially cut-off power-law reflected from neutral matter. At softenergies, when required by the data, intrinsic ab-sorption in terms of simple or/and complex, coldor ionised absorbers have been added and whenspectra showed clear signs of a soft excess, this hasbeen generally fitted with a thermal component. Agaussian line has also been included, to take intoaccount the presence of the iron k α line at around6.4 keV; and when present residuals around 7 keV,these have also been fitted adding a second gaussianline to take into account the iron k β feature.With these spectral parameters Malizia et al.(2014), following Petrucci et al. (2001), have beenable to evaluate the actual physical parameters ofthe Comptonising region by assuming the plasmatemperature to be kT e =E c /2, for optical depth τ ∼
1, and kT e =E c /3 for τ ≫
1. The mostprobable range of plasma temperatures kT e derivedfrom this study was found to be in the range 20 to100 keV (or 2 - 12 × K). Assuming the averagevalue of Γ=1.73 and taking into account both lowand high values of E c , acceptable solutions for τ in the range 1 to 4 have been obtained. These re-sults are in good agreement with those previouslyfound by Petrucci et al. (2001) for a small sampleof Seyfert 1 galaxies with BeppoSAX observations,and by Beckmann et al. (2009) applying a cut-offpower law model to the stacked JEM-X plus IBISspectral data of Seyfert 1 (E c = 86 +21 − keV).Furthermore, the high energy cut-off distributionof the INTEGRAL complete sample of type 1AGN is in good agreement with what found byLubi´nski et al. (2016) employing a more physicalmodel (
COMPPS model) on a sample of 28 brightAGN (type 1 and 2) and analysing
INTEGRAL data together with soft X-ray ones acquired by
XMM-Newton , Suzaku and
RXTE . Lubi´nski et al.(2016) tested several model options assuming athermal Comptonisation of the primary continuumaccompanied by a complex absorption and a Comp-ton reflection. They accurately determined themean temperature of the electron plasma to be 26 ≤ kTe ≤
62 keV for the majority of the sampleobjects with only two sources exhibiting tempera-tures kT e >
200 keV. These low temperatures ofthe electron plasma obtained in all these studies,imply that the template Seyferts spectra used in thepopulation synthesis models of AGN should be re-vised: the most important consequence of a shiftedhigh-energy cut-off will be a considerably smallerfraction of CT AGN needed to explain the peak ofthe CXB spectrum.Most of the works mentioned above, made use ofnon-simultaneous low versus high energy data, anddespite the introduction of cross-calibration con-stants to properly take into account flux variabil-ity (and possible mismatches in the instrumentscalibration), some degree of uncertainty remainssince spectral variability cannot be excluded a pri-ori . Therefore, if one wants to remove this ambi-guity, it is fundamental to have simultaneous ob-servations in both the soft and hard X-ray bandsand this is now achievable with
NuSTAR . In a re-cent work Molina et al. (2019) presented the 0.5 –78 keV spectral analysis of 18 broad line AGN be-longing to the
INTEGRAL complete sample, thosefor which simultaneous
Swift /XRT and
NuSTAR observations were available. Employing the same simple phenomenological model to fit the data asin Malizia et al. (2014), these authors found a meanhigh-energy cut-off of 111 keV ( σ = 45 keV) for thewhole sample, in perfect agreement with what pre-viously found employing INTEGRAL data. Thesefindings also confirm that simultaneity of the ob-servations in the soft and hard X-ray band is notessential once flux and spectral variability are prop-erly accounted for.Finally, compatible values of photon index andcut-off values have been found by Molina et al.(2013) using only high energy data i.e.
INTE-GRAL /IBIS and
Swift /BAT. Furthermore, thisstudy allowed also a cross-calibration between thetwo instruments, finding general good agreementbetween BAT and IBIS spectra, despite a system-atic mismatch of about 22 per cent in flux normal-isation.These and other high-energy spectral studiesprompted Fabian and collaborators (Fabian et al.,2015, 2017) to propose the pair thermostat modelto explain the observations. In the compact-ness/temperature diagram, AGN coronae which arehot and radiatively compact, are located close tothe boundary of the region which is forbidden dueto runaway pair production. Pair production andannihilation can be considered essential ingredi-ents in AGN corona physics and strongly affect theshape of the observed spectra. In fact, if photonsare energetic enough, the subsequent increase inluminosity produces electron-positron pairs ratherthan an increase in temperatures, until a point ofequilibrium is reached. At this point, pair pro-duction consumes all the available energy, there-fore limiting the coronal temperature; electron-positron pair production becomes a runaway pro-cess thus acting as a sort of thermostat. Pair pro-duction from the non-thermal component (hybridplasma compared to pure thermal plasma) can re-duce the temperature leading to a much wider rangeof values, more consistent with present observations(Fabian et al., 2017).
As said before, an obscuring ’torus’ is believedto be responsible for the type 1 and 2 division ofSeyfert galaxies and quasars. What is the actualstructure of the torus and what is its physical rela-tion to other structural components of AGN such asthe accretion disc and broad-line region, is still un-clear although progress in understanding this issuehas been recently made (Ricci et al., 2017; H¨onig,10019). These are some of the central questions inAGN research and hard X-ray surveys provide amore direct answer than studies at lower energiesbecause these surveys are not affected by absorp-tion bias except for very heavy absorption.When an X-ray telescope observes an AGNthrough the torus, the measured spectrum will ex-hibit a characteristic low-energy cutoff due to thephotoabsorption of the soft radiation in the gas anddust of the torus (and perhaps also in the enclosedbroad-line region), which allows one to estimate theabsorption column density, N H , along the view-ing direction. A statistical analysis of absorbingcolumns determined in this way (or by a similarbut more model-dependent method for Compton-thick AGN) for a representative sample of objectsmakes it possible to infer the typical optical depthand covering fraction of the torii in the AGN pop-ulation. With this understanding, a lot of effortshave been put into follow-up X-ray spectral obser-vations of INTEGRAL (and
Swift ) detected AGN(see the previous sections), which has eventually re-sulted in a unique and well characterised sample oflocal ( z . .
2) hard X-ray selected AGN.One of the earliest attempts to use
INTEGRAL data for AGN absorption studies was undertakenby Beckmann et al. (2006). They made use of asample of 38 Seyfert galaxies detected by IBIS inthe first year of the mission, with absorption in-formation available for 32 of them. Already thislimited statistics provided a hint that the fractionof absorbed ( N H > cm − ) AGN decreaseswith increasing hard X-ray (20–40 keV) luminosity.Although the statistical significance of this resultwas very low, it was consistent with other emerg-ing indications of such a trend both in the local(Sazonov and Revnivtsev, 2004; Markwardt et al.,2005) and distant (Steffen et al., 2003; Ueda et al.,2003) Universe. The Beckmann et al. (2006) sam-ple included 4 Compton-thick ( N H > cm − )objects, all previously known (NGC 1068, Mrk 3,NGC 4945 and Circinus galaxy), implying that theobserved fraction of such AGN is ∼
10% or some-what higher, taking into account the absence of ab-sorption information for several objects.Malizia et al. (2007) carried out a statisticalanalysis of 38 new
AGN discovered by
INTE-GRAL /IBIS (some of these objects have also beenindependently found by
Swift /BAT) and followedup with
Swift /XRT. Sixteen objects proved to beabsorbed AGN and three others were suggested tobe Compton-thick based on the low ratios of the
Figure 7: Observed distribution of absorption columns. Un-absorbed ( N H < cm ), weakly absorbed (10 ≤ N H ≤ cm ) and Compton-thick ( N H ≥ cm ) objectsare shown in blue, magenta and red, respectively. From(Sazonov et al., 2015). observed fluxes in the 2–10 keV and 20–100 keVenergy bands. Therefore, the inferred fractionsof absorbed and Compton-thick objects ( ∼ ∼ | b | > ◦ . Theenhanced sample provided increased evidence thatthe fraction of absorbed AGN decreases with in-creasing luminosity (17–60 keV). The observed frac-tion of Compton-thick objects was again found tobe ∼
10% with an upper limit of ∼
20% (takinginto account missing information on the absorptioncolumns for some of the objects).Later on, several more studies (Malizia et al.,2009; Beckmann et al., 2009; Sazonov et al., 2010,2015) have taken advantage of the ever increasingcatalogue of
INTEGRAL detected AGN and follow-up efforts to tighten the constraints on the absorp-tion properties of the local AGN population. Inparticular, using
INTEGRAL selected AGN, it wasassessed for the first time that even at high energy11 igure 8: Observed fraction of absorbed AGN as a functionof observed hard X-ray luminosity. From (Sazonov et al.,2015). a bias in the estimation of the fraction of Comp-ton thick sources still exists (Malizia et al., 2009).Their flux is reduced due to sensitivity limit, butif corrected their fraction turns out to be of ∼ Swift /BAT data (Burlon et al., 2011).An important step forward has been made bySazonov et al. (2015) using a sample of 151 localSeyfert galaxies detected in the 17–60 keV energyband by IBIS (at | b | > ◦ ), from the 7-year all-skycatalogue of Krivonos et al. (2010). This sample ishighly complete in terms of supplementary infor-mation (optical types, distances and X-ray absorp-tion columns) and consists of 67 unabsorbed ( N H < cm − ) and 84 absorbed ( N H > cm − )objects including 17 proven or likely Compton-thick ( N H > cm − ) AGN (see Figure 7 andSemena et al. (2019)). The observed fractions, i.e.without the correction for the absorption bias, ofabsorbed and Compton-thick objects turned out tobe nearly the same ( ∼
60% and ∼ ∼ Swift /BAT survey (Ricci et al., 2015). Sazonov et al. (2015) decisively ascertained thedeclining trend of the observed fraction of absorbedAGN with increasing luminosity (see Figure 8).A similar dependence has been independently es-tablished for the local AGN population using the
Swift /BAT hard X-ray survey (Burlon et al., 2011)and for higher-redshift AGN with surveys con-ducted in the standard X-ray band (correspondingto the hard X-ray band in the rest-frame of quasarsat z &
1) (Ueda et al., 2014). Sazonov et al. (2015)suggested that this may be at least partially a se-lection effect, because not only hard X-ray, flux-limited surveys are negatively biased with respectto Compton-thick AGN, but hard X-ray selectionis also positively biased with respect to unabsorbedones (due to the reflection of part of the cen-tral source’s hard X-ray radiation towards the ob-server). This implies that the intrinsic fraction ofabsorbed sources at a given luminosity must behigher than the observed one. Sazonov et al. (2015)further speculated that there is possibly no intrin-sic declining trend of this fraction with luminosityif the hard X-ray emission from the accretion disc’scorona is weakly collimated along its axis, as canwell be the case. If so, the covering fractions of thetorii in the local Seyfert galaxies should typicallybe as high as ∼ Together with
Swift /BAT,
INTEGRAL /IBISmulti-year observations have enabled an accuratedetermination of the hard X-ray luminosity func-tion of local Seyfert galaxies. Apart from statis-tically characterising the supermassive black holeactivity in present-day galactic nuclei, such a mea-surement is crucial for studying the cosmic his-tory of black hole growth, since it provides an all-important z = 0 reference point for the models ofAGN evolution based on deep, pencil-beam X-raysurveys.As already mentioned, shortly preceding the IN-TEGRAL measurements in the hard X-ray band,the AGN luminosity function was measured inthe softer band of 3-20 keV in the XSS survey(Sazonov and Revnivtsev, 2004). This result was12ased on 95 local Seyfert galaxies and representeda substantial improvement over previous estimatesobtained at energies below 10 keV thanks to thesignificantly reduced bias with respect to absorbedAGN and fairly large sample of objects. Nev-ertheless, XSS was still strongly biased againstsources with N H > cm − and it was clear thatyet harder (above 15 keV) large-area surveys wereneeded for a robust measurement of the luminosityfunction of local AGN.The first AGN luminosity function basedon INTEGRAL results was computed byBeckmann et al. (2006) in the 20–40 keV en-ergy band using the aforementioned sample of 38non-blazar AGN detected by IBIS over the firstyear of the mission. Most of the objects werenearby (the average redshift is 0.022). The derivedluminosity function could be described by a brokenpower law, an empirical model commonly usedin fitting AGN luminosity functions in variouswavebands. Afterwards, Sazonov et al. (2007)made a more precise measurement of the hardX-ray luminosity function using a larger sample ofIBIS-detected AGN (66 Seyfert galaxies).The most recent version of the hard X-ray(17–60 keV) luminosity function of local AGNbased on
INTEGRAL /IBIS data is presented inSazonov et al. (2015) (see Figure 9). As mentionedbefore, these authors used a sample of 151 Seyfertgalaxies, which, compared to previous ones, cov-ers broader ranges of distances and luminosities: z ∼ . . L ∼ × –2 × erg s − .It may nevertheless be considered a local one sincemost of the objects are located at z < .
1. The de-rived luminosity function is in good agreement withthat based on the
Swift /BAT survey (Ajello et al.,2012). Apart from further tightening the parame-ters of the observed hard X-ray luminosity function,Sazonov et al. (2015) have also reconstructed the intrinsic luminosity function of local AGN by cor-recting for the observational biases related to X-rayabsorption and reflection discussed in the precedingsection. As a consequence, they determined the to-tal intrinsic hard X-ray (17–60 keV) luminosity den-sity of local AGN (with luminosities between 10 . and 10 . erg s − ): ∼ . × erg s − Mpc − .It is important to note that although there issubstantial room for further improvement of localAGN statistics at the low end of the luminosityfunction (below ∼ erg s − ), in particular withthe eROSITA and ART-XC telescopes aboard therecently launched Spektr-RG satellite, there is no
Figure 9: Observed hard X-ray luminosity function of localAGN (circles) fitted by a broken power law (solid line). Forcomparison, the luminosity function based on the
Swift /BATsurvey (Ajello et al., 2012) is shown by the dashed line. From(Sazonov et al., 2015). such possibility at the high end ( L & erg s − )since INTEGRAL /IBIS and
Swift /BAT have al-ready probed the whole Universe out to z ∼ . For a significant fraction of the extragalactic sky,in particular in the M81, LMC and 3C 273/Comafields, IBIS observations have now reached a depthof ∼ . N –log S function of local Seyfertgalaxies down to ∼ × − erg s − cm − (17–60 keV) (see Figure 10, Mereminskiy et al. (2016)).Although just ∼
2% of the CXB is resolved intopoint sources at these fluxes, the census of AGNconducted locally (at z . .
2) by
INTEGRAL and
Swift is now nicely complemented by
NuSTAR athigher redshifts ( z .
1) (see Figure 10). Togetherwith findings from deep extragalactic surveys per-formed in the standard X-ray band, these resultshave substantially tightened the constraints on thecomposition of the CXB.In this context, an important result was pre-sented by Sazonov et al. (2008a), who performed13 igure 10: AGN number-flux relations in hard X-rays measured by
NuSTAR (Harrison et al., 2016),
Swift /BAT(Ajello et al., 2012), in the
INTEGRAL /IBIS all-sky survey (Krivonos et al., 2010) and in the
INTEGRAL /IBISdeep fields (Mereminskiy et al., 2016). The shaded area demonstrates a flux region not yet probed by hard X-raymissions. The inset is a zoom of the range of the used data, see Mereminskiy et al. (2016). a stacking analysis of the X-ray spectra of AGN de-tected in the all-sky surveys performed by
INTE-GRAL /IBIS and
RXTE (the aforementioned XSSsurvey), taking into account the space densities ofAGN with different luminosities and absorbing col-umn densities, i.e. the luminosity function and N H distributions discussed above. They obtained thebroad-band (3–300 keV) spectral energy distribu-tion of the summed emission of the local AGN (seeFigure 11). It exhibits (albeit with low significance)a cutoff at energies above 100–200 keV, in line withresults obtained from broad band spectral studiesof local AGN (see previous section (4.1)). It turnedout that this locally determined spectrum is con-sistent with the CXB spectrum if the AGN popu-lation has experienced a pure luminosity evolution(so-called downsizing) between z ∼ z = 0, asis indeed indicated by the results of Chandra and
XMM-Newton deep X-ray surveys (Barger et al.,2005). This nicely demonstrates that the popularconcept of the CXB being a superposition of AGNis generally correct.
In the present-day Universe, matter is distributedvery inhomogeneously on scales smaller than 100–
Figure 11: Cumulative spectrum of the local Seyfert galaxies,based on
INTEGRAL and
RXTE data (solid line with thecorresponding hatched uncertainty region) The dashed anddotted lines show the contributions of low- ( < . erg s − )and high- ( > . erg s − ) luminosity AGN, respectively.The estimated contribution of blazars is also shown. From(Sazonov et al., 2008a). igure 12: 2D-map (in Galactic coordinates) of the AGNnumber density in the local Universe compared to that ofnormal galaxies. See (Krivonos et al., 2007) for details.
200 Mpc. The contrast in matter density be-tween large galaxy concentrations (superclusters)and voids can reach an order of magnitude andmore. The vast majority of galaxies in the localUniverse is believed to contain supermassive blackholes in their nuclei. Most of them are currentlydormant or only weakly active but a significant frac-tion ( ∼ INTEGRAL all-sky hard X-raysurvey provides a virtually unobscured view of theAGN population out to a few hundred Mpc, we havea unique opportunity to verify that AGN trace thecosmic large-scale structure.Such a test has been done by Krivonos et al.(2007). They focused on the local volume of 70 Mpcradius, where maximal contrasts in galaxy countsare expected. Based on a subsample of ∼
40 Seyfertgalaxies detected by IBIS, they demonstrated thatAGN do follow closely the large-scale structure ofthe Universe, strongly concentrating in such well-known structures as the Virgo cluster, Great At-tractor and Perseus-Pisces supercluster (see Fig-ure 12). This result has subsequently been con-firmed using the
Swift /BAT survey (Ajello et al.,2012).
Hard X-ray observations primarily probe theemission produced in the hot corona of the accre-tion disc. Although this high-energy componentconstitutes a significant fraction of the bolomet-ric luminosity of an accreting supermassive black hole, it is subdominant with respect to the softer(mainly UV) emission produced in the disc itself.A large fraction of the latter is in turn convertedinto even softer, infrared radiation in the surround-ing dusty torus. In order to better understand theinternal structure of AGN and physical processestaking place there, it is important to measure howthe AGN bolometric luminosity is partitioned be-tween these three main emission components (in ra-dio galaxies and blazars, there may be an additionalsignificant contribution from a relativistic jet, seebelow).To this end, Sazonov et al. (2012) have utilisedproprietary and archival data of
Spitzer infrared ob-servations for a sample of 68 local Seyfert galaxiesdetected by IBIS. They found a clear correlationbetween their hard X-ray and mid-infrared (MIR)luminosities: L µm ∝ L . ± . , where L µm isthe monochromatic luminosity at 15 µ m and L HX is the luminosity in the 17–60 keV energy band. As-suming that the observed MIR emission is radiationfrom an accretion disc reprocessed in a torus thatsubtends a solid angle decreasing with increasingluminosity (as inferred from the declining fractionof absorbed AGN, see above), the authors inferredthat the intrinsic disc luminosity, L disc , is approxi-mately proportional to the luminosity of the corona, L corona , namely L disc / L corona ∼
2. This ratio isa factor of ∼ L bol , of Seyfert galaxies, with a typical ratio L bol /L HX ∼
9. This, together with black hole massestimates available for the same sample of AGN,was used by Khorunzhev et al. (2012) to infer theEddington ratios of these objects, which turned outto lie between 1 and 100% for the majority of them.Finally, Sazonov et al. (2012) estimated the cumu-lative bolometric luminosity density of local AGN,which turns out to be ∼ × erg s − Mpc − .
5. Main Results: Radio Galaxies
Radio galaxies are sources showing on radio mapsan extended structure with lobes and jets. Histor-ically, they have been divided into two classes onthe basis of their radio morphology: FRI, havingbright jets in the centre and low total luminosityand FRII, having faint jets but bright hot spots atthe ends of the lobes and high total luminosities15Fanaroff and Riley, 1974). The different morphol-ogy probably reflects the method of energy trans-port in the radio source: FRIIs appear to be able totransport energy efficiently to the ends of the lobes,while FRI beams are inefficient in the sense thatthey radiate a significant amount of their energyaway as they travel. The cause of the FRI/FRIIdifference is still unknown and both external prop-erties (environment, host galaxy, merging history,etc.) or intrinsic factors (different accretion pro-cesses) have been used to explain this dichotomywithout reaching up to now firm conclusions.A further subdivision among radio galaxies hasrecently come from their optical spectroscopic prop-erties (Buttiglione et al., 2010): in general, objectswith and without high-excitation emission lines intheir optical spectra are referred to as High Excita-tion Radio Galaxies (HERG) and Low ExcitationRadio Galaxies (LERG) respectively. HEG accretein a radiatively efficient manner due to high Ed-dington ratios ( ≥ ≤ INTEGRAL /IBIS up to 2016, (Malizia et al., 2016)only 32 (i.e. 8% of the sample) are radio galaxies.These
INTEGRAL selected radio galaxies cover alloptical classes, are characterised by high 20-100 keVluminosities (10 –10 erg s − ) and high Edding-ton ratios (typically larger than 0.01). Most of theseobjects display a FRII radio morphology and areclassified as HERG. Several studies have been per-formed on INTEGRAL high energy selected sampleof radio galaxies in order to investigate their spec-tral characteristics.Panessa et al. (2016) studied the absorptionproperties of this sample with the addition of radiogalaxies detected by
Swift /BAT and found that thecolumn density distribution is consistent with theunified model of AGN with those optically classi-fied as type 2 being absorbed and those optically classified as type 1 not. However, there seemsto be no evidence for the presence of Comptonthick absorption in hard X-ray selected radio galax-ies (Ursini et al., 2018a). Also a significant anti-correlation between the radio core dominance pa-rameter (taken as an orientation indicator) and theX-ray column density is found, again in line withexpectations from the AGN unified theory: coredominated sources are unabsorbed in X-rays sincethey emit their radiation in a direction closer tothe line of sight and therefore not intercepted bythe torus.The broad band spectra of some
INTEGRAL detected radio galaxies have been studied overthe years by various authors (Beckmann et al.,2011; Malizia et al., 2014; Molina et al., 2014;Lubi´nski et al., 2016; Molina et al., 2015;Ursini et al., 2018b) and the overall result isthat they behave very similarly to radio quietAGN in terms of primary continuum, presenceof complex absorption and soft excess, with thepossible exception of the reflection features (10–30keV bump and iron line) which tend to be weak inthese objects.These observational results confirm that the highenergy properties of these sources are consistentwith an accretion-related emission, likely originat-ing from a hot corona coupled with a radiativelyefficient accretion flow.The radio size distribution of the entire
INTE-GRAL sample of radio galaxies shows an almostcontinuous coverage, from around 50 kpc (inPKS 0521-365) up to 1.5 Mpc (in IGR J14488-4008), with many sources displaying values abovefew hundred kpc; indeed 56% of the objects in thesample have sizes above 0.4 Mpc. If we considerthe classical threshold to define a giant radiogalaxy (GRG), i.e. a size larger than 0.7 Mpc, thenin the
INTEGRAL sample of 32 radio galaxies,8 are giants, i.e. 25% of the sample. It may bethe case that high-energy surveys could be moreefficient in searching for new GRG as comparedto radio surveys, where, for example, in the wellstudied 3CRR sample, only 6% of the sources areidentified as GRG (Bassani et al., 2016). That alarge fraction of hard X-ray selected radio sourcesbecome radio giants is also evident by the discoveryof two completely new GRG among
INTEGRAL
AGN: IGR J17488-2338 (Molina et al., 2014) andIGR J14488-4008 (Molina et al., 2015), whichdisplay peculiar and interesting radio as well assoft and hard X-ray properties. In Figure 13 the16adio 610MHz GMRT full resolution image ofIGR J14488-4008 as published by Molina et al.(2015) is displayed: evident from the figure are thelarge size of the source, its FRII morphology andalso its interesting environment.
Figure 13: Radio 610MHz GMRT full resolution image ofIGR J14488-4008 as in Molina et al. (2015)
All 8
INTEGRAL selected GRG have been thefocus of an intense observational campaign espe-cially at radio frequencies to probe the possibilityof restarting activity in their nuclei. In fact in thesegiants, the luminosity of the radio lobes and the es-timated jet power are relatively low compared withthe nuclear X-ray emission (Ursini et al., 2018b).This indicates that either the nucleus is more pow-erful now than was in the past, consistent with arestarting of the central engine, or that the giantlobes are dimmer due to expansion losses. Thefirst scenario is backed up by the finding in the ra-dio band that 6 objects (75% of the sample) hosta core with a self absorbed spectrum (peaking inthe range from 2 to 10 GHz or above) typical of young radio sources (ages of kyears) while their ex-tended structure must be very old and evolved (agesof Myears): these young nuclei are probably un-dergoing a restarting activity episode, suggesting alink between the detected hard X-ray emission, dueto the ongoing accretion, and the reactivation oftheir jets (Bruni et al., 2019). Of the two sourcesnot showing a young radio core spectrum, one hasthe radio lobes embedded in an extended low sur-face brightness cocoon, that is likely the resultof a previous period of activity (We˙zgowiec et al.,2016). The other source presents instead a discon-tinuity between the extended lobes and the arcseccore-jet structure again pointing towards the pres-ence of different activity phases (Giovannini et al.,2007). These findings support the scenario origi-nally proposed by Subrahmanyan et al. (1996), inwhich multiple episodes of activity would favourthe growth of radio sources up to the extreme sizeof GRGs but also underline the importance of ex-tracting hard X-ray selected samples of radio galax-ies with the purpose of studying the duty cycle ofAGN.
6. Main Results: Blazars
Among radio-loud AGN, blazars are the most lu-minous and variable. This is because strong rela-tivistic aberration effects, primarily light magnifica-tion and time intervals foreshortening, take place inthe plasma flowing at speeds close to that of light intheir powerful jets, that are oriented only a few de-grees away from the line of sight. They include bothflat-spectrum radio quasars (FSRQ) and BL Lac-ertae objects (BL Lac). BL Lacs are further clas-sified into low/intermediate/high-frequency-peakedBL Lac objects (LBL, IBL, HBL) depending on thelocation of the first characteristic peak frequency:below 10 Hz, at 10 − Hz, or above 10 Hz, re-spectively. Both blazar classes share the proper-ties of a nonthermal continuum, but FSRQ havestrong and broad optical emission lines, while inBL Lac the optical lines are weak or absent. FSRQhave higher bolometric luminosities than BL Lac(Sambruna et al., 1996) and can exhibit signs ofthermal activity possibly related to an accretiondisc in their optical and UV spectra (Smith et al.,1988) in contrast to BL Lac, which have smoothcontinua.Multiwavelength studies of blazars in the last 25years have identified a characteristic broad-band17pectral shape, with two ”humps” in a νf ν rep-resentation (Falomo et al., 2014). The first hump,peaking at mm to soft X-ray frequencies (depend-ing on the source, and varying even in the samesource during different emission states), is producedby synchrotron radiation, while the origin of thesecond hump, that has a maximum between MeVand GeV energies, is complex, with various sce-narios contemplating a pure leptonic compositionof the jet or a lepto-hadronic composition. In theleptonic case, the high energy hump can be dueto inverse Compton scattering of relativistic elec-trons or positrons off synchrotron photons (inter-nal Compton or self-Compton) or off ambient pho-ton fields, if these are relevant, as in the case ofFSRQ that, as said before, host luminous accretiondiscs and broad emission lines (external Compton).For the lepto-hadronic scenario, if sufficient energyis injected in the jet to trigger photo-pion produc-tion, synchrotron-supported pair cascades will de-velop (Boettcher (2010); Murase (2017) and refer-ences therein). These initiate showers of mesons,leptons, neutrinos and high energy radiation.According to the location of the two radiationpeaks, blazars form a ”sequence” (Fossati et al.,1998; Ghisellini et al., 1998) whereby sources withpeaks at lower frequencies have larger luminositiesand larger ratios between the high-energy and low-energy components (see however Padovani et al.(2012)). The blazars with largest bolometric lu-minosities, largest dominance of gamma-ray lumi-nosities, lowest frequencies of the synchrotron andhighest energy Compton hump, generally coincidewith the FSRQ, in whose jets relativistic particlescool rapidly by losing energy in Compton upscat-tering disc and line optical-UV photons. The less-efficiently cooling, less luminous sources coincidewith the BL Lac objects.Blazars represent almost 70% of allsources detected at energies larger than100 MeV by the Fermi /LAT instrument(The Fermi-LAT collaboration, 2019). This,together with the fact that most blazars arebright X-ray sources, makes them also excellenttargets for hard X-rays studies. Indeed various
INTEGRAL surveys list a conspicuous number ofthem.For example considering all the AGN detectedby
INTEGRAL (see Malizia et al. (2012);Malizia et al. (2016) and more recent updates), wecount 29 FSRQ and 19 BL Lac, around 11% of theentire
INTEGRAL
AGN population (see Figure 4).
Figure 14: Light curve of MKN 421 in April 2013, seePian et al. (2014)
Interestingly all 8 blazars listed in the completesample of AGN discussed by Malizia et al. (2009)have now a counterpart at GeV energies, with 3also having a TeV association: this indicates thatthese are bright enough for detection up to thehighest observable energies.Blazars detected by
INTEGRAL have been usedover the years to provide information on theirhigh-energy variability patterns (Beckmann et al.,2007), to monitor individual targets over a periodof intense activity (see for example Figure 14 whichshows the light curves of MKN 421 during a flarerecorded in April 2013 by Pian et al. (2014)), orto study their broad band spectral characteristicssuch as in the case of 4C 04.42, where excess emis-sion observed in the soft X-ray band was interpretedas due to bulk Compton radiation of cold electrons(de Rosa et al., 2008).According to the locations of the two spectralpeaks in blazar energy distributions, the hard X-ray region represents either the tail of the syn-chrotron spectrum at energies higher than the cool-ing break, or the rising portion of the inverseCompton (leptonic case) or proton-synchrotron(lepto-hadronic case) spectrum. HBL, and inparticular those peaking in X/hard X-rays, areprominent in the 20-200 keV band, and havingthe lowest jet powers, represent the extreme endof the blazar sequence, opposite to FSRQ. One18uch example of extreme blazar was discoveredby
INTEGRAL in the source IGR J19443+2117(Landi et al., 2009) which was later detectedalso by the Cherenkov telescope
HESS asHESS J1943+213 (H. E. S. S. Collaboration et al.,2011; Archer et al., 2018): the source displays asynchrotron peak at around 10 keV and an inverseCompton peak above few hundred of GeV.During bright outbursts of ”extreme” HBL, thesynchrotron spectrum, normally peaking at soft X-rays, flattens and reaches a peak at energies equalor higher than 100 keV, as observed first in theBL Lac object MKN 501 in 1997 with
BeppoSAX (Pian et al., 1998). Thus, depending on whetherthe blazar has a high-frequency or low-frequencysynchrotron peak, IBIS observations will samplethe rising part of the inverse Compton componentor the tail of the synchrotron, thus allowing oneto locate more precisely the peaks of these ra-diation components and to extract precise infor-mation on the energies of the emitting particles(Bottacini et al., 2016).Equally important for hard X-ray studies areFSRQ located at high redshifts displaying a Comp-ton peak in the sub-MeV region which alsofavour their detection by instruments like
INTE-GRAL /IBIS. These sources have the most pow-erful jets, the largest black hole masses and themost luminous accretion discs.
INTEGRAL hasplayed a role in the discovery of such high red-shift blazars like IGR J22517+2217 (Bassani et al.,2007; Lanzuisi et al., 2012), Swift J1656.3-3302(Masetti et al., 2008a) and IGR J12319-0749(Bassani et al., 2012). So far 17 objects have beendetected at redshift greater than 1, with 3 having z above 3. The most distant AGN so far detectedby INTEGRAL , the FSRQ IGR J22517+2217, hasbeen the target of intense studies after the
INTE-GRAL detection (Lanzuisi et al., 2012) which leadto various results: the discovery of a strong flareepisode, the study of the source SED over flar-ing and quiescent states (see Figure 15) and themeasurement of a flare jet power luminosity whichturned out to be around 30 times more powerfulthan the accretion disc luminosity.
INTEGRAL /IBIS’s sensitivity of ∼
10 mCrab inthe 20-100 keV range implies that most blazars canbe detected with a high statistical significance onlywhen they are in a high emission state. There-fore, they are generally observed with
INTEGRAL during outbursts, after a notification from a largefield of view X-ray or gamma-ray camera, like e.g.
Figure 15: Top: spectral energy distribution of IGRJ22517+2217. Grey circles and arrows represent archival ra-dio/optical/UV data from NED. Red triangles and magentasquares represent XIS 0 and XIS 3 data, green circles andorange pentagons represent PIN and BAT quiescent datarespectively, while black arrows are Fermi upper limits infive bands. Filled violet squares represent XRT data, filledcyan diamonds and blue pentagons represent IBIS and BATflare data respectively. The solid cyan and orange curves arethe results of modelling of the quiescent and flaring states,respectively. With grey lines we show the different compo-nents of non-thermal emission: synchrotron (dotted), syn-chrotron self-Compton (long-dashed) and external Compton(dotdashed). The black dashed line corresponds to the ther-mal emission of the disc, the IR torus and the X-ray disccorona. The model does not account for radio emission, pro-duced from much larger regions of the jet. Bottom: enlarge-ment in the X-ray energy range for the two SEDs. Symbolsas in the top panel. Figure from Lanzuisi et al. (2012) wift /BAT, MAXI , Fermi /LAT, or
AGILE /GRID.This ”Target-of-Opportunity” strategy is rather ef-fective, but implies that
INTEGRAL can typi-cally cover the declining phase of the outburst,owing to the characteristic ∼ INTEGRAL , primarily with IBIS, but in somecases also with the JEM-X cameras (3-30 keV),and other multiwavelength facilities have set co-gent constraints on the physics that governs blazarcorrelated flux and spectral variability over thewhole electromagnetic spectrum (e.g. Pian et al.(2006); Ghisellini et al. (2007); Vercellone et al.(2011); Collmar et al. (2010); Pian et al. (2011);Castignani et al. (2017)).Owing to their luminosities, blazars are, aftergamma-ray bursts, the farthest detectable sourcesat all wavelengths and therefore potential beaconsof the early Universe. The cosmological relevanceof blazars was not missed by the
INTEGRAL mis-sion, that enabled not only the detection of pre-viously unknown ones at high redshift but alsothe monitoring of a few previously known distantones as in Pian et al. (2005); Bianchin et al. (2009);Bottacini et al. (2010a); Giann´ı et al. (2011).The
IceCUBE detection of a ∼
200 TeV neu-trino from blazar TXS0506+056 ( z = 0.336) on22 September 2017 confirmed many expectationsthat powerful extragalactic jetted sources may bethe origin of high-energy (i.e. E >
INTEGRAL among the many space-based facilitiesthat were employed (IceCube Collaboration et al.,2018). The resulting spectra and light curvesand their correlations, combined with the neu-trino event, were used to test blazar lepto-hadronicmodels, in an attempt to determine the mutualrole of leptons and hadrons in producing radiation(Keivani et al., 2018; Gao et al., 2019; Righi et al.,2019). While neutrino events accompanying blazarsare rarely detected with the present instrumenta-tion, they represent a formidable diagnostic of thestructure and mechanisms in relativistic jets.
7. keV to GeV/TeV connection: a casestudy
The connection between the hard X-ray region ofthe spectrum and the GeV/TeV domain is criticalfor the study of many AGN, blazars in primis (seeprevious section), but also for radio galaxies andSeyferts galaxies. In these last two types of objects,it is important to understand how to connect theemission due to the disc/corona which dominatesat lower energies, to the one related to the jet orsome other mechanism (starburst?) which insteaddominate at higher frequencies.Ubertini et al. (2009) presented for the first timethe result of the cross correlation between thefourth
INTEGRAL /IBIS soft gamma-ray catalogueand the
Fermi /LAT bright source list of objectsemitting in the 100 MeV–100 GeV range. Surpris-ingly, from this initial cross-correlation between lowand high energy gamma-ray sources emerged thatonly an handful of objects were common to bothsurveys; they comprised blazars (10) both of FSRQand BL Lac types, and no X-Ray Binaries, with theonly exception of two microquasars.This initial approach can now be applied furtherby finding objects, among
INTEGRAL detectedAGN, that emit also at GeV and/or at TeV en-ergies.This can be done by cross-correlating our list of440 AGN with the most recent
Fermi /LAT cat-alogue and the TeV on line catalogue . Fromthis cross correlation, interesting results have beenfound and reported here to highlight INTEGRAL ’scapability in this field. The standard statisticaltechnique developed by Stephen et al. (2005) hasbeen used. It consists of simply calculating thenumber of GeV/TeV sources for which at least oneINTEGRAL counterpart was found within a speci-fied distance, out to a distance where all GeV/TeVsources had at least one soft-gamma ray counter-part. To have a control group, a list of ’antiGeV/TeV sources’, mirrored in Galactic longitudeand latitude, has been created and the same cor-relation algorithm applied. In both cases a cor-relation out to about 300 arcsec has been found,while at further distances, chance correlations be-come increasingly more important. At GeV ener-gies 58
INTEGRAL
AGN (13% of the sample) have https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermilpsc.html http://tevcat.uchicago.edu igure 16: Spectral Energy Distribution ofIGR J20569+4940 obtained assembling unpublished NuSTAR observation performed in November 2015 withthe average
INTEGRAL and
Fermi /LAT spectra with theaddition of the reported TeV data (courtesy of F. Ursini). emission above 1 GeV, while only 15 objects (3%of the sample) emit up to TeV energies, confirm-ing that the emission of the majority of hard X-ray selected AGN drops exponentially above 100-200 keV. The GeV sample is largely dominatedby blazars: there are 18 BL Lac and 24 FSRQ.Other sources include 9 radio galaxies, 3 Seyfert 2(NGC 1068, NGC 4945 and the Circinus galaxy),one NLS1 galaxy (1H 0323+342), one peculiar ob-ject (IGR J20569+4940) and two sources of unclearclass (IGR J18249-3243 and IGR J13109-5552).Two radio galaxies detected in the GeV band arealso TeV emitters, while none of the 4 Seyfert galax-ies have so far being detected at TeV energies. In-terestingly of the 18 BL Lac objects detected by
Fermi /LAT, 11 have been seen also by Cherenkovtelescopes: they are mostly high frequency peakedobjects, 8 HBL and 2 IBL. Their Compton peak isgenerally just below 1 TeV but can be as high as10 TeV or more like in H 1426+428 (Foffano et al.,2019).A peculiar source is IGR J20569+4940.It is certainly a blazar but still of un-known class and redshift: the source hasrecently been announced as a VHE emitter(Benbow and VERITAS Collaboration, 2017) afterobservations with
VERITAS performed in Novem-ber 2016. It is most likely a BL Lac (Chiaro et al.,2016) with a high synchrotron peak (Fan et al.,2016). In Figure 16, we assembled high energydata on this source using an unpublished
NuSTAR observation performed in November 2015 with theaverage
INTEGRAL and
Fermi /LAT spectra plusthe reported TeV data. As evident from the figure,two peaks can be located, one below the
NuSTAR low energy threshold, at around few keV, likelyassociated to the synchrotron part of the SED,and the other at around 0.1-0.3 TeV, probablyrelated to the Compton part of the SED. Thisqualifies IGR J20569+4940 to be another HBL oralternatively another extreme BL Lac.As for the FSRQ, the set of
INTEGRAL objectswith GeV emission spans a wide range of redshifts(0.3-3.1) and black hole masses (Log (M/M ⊙ ) =8-9.5). As discussed in the previous section, thesesources display the most powerful jets, they are verymassive, host the largest black hole masses and themost luminous accretion discs; in other words theyare more extreme than blazars selected in otherwavebands, like, for example, the only one exploredby Fermi /LAT (Bassani et al., 2013).Only 4 of the 24 GeV FSRQ in our list have a de-tection at TeV energies, namely 3C 279, PKS 1510-089, PG 1222+216 and SWIFT J0218+7348. Thislow number of detections is expected since FSRQare generally faint at very high energies for a num-ber of reasons: the location of their Compton peakat the lower end of the high energy gamma rayband implies a low flux at these energies; the com-pactness of the source emission region indicatesstrong attenuation by the broad line region pho-ton field and, finally, their rather high redshifts(H. E. S. S. Collaboration et al., 2019).However, the same reasons indicate that it is impor-tant to study the SED of these peculiar FSRQ andsoft gamma ray observations can be of great helpto better define their spectral energy distribution.Among
INTEGRAL
AGN with GeV emission,there are a couple of sources of unclear class:IGR J18249-3243 and IGR J13109-5552. Opticallythey are both classified as Seyfert 1. They are bothbright in the radio waveband and, although repeat-edly observed, are poorly studied. Their structureseems to be extended but a dedicated observationalcampaign is necessary to confirm the presence of jetand related features. The broad band (
XMM plusIBIS/BAT data) properties of both objects havebeen explored by Malizia et al. (2014). Unlike otherSeyfert galaxies and similarly to what we observein blazars neither source shows a low energy cut-off( ≥
357 keV in IGR J13109-552 and not required inIGR J18249-3243). Furthermore, in IGR J18249-3243 partial covering absorption is present, like in21n ordinary radio quiet Seyfert, making the pictureeven more confused. Clearly both sources deservefurther investigation, possibly through a multiwave-band campaign that will be able to clarify their na-ture.
Nearly all identified extragalactic TeV emittersare blazars (about 70 sources), exceptions beinga handful of nearby radio galaxies and starburstgalaxies and a recently detected long gamma-rayburst at z = 0.4. The brightest blazars in theTeV domain are the HBL, i.e. those whose highenergy component peaks in or just beyond thatband. These sources are ideal targets of coordi-nated hard X-ray and TeV observing campaigns(e.g. Ahnen et al. (2016, 2018); Chevalier et al.(2019) and references therein).Prototype blazars with these characteristic spec-tral energy distributions are MKN 421 andMKN 501, that are also the brightest objectsof this class. They may reach ”extreme” stateswhereby the synchortron spectrum extends to ener-gies higher than 100 keV during flares and the TeVemission increases by more than one order of mag-nitude with respect to the quiescent state. Recentsimultaneous monitoring at hard X-rays and TeVenergies with NuSTAR and the
Veritas
Cherenkovtelescope respectively, combined with observationsat optical and radio frequencies, yielded detailedinsight into the physics of these sources, with accu-rate mapping of critical parameters such as electronenergies and Lorentz factor (Furniss et al., 2015;Sinha et al., 2015; Balokovi´c et al., 2016). Re-peated observations of HBLs in bright states, astriggered by
Swift /XRT or atmospheric Cherenkovradiation telescopes, and in particular of the nearbyBL Lac object MKN 421 with
INTEGRAL , sam-pled bright states and collected detailed light curvesand spectra (Lichti et al., 2008; Pian et al., 2014).The type of X-ray variability suggests a complex be-haviour and a correlation among different frequen-cies. However, so far, although IBIS detected ratherhard spectra, no clear evidence was found that thesestill rise (in νf ν ) beyond 15-20 keV. Investigationwith IBIS of the BL Lac object MKN 501, in searchof a shift of the synchrotron peak to hard X-raysduring outbursts in 2013 and 2014 is underway.A few dimmer BL Lac share the ”extreme” proper-ties of MKN 421 and MKN 501 and are potentialideal targets for coordinated hard X-ray and TeVmonitoring (Costamante et al., 2018). In the near future, these can be followed-up with the sensitiveCherenkov Telescope Array for more accurate as-sessment of their physical parameters than it is pos-sible with present state-of-the-art Cherenkov tele-scopes.Particularly interesting are the so called ”or-phan” gamma-ray flares, whereby some sources ex-hibit strong TeV outbursts that are not accompa-nied by simultaneous X-ray flux increase at anyappreciable level (e.g. Krawczynski et al. (2004);Bottacini et al. (2010b); MacDonald and Mullan(2017)).
8. INTEGRAL heritage and future perspec-tives
INTEGRAL ’s cumulative exposure has now ex-ceeded 10 Ms in several extragalactic fields with atotal area of ∼ ∼
100 AGN (known or presumed) with fluxes downto 3 × − erg s − cm − (17–60 keV) in theseregions. This sample is a valuable part of the IN-TEGRAL – Swift legacy, since it widens the AGNparameter space to lower luminosities and largerdistances. In particular, a significant fraction ofAGN detected at these (low) fluxes are expectedto be located at z ≃ .
2, which makes it possibleto begin studying the evolution of the AGN popu-lation over the last ∼ . ∼
15 Ms and thecorresponding flux limit to ∼ × − erg s − cm − (17–60 keV). Some 50 new AGN are expected to bedetected upon completion of this programme (andthere are suggestions to continue it afterwards).Moreover, there is expected to be unique synergybetween these INTEGRAL observations and theupcoming all-sky X-ray survey by the
Spektr-RG observatory (with its eROSITA and ART-XC tele-scopes). The latter is expected to reach recordsensitivities ∼ − , ∼ ∼ − and ∼ a few220 − erg s − cm − in the 0.5–2, 2–10 and 5–11keV energy bands, respectively, after 4 years of ob-servations, while shallower maps will be availablealready after the first 6-month scan of the sky. IN-TEGRAL hard X-ray observations and
Spektr-RG data at lower energies in the M81 field will be com-plementary in a number of ways, in particular re-garding the detection and identification of Comptonthick AGN.It is worth noting that, although deep surveys onspecific sky regions are important to probe lowerluminosity objects and further distance scales, itis only through large scale mapping that is possi-ble to enlarge the
INTEGRAL
AGN database. Byextrapolating from previous surveys, it is also pos-sible to estimate how many AGN
INTEGRAL willbe able to detect in the future. From the sky cov-erage obtained in 2000 orbits (September 2018), weexpect to double the number of the detected AGNand we foresee that we will be able to observe morethan 1200 AGN by revolution 2500, likely by theend of the mission. Even more interesting is theongoing monitoring of the Galactic Plane region by
INTEGRAL . As already mentioned, this is a regionwhich is generally avoided by extragalactic studiesand where
INTEGRAL is playing an even greaterrole than
Swift /BAT. In this region, deep sky cov-erage combined with large scale mapping, will allowa unique view of the extragalactic sky behind ourMilky Way across the years, a legacy which willbe extremely useful for future studies of this regionat other wavelengths, especially for the
CherenkovTelescope Array (CTA) future surveys.On the other hand, the advent of the
CTA in thenear future and the sensitivity upgrades of neutrinodetectors will result in a boost of multiwavelengthand multi-messenger studies of high energy extra-galactic sources, and help unveil the physics of theircomplex central engines.
List of abbreviations
List of definitions of abbreviations used in thepaper.NLS1: Narrow Line Seyfert 1XBONG: X-ray Bright Optically Normal GalaxiesLINERs: Low-Ionization Nuclear Emission RegionsFSRQ: Flat Spectrum Radio QuasarBL Lac: BL LacertaeLBL, IBL, HBL: low/intermediate/high-frequency-peaked BL Lac objectsRG: Radio Galaxies FRI: Fanaroff type IFRII: Fanaroff type IIHERG: High Excitation Radio GalaxiesLERG: Low Excitation Radio GalaxiesGRG: Giant Radio Galaxies
Acknowledgements
We acknowledge all the scientists which con-tributed over the years to the analysis and the inter-pretation of
INTEGRAL
AGN data. In particularwe would like to thank Nicola Masetti for his funda-mental work in leading the optical follow-up cam-paigns of the unidentified
INTEGRAL sources. Wethank also J. B. Stephen for his contribution in thecorrelation studies presented in this work. AM andLB acknowledge financial support from ASI undercontract INTEGRAL ASI/INAF n.2019-35-HH; SSand IM acknowledge support from the Russian Sci-ence Foundations grant 19-12-00396 in working onthis review.
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