15 years of Galactic surveys and hard X-ray Background measurements
Roman A. Krivonos, Antony J. Bird, Eugene M. Churazov, John A. Tomsick, Angela Bazzano, Volker Beckmann, Guillaume Belanger, Arash Bodaghee, Sylvain Chaty, Erik Kuulkers, Alexander Lutovinov, Angela Malizia, Nicola Masetti, Ilya A. Mereminskiy, Rashid Sunyaev, Sergey S. Tsygankov, Pietro Ubertini, Christoph Winkler
115 years of Galactic surveys and hard X-ray Background measurements
Roman A. Krivonos a, ∗ , Antony J. Bird b , Eugene M. Churazov a,c , John A. Tomsick d , Angela Bazzano e , Volker Beckmann f ,Guillaume B´elanger g , Arash Bodaghee h , Sylvain Chaty i,j , Erik Kuulkers k , Alexander Lutovinov a , Angela Malizia l , NicolaMasetti l,m , Ilya A. Mereminskiy a , Rashid Sunyaev a,c , Sergey S. Tsygankov n,a , Pietro Ubertini e , Christoph Winkler k a Space Research Institute (IKI), Profsoyuznaya 84 /
32, Moscow 117997, Russia b School of Physics and Astronomy, University of Southampton, SO17 1BJ, UK c Max-Planck-Institut f¨ur Astrophysik (MPA), Karl-Schwarzschild-Strasse 1, Garching 85741, Germany d Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA 94720-7450, USA e INAF - Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, Roma, I-00133, Italy f Minist`ere de l’Enseignement sup´erieur, de la Recherche et de l’Innovation, 1 rue Descartes, 75005 Paris, France g European Space Astronomy Centre - ESA / ESAC, Villanueva de la Ca˜nada, Madrid, Spain h Georgia College & State University, CBX 82, Milledgeville, GA 31061, USA i AIM, CEA, CNRS, Universit´e Paris-Saclay, Universit´e de Paris, F-91191 Gif-sur-Yvette, France j Universit´e de Paris, CNRS, Astroparticule et Cosmologie, F-75006 Paris, France k ESA-ESTEC, Research and Scientific Support Dept., Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands l INAF - Osservatorio di Astrofisica e Scienza dello Spazio, via Piero Gobetti 101, I-40129, Bologna, Italy m Departamento de Ciencias F´ısicas, Universidad Andr´es Bello, Fern´andez Concha 700, Las Condes, Santiago, Chile n Department of Physics and Astronomy, FI-20014 University of Turku, Finland
Abstract
The
INTEGRAL hard X-ray surveys have proven to be of fundamental importance.
INTEGRAL has mapped the Galactic plane withits large field of view and excellent sensitivity. Such hard X-ray snapshots of the whole Milky Way on a time scale of a year arebeyond the capabilities of past and current narrow-FOV grazing incidence X-ray telescopes. By expanding the
INTEGRAL
X-raysurvey into shorter timescales, a productive search for transient X-ray emitters was made possible. In more than fifteen years ofoperation, the
INTEGRAL observatory has given us a sharper view of the hard X-ray sky, and provided the triggers for many follow-up campaigns from radio frequencies to gamma-rays. In addition to conducting a census of hard X-ray sources across the entire sky,
INTEGRAL has carried out, through Earth occultation maneuvers, unique observations of the large-scale cosmic X-ray background,which will without question be included in the annals of X-ray astronomy as one of the mission’s most salient contribution to ourunderstanding of the hard X-ray sky.
Keywords:
Contents1 Introduction 12
INTEGRAL hard X-ray surveys 2
INTEGRAL . . . . . . . . 22.2
INTEGRAL performance for surveys . . . . . . 32.3 Brief overview of INTEGRAL surveys . . . . . 32.4 Time domain . . . . . . . . . . . . . . . . . . 9
INTEGRAL in the 4-200 keV band 145 Conclusions 16 ∗ Corresponding author
Email address: [email protected] (Roman A. Krivonos)
1. Introduction
A wide variety of astrophysical phenomena cannot be su ffi -ciently well investigated via observations of individual sources,but requires instead a systematic approach based on large sta-tistical samples. The last few decades of X-ray astronomy haveprovided us with great opportunities for studies of the popu-lations of compact X-ray sources (white dwarfs, neutron stars,black holes) in our Galaxy and beyond, with the use of newlong-lasting facilities.Two powerful and currently active hard X-ray missions,ESA’s INTEGRAL observatory (Winkler et al., 2003a) andNASA’s Neil Gehrels Swift Observatory (Gehrels et al., 2004)are performing some of the deepest and widest serendipitous X-ray surveys ever undertaken at energies E >
20 keV. In contrastto
Swift , with a nearly uniform all-sky survey, which is espe-cially useful for studies of active galactic nuclei (AGN; Tuelleret al., 2010; Cusumano et al., 2010; Ajello et al., 2012; Baum-gartner et al., 2013; Oh et al., 2018), the
INTEGRAL observa-tory provides a sky survey with exposures that are deeper in theGalactic plane (GP) and Galactic Centre (GC) regions and with
Preprint submitted to Journal of L A TEX Templates January 25, 2021 a r X i v : . [ a s t r o - ph . H E ] J a n igure 1: Typical observational strategy of the INTEGRAL
GPS scans (filledsymbols). The fully coded fields of view (FCFOV, SPI: 16 ◦ , IBIS: 9 ◦ × ◦ ,JEM-X: 5 ◦ ), and some known high energy targets are shown for illustration.Adopted from Winkler et al. (2003b). higher angular resolution, which is essential in these crowdedregions. It allowed to study in depth di ff erent populations ofgalactic binary systems, such as low- and high-mass X-ray bi-naries, cataclysmic variables, symbiotic systems, etc. (see, e.g.,Revnivtsev et al., 2008; Bodaghee et al., 2012; Lutovinov et al.,2005a, 2013b; Kretschmar et al., 2019). This makes the Swift and
INTEGRAL surveys complementary to each other. In thisreview we concentrate on the valuable contribution of the
IN-TEGRAL observatory to the surveying of the hard X-ray skyover the last 15 years.The
INTEGRAL observatory, selected as the M2 missionwithin ESA’s Horizon 2000 program, has been successfullyoperating in orbit since its launch in 2002. Due to the highsensitivity and relatively good angular resolution of its instru-ments, in particular the coded-mask telescope IBIS (Ubertini etal. 2003), surveying the sky in hard X-rays is one of the missionprimary goals. INTEGRAL hard X-ray surveys
Since the beginning of X-ray astronomy many X-ray surveyshave been successfully carried out with the aim of both discov-ering new types of X-ray emitters and to investigate the natureof the Cosmic X-Ray Background. A brief review of the hardX-ray surveys before the
INTEGRAL era is presented hereafter.A more detailed overview of the Hard X-ray / Soft gamma-rayexperiments and missions can be found in Cavallari and Fron-tera (2017).Markert et al. (1979) described observations of the cosmicX-ray sky performed by the MIT 1–40 keV X-ray detectors onthe OSO 7 satellite between 1971 October and 1973 May. Theauthors made intensity determinations or upper limits for 3rdUhuru (Giacconi et al., 1974) and OSO 7 (Markert et al., 1976,1977) cataloged sources in di ff erent energy bands, including the15–40 keV range. Uhuru , also known as the Small Astronomical Satellite 1(SAS-1) provided the first comprehensive and uniform all skysurvey with a sensitivity of 10 − the Crab intensity. Formanet al. (1978) presented the list of detected 339 X-ray sources with measured 2–6 keV intensities. The major classes of iden-tified objects included binary stellar systems, supernova rem-nants, Seyfert galaxies and clusters of galaxies.The first ever attempt to survey the sky at high energies (26 − Ariel V (Coe et al., 1982), which provided thefirst galactic Log N -Log S relation above 100 keV.Skinner et al. (1987b) reported observations of the GalacticCentre made with a coded mask X-ray telescope flown on theSpacelab 2 mission, providing the first images of the GC inhigh-energy X-rays up to 30 keV.In the hard X-ray range (2 . −
30 keV), an all-sky surveywas conducted with the BeppoSAX Wide Field Camera (WFC;Jager et al., 1997). The WFC discovered 21 transients in the GCregion and more than 50 transient and recurrent sources alongthe Galactic plane.Levine et al. (1984) presented the first systematic study ofX-ray sources at high X-ray energies (13–180 keV) over thewhole sky. This all-sky survey was based on data obtained withthe UCSD / MIT Hard X-Ray and Low-Energy Gamma-Ray In-strument A4 on board the HEAO 1 satellite from August 1977until January 1979. The survey catalogue contains 72 sourcesat a flux sensitivity of ∼ −
15 mCrab.The Galactic Center region was observed with theTTM / COMIS coded mask imaging spectrometer on the Kvantmodule of the MIR orbital station in 1989 (Syunyaev et al.,1991a; Sunyaev et al., 1991). Several observations of Galacticsources were performed, deriving the hard X–ray componentof their emission (Sunyaev et al., 1988; Syunyaev et al., 1989,1991b; Borkous et al., 1997; Kaniovsky et al., 1997). Sun-yaev et al. (1989) presented the hard X-ray (2–30 keV) observa-tions of the Large Magellanic Cloud (LMC) performed in 1988-1989 with the TTM / COMIS instrument, reporting the results ofmonitoring and spectral observations of LMC X–1, LMC X–2,LMC X–3, LMC X–4 and PSR 0540-693. Emelyanov et al.(2000) assembled a catalog of 67 X-ray sources observed bythe TTM / COMIS telescope in 1988-1998 at a confidence levelhigher than 4 σ .In 1990-1992 more than 400 sky fields were observed withthe ART-P coded-mask telescope aboard the GRANAT obser-vatory in the 2.5–60 keV energy band (Sunyaev et al., 1990).ART-P provided a good 5 (cid:48)
FWHM angular resolution within3 . ◦ × . ◦ field of view (FOV), which made it especially use-ful for studying the crowded field of the GC. Pavlinsky et al.(1994) reported a detection of 12 point X-ray sources during a ∼ ◦ × ◦ survey of the GC with the sensitivity of ∼ −
17 keV energy range.At higher energies, in the period of 1990–1998 the SIGMAtelescope on board
GRANAT observed more that one quarter ofthe sky with the sensitivity better than 100 mCrab (Revnivtsevet al., 2004a). The SIGMA telescope (Paul et al., 1991), de-signed in a coded-mask paradigm, allowed to reconstruct firstimages of the hard X-ray sky in the energy band 35 − ∼ (cid:48) . During its operation timethe SIGMA telescope detected 37 hard X-ray sources in the 40–100 keV energy band (Revnivtsev et al., 2004a).2 .2. INTEGRAL performance for surveys The IBIS telescope (Ubertini et al., 2003) is the most suitablefor the imaging surveys in the hard X-ray band among the majorinstruments on board
INTEGRAL . This instrument provides thebest combination of field of view, sensitivity and angular reso-lution needed to conduct a wide-angle survey of the sky in a rea-sonable amount of time. This was optimised with the scientificgoal to regularly monitor a large fraction of the Galactic planeand to discover most of the expected transient sources, whoseexistence was anticipated by X-ray missions like
BeppoSAX and
RXTE , operating at lower energies and / or coarser spatialresolutions in the ’90. The low-energy detector layer, ISGRI(INTEGRAL Soft Gamma Ray Imager; Lebrun et al., 2003) ismade of a pixelated 128 ×
128 CdTe solid-state detector thatviews the sky through a coded aperture mask. IBIS generatesimages of the sky with a 12 (cid:48)
FWHM resolution over a 28 ◦ × ◦ field of view in the working energy range 15 − / ISGRI analysis is presented in the paper byGoldwurm et al. (2003).Thanks to the coded-aperture design, the IBIS telescope in-corporates a very large fully-coded FOV of 9 ◦ × ◦ (all sourceradiation is modulated by the mask) and partially-coded FOVof 28 ◦ × ◦ (only a fraction of source flux is modulated by themask). In addition to that, the “dithering” pattern around thenominal target position, a controlled and systematic spacecraftdithering manoeuvres introduced in order to minimize system-atic e ff ects due to spatial and temporal background variationsin the spectrometer’s (SPI) detectors, results in an even largersky coverage. The combination of the standard 5 × INTEGRAL pointings, via the approvedGuest Observer Program at the Galactic X-ray sources makesthe e ff ective latitude coverage of the Galactic plane | b | < . ◦ (Krivonos et al., 2012). As a result, INTEGRAL can conducttime-resolved mapping of the Galactic plane on the time-scaleof a year. This leads to the unique possibility of taking snap-shots of the whole Milky Way in hard X-rays, which is not pos-sible with narrow-FOV grazing X-ray telescopes.The energy response of the IBIS telescope at hard X-rays( E >
20 keV) opens another possibility: to detect highly ob-scured objects. This makes the
INTEGRAL / IBIS survey of theGalaxy unbiased against line-of-sight (or intrinsic to the source)attenuation of X-ray photons. Going to even higher energies,the ISGRI detector of
INTEGRAL / IBIS can provide a census ofGalactic hard X-ray emitters at energies above 100 keV (Baz-zano et al., 2006; Krivonos et al., 2015).
During the first years of operations,
INTEGRAL conductedits so-called Core Programme (CP), i.e., a set of guaranteedobservations dedicated to frequent monitoring of the Galactic plane in order to detect transient sources, and to perform timeresolved mapping of the Galactic plane (Winkler, 2001). TheCP consisted of a deep exposure of the central Galactic radian(GCDE: Galactic Centre Deep Exposure), regular scans of theGalactic plane (GPS: Galactic Plane Scan, Fig. 1) and pointedobservations. This program resulted in a first deep survey of theGalactic plane (Winkler et al., 2003b), which listed 100 knownX-ray emitters and 10 new X-ray sources have been detectedwith IBIS / ISGRI for the first time. Fig. 2 shows the first
INTE-GRAL wide-angle Galactic sky maps obtained during the GPSscans with SPI, IBIS / ISGRI, and JEM-X. During the first yearof observations,
INTEGRAL demonstrated its strength in dis-covering new transient sources. 10 new sources were foundover a period of about 4 months, giving a discovery rate of morethan 2 per month.Deep surveys of the Galactic center region (Revnivtsev et al.,2004b), made within the Russian quota time, were a valuableaddition to the
INTEGRAL
CP and demonstrated the excellentcapabilities of the
INTEGRAL telescopes to construct sensitivesky maps. In total, 60 sources with a flux higher than 1.5 mCrabwere detected, including 2 sources detected at energies above20 keV for the first time.Soon after, the
INTEGRAL hard X-ray cartography of theGalactic plane has been extended by deep observations of theSagittarius spiral arm (Molkov et al., 2004), a region very richof young massive stars and remnants of their evolution. Thispart of the Milky Way contains the brightest well-known mi-croquasar GRS 1915 +
105 (Hannikainen et al., 2003), the pecu-liar object SS433 (Cherepashchuk et al., 2003), soft gamma-rayrepeaters, supernova remnants, about dozen of persistent andtransient X-ray bursters (e.g. 4U 1915-05, Ser X-1, Aql X-1),and X-ray pulsars. As a result of the
INTEGRAL
Sagittariusarm survey, Molkov et al. (2004) reported the significant de-tection of 28 X-ray sources with a flux level above 1.4 mCrab,including three previously unknown X-ray emitters.Next step in the deep surveys of galactic spiral arms wasmade in 2005 with observations of the Galactic plane regionin Crux (Revnivtsev et al., 2006). In total it detected 47 hardX-ray sources, with 15 of them being new ones. Among theidentified sources there were 12 active galactic nuclei, and 11and 6 galactic binary systems with high-mass and low-mass op-tical companions, respectively.In February 2005 the program of monitoring the source activ-ity in the Galactic bulge region started . Kuulkers et al. (2007)presented a detailed study of a homogeneous (hard) X-ray sam-ple of 76 sources in the Galactic bulge region, based on the first1.5 years of the program. The authors showed that almost allthe sources in the Galactic bulge are variable. Additionally, 6new hard X-ray sources were discovered.Also, den Hartog et al. (2006) presented results based on1.6 Ms INTEGRAL observations of the Cassiopeia region. Theanalysis of the IBIS / ISGRI data resulted in detection of 11sources at energies above 20 keV, including 3 new hard X-raysources. http://isdc.unige.ch/Science/BULGE/ igure 2: Examples of Galactic sky maps obtained during the GPS observations. Top panel : SPI significance 20 −
40 keV sky map in galactic coordinates (December2002 – March 2003).
Middle panel : 15 −
30 keV IBIS / ISGRI skymap in equatorial J2000 coordinates (12 March 2003).
Bottom panel : 3 −
15 keV JEM-X sky mapclose to the Galactic Centre (24 March 2003). Adopted from Winkler et al. (2003b). +
61 up to 150 keV demonstrated the uniquecapabilities of the IBIS / ISGRI imager.Thanks to the continuing observations and, consequently,rapidly growing exposure, Bird et al. (2004) released a firstsystematic survey of hard X-ray sources detected with the IBIStelescope, based on 5 Ms of total exposure time. This initial sur-vey has revealed the presence of ∼
120 sources detected withthe unprecedented sensitivity of ∼ −
100 keV. The survey contains also 28 objects of unknownnature.A sequence of IBIS / ISGRI survey catalogues has been re-leased at regular basis as more data have become available. Thesecond
INTEGRAL / IBIS / ISGRI catalogue (Bird et al., 2006)used a greatly increased data set of 10 Ms to unveil a softgamma-ray sky comprising 209 sources, again with a substan-tial component (25%) of new and unidentified sources.In the meantime the first sensitive survey of the central part ofthe Galaxy was performed at very high energy gamma-rays (E >
100 GeV) with the
High Energy Stereoscopic System (HESS,Aharonian et al., 2005). This survey revealed a new popula-tion of previously unknown sources emitting at very high en-ergies. At least two had no known radio or X-ray counter-part. One year later the extended HESS survey of the Galaxy( ± ◦ in longitude and ± ◦ in latitude) confirmed the detec-tion of 14 galactic sources (Aharonian et al., 2006). Most ofthem had no known radio or X-ray counterpart and were hy-pothesised to be representative of a new class of dark nucle-onic cosmic sources. In fact, high energy gamma-rays withenergies E > eV were, and actually are, the best proofof non-thermal processes in the universe and provide a directin-site view of matter-radiation interaction at energies by fargreater than producible in human-made accelerators. INTE-GRAL survey results were a powerful tool to immediately in-vestigate the nature of this new type of galactic cosmic ac-celerator (Ubertini, 2005) thanks to the unprecedented
INTE-GRAL / IBIS angular resolution and Point Source Location Ca-pability of about 1-2 arcminute between 15 keV and a fewMeV. Furthermore,
INTEGRAL had already performed deepobservations of almost all HESS detected sources: the new IN-TEGRAL source IGR J18135 − −
178 (Ubertini et al.,2005) and AX J1838.0 − / gamma-ray counterpartof HESS J1837 −
069 (Malizia et al., 2005), as shown in Fig. 3.It was immediately obvious that most of the common
INTE-GRAL / HESS sources identified belonged to SNR and PulsarWind Nebulae.With the aim to provide a prompt release of the
INTEGRAL hard X-ray survey information to the community, Bird et al.(2007) released a third
INTEGRAL survey that covers > INTEGRAL exposure,comprises more than 400 high-energy sources detected in theenergy range 17 −
100 keV, including both transients and faintpersistent objects revealed on time-averaged maps.Using alternative IBIS / ISGRI sky reconstruction software,Krivonos et al. (2007) released the first
INTEGRAL / IBIS / ISGRI survey with all-sky coverage. The catalogue of detected sourcesincludes 403 objects, 316 of which exceed a 5 σ detectionthreshold on the time-averaged map of the sky, with the restdetected in various subsamples of exposures. Since this wasthe first all-sky survey in hard X-ray energy band with unprece-dented sensitivity (a factor of 10 deeper than the extragalacticpart of the HEAO 1 A-4 source catalog, Levine et al., 1984),the statistical properties of extragalactic X-ray source popula-tion (mainly AGNs) have been significantly improved. In par-ticular, using 68 AGNs detected by INTEGRAL , Krivonos et al.(2007) presented evidence of a strong inhomogeneity in the spa-tial distribution of nearby ( <
70 Mpc) AGNs, which reflectsthe large-scale structure in the local Universe. This findinghas been later confirmed and significantly improved with ∼ Swift / BAT all-sky surveyby Ajello et al. (2012).In the following years,
INTEGRAL continued to accumulateexposure time within the Galactic plane. However, the growingexposure time was not reflected by a corresponding increase insurvey sensitivity, since the observations are strongly a ff ectedby the systematics related to the crowded field of the Galacticcenter. Krivonos et al. (2010a) developed an improved methodof sky image reconstruction for the IBIS telescope, which al-lowed them to significantly suppress the systematic noise inthe deep images of the Galactic center (see Fig. 4), and prac-tically remove non-statistical noise from the high-latitude skyimages. This improved method of sky reconstruction was usedby Krivonos et al. (2010b) to conduct the most sensitive sur-vey of the Milky Way above 20 keV at that time. The minimaldetectable flux with a 5 σ detection level reached the value of3 . × − erg s − cm − , which corresponds to ∼ .
26 mCrabin the 17 −
60 keV energy band. The catalogue of detectedsources includes 521 objects, 449 of which exceed a 5 σ de-tection threshold on the time-averaged map of the sky, and 53were detected in di ff erent periods of observations. Among theidentified sources, 262 are Galactic and 221 are of extragalacticorigin.Bird et al. (2010) presented the fourth soft gamma-ray sourcecatalogue obtained with IBIS / ISGRI based on 70 Ms of high-quality observations performed during the first five and a halfyears of the Core Program and public observations. The cata-logue includes a substantially increased coverage of extragalac-tic fields, and comprises more than 700 high-energy sourcesdetected in 17 −
100 keV energy range. The authors performedcareful analysis of IBIS data using the latest o ffi cial OSA soft-ware and source detection techniques. Particular care has beentaken to optimize the detection of the transient sources that arecommon to find both transients and faint persistent objects thatcan only be revealed with longer exposure times.Six years later, Bird et al. (2016) reported an all-sky cata-logue of soft gamma-ray sources based on IBIS observationsduring the first 1000 orbits of INTEGRAL . This legacy-levelsurvey contains all good-quality data acquired from the launch The
INTEGRAL O ff -line Scientific Analysis (OSA) package is providedby INTEGRAL
Science Data Center (ISDC, Courvoisier et al. (2003)) to thecommunity to reduce and analyze data collected by the
INTEGRAL satellite. igure 3: Left:
The IBIS 20 −
300 keV sky region map containing AX J1838.0 − −
069 (white circle) andthe position determined by the
Einstein telescope (black cross). Figure adapted from Ubertini (2005) and Malizia et al. (2005).
Right panel:
The emission regionof HESS J1837-069 overlapped to the ASCA source AX J1838.0 − in 2002, up to the end of 2010 and contains 110 Ms of sci-entific public observations, with a concentrated coverage on theGalactic Plane and extragalactic deep exposures. The catalogueincludes 939 sources above a 4 . σ significance threshold de-tected in the 17 −
100 keV energy band. The list of previouslyundiscovered soft gamma-ray emitters contains 120 sources.Substantial e ff orts have been taken to detect transient sourceson di ff erent time scales as described in Section 2.4.In June 2008 the Fermi Gamma-ray Space Telescope wassuccessfully launched and soon after the first high energy cat-alogue was published. Ubertini et al. (2009) reported the re-sult of the cross correlation between the 4th
INTEGRAL / IBISsoft gamma-ray catalogue Bird et al. (2010), in the range 20–100 keV, and the
Fermi / LAT bright source list of objects emit-ting in the 100 MeV – 100 GeV range. Surprisingly, the mainresult was that only a minuscule part of the 720 sources de-tected by INTEGRAL were present among the 205
Fermi / LATsources (Fig. 5). This result was not expected due to the mCrab
INTEGRAL sensitivity and the
Fermi breakthrough at MeV–GeV energies. Most of the
Fermi / LAT gamma-ray sourcespresent in the 4th INTEGRAL / IBIS catalogue were opticallyidentified as AGNs (10) complemented by 2 isolated pulsars(Crab and Vela) and 2 High Mass X-Ray Binaries (HMXB,LS I + ◦
303 and LS5039). Two more possible associationswere found: one is 0FGL J1045.6–5937, possibly the counter-part at high energy of the massive colliding wind binary systemEta Carinae, discovered to be a soft gamma-ray emitter. For theremaining 189
Fermi / LAT sources no
INTEGRAL counterpartswere found.This initial (unbiased) cross-correlation between low andhigh energy gamma-ray sources showed that MeV / GeV
Fermi sources were usually not associated with IBIS / ISGRI sources inthe range 20 – 100 keV. The handful of objects common to bothsurveys comprised only Flat Spectrum Radio Quasars (FSRQ) and BL Lac objects, but no X-Ray Binaries, with the excep-tion of the two microquasars mentioned. Also absent werethe AXP, which were known to be strong emitters in the keV-MeV range with a total energy rising in ν F ν and no cut-o ff de-tected in their spectra up to a few hundreds of keV (Kuiper andHermsen, 2009), implying some kind of switch-o ff mechanismin the MeV regime. Similarly, SGR and Magnetars, detectedeven in quiescence mode by IBIS / ISGRI (Rea et al., 2009) andamong the brightest sources of the hard X-ray sky when flaring(Kouveliotou et al., 1999; G¨otz et al., 2006; Israel et al., 2008)were not detected in the high energy gamma-ray range.As mentioned earlier, most of the observing time of
IN-TEGRAL is dedicated to Galactic source population studies,making possible the deepest Galactic survey in hard X-raysever compiled. Using sky reconstruction algorithms especiallydeveloped for the high quality imaging of IBIS / ISGRI data(Krivonos et al., 2010a), Krivonos et al. (2012) published an
INTEGRAL
Galactic plane survey based on nine years of obser-vations, from December 2002 to January 2011. As seen fromthe range of the used spacecraft revolutions 26-1013, the timespan of this survey is similar to that covered by 1000-orbits sur-vey by Bird et al. (2016). Krivonos et al. (2012) presented skyimages, sensitivity maps, and catalogues of detected sourcesin di ff erent energy bands energy bands (17 −
60, 17 −
35, and35 −
80 keV) within the Galactic plane ( | b | < . ◦ ). Usingthe extended data set, Krivonos et al. (2017) reported on a cat-alogue of new hard X-ray source candidates based on the skymaps comprising 14 years of data acquired with the IBIS tele-scope within the Galactic Plane ( | b | < . ◦ ). The catalogueincludes in total 72 hard X-ray sources detected at S / N > . σ and not known to previous INTEGRAL surveys.Most of the INTEGRAL surveys have been conducted in the17 −
100 keV regime where the IBIS / ISGRI sensitivity is op-timal in search for point sources. However, energy response6 igure 4: Map of the central part of the Galaxy obtained with IBIS / ISGRI in the 17 −
60 keV energy band using the improved sky reconstruction method (Krivonoset al., 2010a). The total exposure is about 20 Ms in the region of the Galactic center. To highlight background fluctuations, the image is shown in significance witha squared root color map ranging from 0 to 25. As a consequence of the chosen color scheme, the apparent diameters of the source images partially scale as theX-ray brightness of the sources. Figure adapted from Krivonos et al. (2010a).Figure 5: Gamma-ray flux (100 MeV – 1 GeV) of each
Fermi / LAT source asa function of the corresponding 20–40 keV IBIS / ISGRI flux. The colouredpoints refer to the IBIS detections, specifically red points are blazars, dark blueare pulsars, green are HMXBs, yellow is Eta Carinae and finally light blue isIGR J17459–2902. The black points refer to IBIS non-detections (2 σ upperlimit). Figure adapted from Ubertini et al. (2009). of ISGRI detector allows to e ff ectively detect photons at evenhigher energies, as seen from many studies of bright X-raysources of di ff erent nature (see e.g., Lutovinov et al., 2012a;Revnivtsev et al., 2014; Natalucci et al., 2015; Churazov et al.,2015; Kajava et al., 2016; Lubi´nski et al., 2016; De Falco et al.,2017). The first systematic study of X-ray emitters detectedwith IBIS / ISGRI in soft gamma-ray band 100 −
150 keV hasbeen conducted by Bazzano et al. (2006) based on the Core Pro-gram and public open-time observations up to 2005 April. Thecatalogue includes 49 sources detected in the 100 −
150 keVband, of which 14 are also seen in the 150 −
300 keV range.The source types in 100 −
150 keV band are dominated byGalactic low and high mass X-ray binary systems, and also in- clude active galaxies (10). Among the binary systems that aredetected above 150 keV, more than 50% are associated withblack hole candidates. Bazzano et al. (2006) constructed thefirst 100 −
150 keV Galactic and extragalactic Log N -Log S re-lation, predicting at E >
100 keV around 200 Galactic sourcesand almost 350 active galaxies at a flux above 1 mCrab.Ten years later, using significantly increased exposure time,Krivonos et al. (2015) published an
INTEGRAL all-sky surveyat energies above 100 keV. The catalogue of detected sourcesincludes 132 objects, which significantly increases the high-energy source sample compared to the work of Bazzano et al.(2006). The whole sky map of all the detected sources in thesurvey, discriminated in four basic source classes, is shown inFig. 6. The survey is dominated by 97 hard X-ray sources ofGalactic origin (mainly Low-Mass X-ray Binaries – LMXBsand HMXBs, 83 in total) in comparison with the extragalac-tic source population, represented by 35 AGNs. Compared toBazzano et al. (2006), the Log N -Log S was extended down tofainter fluxes by a factor of 1.4 and has a steeper slope. INTEGRAL regular observations of the Galactic plane makeit possible to address non-standard questions: for instance, Tsy-gankov et al. (2016) performed the deepest systematic searchfor the nuclear de-excitation lines of titanium-44 ( Ti) at 67.9and 78.4 keV, as a tracer of core-collapse supernova explosionsin the Galaxy. The peak sensitivity of this Ti survey reachedan unprecedented level of 4 . × − ph cm − s − that improvedthe sensitivity of the survey done by Compton Gamma-Ray Ob-servatory / COMPTEL (Iyudin et al., 1999) by a factor of ∼ Ti emission from any known SNRusing existing and prospective X- and gamma-ray telescopes.Thanks to large observational campaigns of the extragalac-7 igure 6: All-sky map showing the four basic X-ray source types detected in the 100 −
150 keV survey by Krivonos et al. (2015): 65 LMXBs, 19 HMXBs, 13 PSRs(mostly in the Galactic plane) and 35 AGNs. Adapted from Krivonos et al. (2015). tic sky,
INTEGRAL accumulated a number of deep fields athigh Galactic latitudes. The first
INTEGRAL extragalactic sur-vey was conducted in the direction toward the Coma cluster ofgalaxies by Krivonos et al. (2005), who detected 12 serendip-itous sources with statistical significance > σ and extendedthe extragalactic source counts in the 20 −
50 keV energy banddown to a limiting flux of ∼ ∼ ,
000 deg sky coverage down to a limiting flux of3 × − erg s − cm − in 20 −
40 keV. The sample of 38 AGNswas used to construct Log N -Log S and to produce the first lumi-nosity function of AGNs in the 20 −
40 keV energy range. Thecensus of nearby AGNs and their statistical properties was laterextended by Sazonov et al. (2007b) using a representative sam-ple of 127 AGN from Krivonos et al. (2007). Later on, Paltaniet al. (2008) presented an analysis of a deep hard X-ray surveyof the 3C 273 / Coma region with sky coverage of about 2500deg , resulting in a list of 34 candidate sources detected in themosaic with a significance σ >
5. Another extragalactic field,that of the LMC, was scrutinized during the large observationalcampaign aimed at detecting the emission lines from the de-cay of Ti in the remnant of SN1987A (Grebenev et al., 2012).The catalogue of sources in the LMC region was published byGrebenev et al. (2013) and consisted of 21 sources, 4 of whichwere detected in hard X-rays for the first time. Later, a num-ber of deep extragalactic fields, including M81, LMC and 3C273 / Coma, were studied in Mereminskiy et al. (2016), who de-tected 147 sources at S / N > σ , including 37 sources observedin hard X-rays for the first time.The SPI spectrometer with its comparably large field of viewprovides the opportunity to expand the energy range of the IN- TEGRAL surveys up to a few MeV. Based on only the firstyear’s data, Bouchet et al. (2005) detected 63 sources at ener-gies below 100 keV and four above 300 keV. The main contri-bution made by SPI was done in studies of the di ff use emissionof the Galaxy, which are beyond the scope of the current review(see review by Diehl, 2014). However, the positron annihilationline at 511 keV may have not only di ff use origin but can alsooriginate from the very vicinity of compact objects. A system-atic search for outbursts in the narrow positron annihilation lineon various time scales based on the INTEGRAL / SPI data wasperformed by Tsygankov and Churazov (2010). As a result,upper limits on the rate of outbursts with a given duration andflux in di ff erent parts of the sky were provided.The two JEM-X telescopes on-board INTEGRAL have asmaller (partially coded) field of view (10 ◦ in diameter) com-pared to the IBIS / ISGRI partially coded FOV of 28 ◦ × ◦ ,which restricted their ability to conduct wide-angle surveys;however, Grebenev and Mereminskiy (2015) released an X-raysurvey of the GC region based on ∼
10 years of observations(2003-2013).Table 1 summarizes the
INTEGRAL surveys conducted withIBIS / ISGRI, JEM-X, and SPI, listing important survey charac-teristics such as limiting flux, sky coverage, total number ofdetected sources and completeness. Since this table is sorted byyear of publication, one can see how sensitivity improves withtime, resulting in the growing number of detected sources. It isnot trivial to count all hard X-ray sources discovered with
IN-TEGRAL in di ff erent surveys by di ff erent research groups; how-ever, as seen in Table 1, the total number of new IGR sourcescan easily reach several hundreds of objects, which demon-strates great impact of INTEGRAL in surveying the hard X-raysky.The on-going survey of the Galactic plane with
INTEGRAL provides a continuous improvement in sensitivity, which makes8aper by
INTEGRAL ∆ E Sensitivity Sky Total number IGR Completeness d ) telescope [keV] [mCrab ( σ )] coverage of sources sources c ) Winkler et al. (2003b) IBIS / ISGRI 15 −
40 36 a ) (5 σ ) 110 10SPI 20 −
40 62 a ) (5 σ ) 33 3JEM-X 5 −
20 20 a ) (5 σ ) 50Revnivtsev et al. (2004b) IBIS / ISGRI 18 −
60 1 − ∼
900 deg
60 5 10 / / ISGRI 18 −
60 1 . ◦ × ◦
28 7 7 / / ISGRI 20 − ∼ / / ISGRI 20 − ∼ σ ) 40 ◦ × ◦
13 5 5 / / ISGRI 17 −
60 0 . − σ ) 50 ◦ × ◦
46 20 13 / −
150 100 ◦ × ◦
63 8Bird et al. (2006) IBIS / ISGRI 20 − ∼ ∼
50% 209 56 ∼ / ISGRI 100 − ∼ σ ) ∼
50% 49 100%Bird et al. (2007) IBIS / ISGRI 17 − ∼ ∼
70% 421 167 ∼ / ISGRI 17 − ∼ / / ISGRI 20 −
60, 1 b ) (3 σ ) 76 1860 −
150 3 b ) (3 σ ) 76 18JEM-X 3 −
10, 1810 −
25 18Paltani et al. (2008) IBIS / ISGRI 20 −
60 0.5 (5 σ ) 2500 deg
34 34 ∼ / ISGRI 17 −
60 0 .
26 (5 σ ) 100% 521 212 38 / / ISGRI 17 − < ∼ / ISGRI 17 − ∼ . . σ ) | b | < . ◦
402 180 ∼ / ISGRI 20 − ∼ . σ ) 640 deg
21 4 90%JEM-X 3 − ∼ σ ) ∼
100 deg
10 0 100%Krivonos et al. (2015) IBIS / ISGRI 100 − ∼ σ ) 100% 132 100%Grebenev et al. (2015) JEM-X 5 − | l , b | < ◦
105 24Bird et al. (2016) IBIS / ISGRI 17 − < ∼ / ISGRI 64 . − . ∼ . . σ ) | b | < . ◦ / ISGRI 17 − ∼ .
18 (4 σ ) 4900 deg
147 37 25 / / ISGRI 17 − ∼ .
15 (4 . σ ) | b | < . ◦
72 72 46 / Table 1: The list of the
INTEGRAL surveys. a ) Average sensitivity per one GPS scan. b ) Average sensitivity per season. c ) The total number of IGR sources discovered with
INTEGRAL in a given survey or previous works. d ) The completeness column describes the fraction of sources with known nature,if specified with percentile. The numbers shown as a fraction represent the number of unclassified sources with respect to the totalnumber of sources detected.it possible to probe deeper into the Galaxy. Fig. 7 shows aface-on schematic view of the Milky Way and the distances atwhich
INTEGRAL can observe a hard X-ray source of a givenluminosity L HX . One can notice that INTEGRAL can detect allsources with the luminosity L HX > × erg s − at the far endof the Galaxy in the direction towards the Galactic Centre; thedistance range for the luminosity L HX > × erg s − coversmost of the Galactic stellar mass; and the Galactic central bar isfully reachable at luminosities L HX > × erg s − (Krivonoset al., 2017). The deep sensitivity of modern hard X-ray surveys is largelyachieved by stacking large numbers of relatively short expo-sures taken for the same fields over many years. In the case ofIBIS, observations are divided into short pointings, or sciencewindows of typically 2000 s, separated by short slews during which the instrument pointing direction changes by a few de-grees. Each science window can be considered an independentmeasurement of the flux from all points in the field of view forthat pointing.The final outcome of this stacking approach is essentiallyused to derive the weighted mean of many 2000 s of measure-ments of source flux taken over a time period in excess of adecade. The weighted mean is used because the measurementquality is non-uniform, being a ff ected by several factors suchas exposure time, changing position of the source in the field ofview, and the presence of other bright sources in the field. For a persistent emitter, the weighted mean of the flux, and the erroron that weighted mean, is an excellent estimator of the meanflux and how significantly the mean flux is non-zero; this is thedetection significance usually quoted in survey catalogues. Inother words, in the assumption that the source is persistent, thesignificance tells us how confident we can be that we detect a9 igure 7: Face-on view of the Galaxy shown along with the distance range atwhich an X-ray source of a given luminosity L HX (or more) can be detected ac-cording to the 17 −
60 keV sensitivity of the 14-year
INTEGRAL survey (solidlines, Krivonos et al., 2017), compared to the 9-year Galactic plane survey (dot-ted lines, Krivonos et al., 2012). Red, orange and yellow contours correspondto L HX = × , 10 and 5 × erg s − , respectively. The backgroundimage is a sketch of the Galaxy adapted from Churchwell et al. (2009). non-zero flux from a given sky position. But this assumptionfails for variable or transient sources.In order to optimise transient source detection, sources mustbe searched for on di ff erent timescales. This may be done withthe construction of multiple maps covering di ff erent time peri-ods, or directly on light curves of known (or suspected) sources.Inevitably, biases are introduced when we search for emissionon a specific set of timescales, as we must make the problemtractable.The first IBIS survey (Bird et al., 2004) employed a straight-forward stacking analysis, but from the second catalogue (Birdet al., 2006) onward, which analysed ∼
18 months of data, itwas realised that source searches on additional time intervalswould be needed to optimise the detection of sources that onlyemitted on shorter timescales. Consequently, maps were con-structed and searched not only for the full archive, but also foreach revolution (satellite orbit, ∼ ∼ revolution sequences covering any observing period wherethe telescope performed a deep exposure on a single field. Dur-ing the third catalogue construction, it was noticed that sourcesdetected in previous catalogues were becoming di ffi cult to de-tect, and strategies were developed to deal with the increasingbaseline of the dataset. The problem is illustrated by the detec-tion of the gamma-ray burst GRB041219A, one of the bright-est sources ever detected by INTEGRAL. A strong detection ofGRB041219A in the specific science window reduces rapidlyas the observation window lengthens, and if more than ∼ bursticity anal-ysis, a sliding-window analysis that sought to detect sources onwhatever timescale optimised their detection significance. Mostrecently, the bursticity method was refined for the catalogue of1000 orbits (Bird et al., 2016) which had a dataset spanning 8years of satellite operations, and yet searches were performedfor transient emission on timescales down to 0.5 days.Fig. 8 illustrates how the bursticity method aids the recoveryof transient sources in long datasets, plotting the outburst signif-icance against the significance derived from the full light curve.Persistent sources fall along the line of y = x (the diagonaldashed white line), but many sources sit above y = x indicatingthat their significance can be enhanced in a more limited timeperiod. Sources that fall below the global significance thresh-old (the vertical red line) but above a burst detection threshold(horizontal dashed white line) can be recovered into the cata-logue. The level of the burst detection threshold is determinedexperimentally - see below. detection limit for persistent sources Stacked map significance bursticity noise floor (depends on light curve length) p e r s i s t e n t s o u r c e s t r a n s i e n t s o u r c e s ( d e t e c t a b l e i n s t a c k e d m a p s ) t r a n s i e n t s o u r ce s ( r ec o v e r e d b y b u r s t i c i t y ) (sigma) B u r s t i c i t y s i g n i fi ca n ce ( s i g m a ) ~5 ~ - Figure 8: Source recovery by bursticity (explanation in the text).
Bursticity searches, as currently implemented, are still some-what biased because in order to improve the speed of the algo-rithm not all window sizes are tested and the stride (the speedwith which the window passes along the light curve) is quitelarge. This means that not all possible windows are tested,although the assumption is made that the significance is onlyslowly degraded by using a non-optimal window, and only thevery faintest outbursts will be missed in this way.Of more concern is that bursticity is testing a very largenumber of non-independent windows, since the stride is typ-ically ∼
10% of the window length. This makes an analyt-ical determination of the false alarm probability, or the burstdetection threshold, di ffi cult. Furthermore, the burst detectionthreshold depends both on the length and the time structure of10he light curve. For the longest light curves (IBIS sources inthe Galactic Plane) more than 100,000 window tests are per-formed during a search. In practice, monte-carlo simulationsof a flux-randomised light curve with realistic temporal struc-ture are used to establish confidence limits. Such tests are re-ally only valid for light curves containing pure white noise, soany long-term source variability a ff ects the determination of theburst detection threshold. Furthermore, the detection thresholdactually increases with greater exposure as the number of trialsincreases, which is counter to normal expectations for persistentsources.
3. Follow-up campaigns of INTEGRAL surveys
Multiwavelength followup of serendipitously detected X-raysources is crucial to understand the properties of the objectsobserved, resulting in large imaging campaigns from radio fre-quencies to gamma-rays for specific areas of the sky. INTE-GRAL provides input for many follow-up X-ray and opticalcampaigns.
While the
INTEGRAL surveys have been very successful atfinding sources of high-energy emission, for new or previouslypoorly studied IGR sources, follow-up observations are nec-essary to obtain classifications.
INTEGRAL ’s few arcminutepositions typically do not allow for the identification of opti-cal or near-IR counterparts, especially in the crowded GalacticPlane, but follow-up X-ray observations reduce the error circlesto the subarcsecond level (with the
Chandra X-ray Observa-tory ) or the few arcsecond level (with
XMM-Newton or the
NeilGehrels Swift Observatory ), allowing for the correct counter-part to be found. In addition to localizations, soft X-ray spectraprovide important diagnostics for classifying sources, includ-ing the spectral slope in the 1-10 keV bandpass and the columndensity. Finally, for some IGR sources, the angular resolutionin the soft X-rays has led to the discovery of extended emission.Early in the
INTEGRAL mission, it was realized that manyIGR sources were being found in the spiral arm regions of theGalaxy, and this led to the discovery of many HMXBs. The firstIGR source, IGR J16318–4848, constitutes an excellent exam-ple where follow-up
XMM observations provided important in-formation. While the
XMM spectrum showed an extremely highcolumn density N H > cm − , the localization of the sourceindicated a match with a bright near-IR ( K s =
7) star, whichproved that a large amount of material was obscuring the X-raysource (Matt and Guainazzi, 2003; Walter et al., 2003). Spec-troscopy of the near-IR source showed that it is a B[e]-typesupergiant (Filliatre and Chaty, 2004), surrounded by an innercavity and an outer very large disc of gas and dust heated bythis hot star, similar to Herbig Ae / Be stars, the compact objectlikely orbiting close to the rim separating the cavity from thedisk (Chaty and Rahoui, 2012). The existence of this disk wasconfirmed by the detection of flat-topped iron lines originat-ing from a spherically symmetric disk wind, using broad-bandspectroscopy with the ESO / VLT X-Shooter instrument (Fortin et al., 2020). A stellar atmosphere and wind modeling, withthe PoWR code, of the optical to mid-IR spectral energy dis-tribution of this source – adding mid-infrared
Spitzer and
Her-schel data to these X-Shooter observations–, showed that thecentral star likely has an helium-enhanced atmosphere, due toan intense stellar wind shedding part of its hydrogen envelope(Fortin et al., 2020).A new class of obscured HMXBs had thus been discov-ered. IGR J16318–4848, with a likely long orbital period of ∼
80 days (Iyer and Paul, 2017), is in the Norma spiral armregion of the Galaxy, and further soft X-ray observations un-covered more HMXBs in this region. Although we still do notknow the nature of the compact object in IGR J16318–4848,IGR J16320–4751 was found to be an HMXB with a slowly ro-tating (1300 sec period) neutron star using
XMM observations(Lutovinov et al., 2005b; Garc´ıa et al., 2018). Additional IGRHMXBs were uncovered in the Norma region as well as otherpart of the Galaxy, using
Chandra localizations and informa-tion about the optical or near-IR counterpart (Tomsick et al.,2006, 2008, 2009, 2012a, 2016, 2020). Other identificationsmade use of the
Neil Gehrels Swift Observatory (e.g. Rodriguezet al., 2008, 2009b,a), and
XMM-Newton observations providedspectral and timing information about a large number of IGRHMXBs (Walter et al., 2006; Halpern et al., 2014).In addition to HMXBs, other large groups of IGR sourcesare Cataclysmic Variables (CVs) and Low-Mass X-ray Binaries(LMXBs, see e.g. Fortin et al., 2018; Lutovinov et al., 2020),Active Galactic Nuclei (AGN, see e.g. Tomsick et al., 2015),and pulsars or Pulsar Wind Nebulae (PWNe). In some cases,localizations by
Swift provide identifications (see e.g., Landiet al., 2017, and references therein). CVs are often nearby withbright optical counterparts (Landi et al., 2009a), and AGN usu-ally have IR or radio counterparts (Landi et al., 2009b). ForPWNe, extended X-ray emission is present, and IGR J11014–6103 provides a dramatic example (Tomsick et al., 2012b).While soft X-rays have been a critical component to classify-ing IGR sources, firm classifications most often require opticalor near-IR spectroscopy, which is discussed in the following.
With the publication of the 1 st INTEGRAL / IBIS survey (Birdet al., 2004) it was realized that about one third of the cat-alogued hard X-ray sources had no identified nature or hadtoo poor information on their characteristics. This percentageof unidentified or poorly known objects kept nearly constantin the subsequent issues of the all-sky
INTEGRAL / IBIS sur-veys (see Bird et al., 2010, for details). Therefore, the needfor a multiwavelength approach to pinpoint the nature of theseobjects, and ultimately the spectroscopic study of their opti-cal and / or near-infrared (NIR) counterparts was apparent. In-deed, this technique allows the identification of the nature of thenewly-discovered INTEGRAL sources and the characterizationthereof by exploring their spectral features (mainly emissionlines, which generally herald high-energy activity in cosmicsources) and overall continuum, thus permitting the determina-tion of the main physical parameters for these sources, such asdistance, luminosity and chemical composition among others.11owever, the first, straightforward attempts to unravel the na-ture of these emitters involved searching the online multiwave-length repositories, such as SIMBAD , for known conspicuous(i.e., line-emitting) optical objects within the IBIS error circleof unidentified INTEGRAL sources: a paradigmatic example ofthe application of this technique was the case of IGR J12349–6433 ( = RT Cru), for which a symbiotic star nature was first sug-gested by Masetti et al. (2005) on the basis of the localizationof this peculiar optical source inside the hard X–ray positionaluncertainty and of its published optical characteristics (Cieslin-ski et al., 1994): the identification was then confirmed with thedetection of soft X-rays (Tueller et al., 2005) from this opticalobject.Notwithstanding, the X-ray follow-up approach outlined inthe previous subsection, of course, allows a better knowledgeof the position of hard X-ray sources by reducing their errorcircles from a few arcminutes down to some arcseconds or less,thus reducing the search area in the sky by a factor of up to 10 .Indeed, Stephen et al. (2006) showed a low ( < INTEGRAL de-tections and those of softer X-ray sources within the hard X-rayerror circle; similar figures are found using radio surveys (e.g.,Maiorano et al., 2011). This approach largely helps pinpoint-ing the actual optical, as well as NIR, counterpart of the objectresponsible for the hard X–ray emission detected with
INTE-GRAL , which can then be studied through optical / NIR spec-troscopy (see Fig. 9 for a sketch; for details, see Chaty et al.,2008; Zurita Heras and Chaty, 2008; Butler et al., 2009; Coleiroet al., 2013; Fortin et al., 2018; Masetti et al., 2013; Bikmaevet al., 2006, 2008; Burenin et al., 2008, 2009; Lutovinov et al.,2012b, 2013a; ¨Ozbey Arabacı et al., 2012; Karasev et al., 2018,2020, and references therein), and also through mid-IR obser-vations (Rahoui et al., 2008).Fifteen years of optical and NIR spectroscopic follow-upstudies of unidentified
INTEGRAL sources performed by sev-eral groups worldwide led to a host of identifications: to the bestof our knowledge, 265 such objects had their nature identifiedor better described through optical / NIR spectroscopy, with thefollowing percentage breakdown: 58% AGNs, 28% GalacticX–ray binaries (3 / ∼ IN-TEGRAL sources in the first IBIS catalogue Bird et al. (2004)grouped into the same broad classes: this gives AGN, X-raybinary and CV percentages of 4%, 64% (with LMXBs beingmore than twice in number than HMXBs) and 3% and no ac-tive stars, respectively. Thus, the contribution of
INTEGRAL to http://simbad.u-strasbg.fr the advancement of our knowledge of the hard X–ray sky, com-bined with the optical / NIR followup of the unidentified sourcesin its surveys, has been multifold, namely: (1) it allowed theexploration of the extragalactic sky through the so-called ’Zoneof Avoidance’ along the Galactic Plane, where the attenuationinduced by dust and gas is not an hindrance for high-energy de-tectors; (2) similarly, it enhanced our knowledge on HMXBs byincreasing the source statistics; (3) it allowed the detection ofa siginficant number of (mostly magnetic) hard X–ray emittingCVs, which was unexpected (but not unprecedented in hind-sight, see, e.g., recent review of Lutovinov et al., 2020, andreferenes therein).Although in-depth presentations will be given in other contri-butions within this group of reviews, we would like to concludethis section by focusing on a few issues raised thanks to theoptical / NIR follow-up of
INTEGRAL sources: these considera-tions are connected with the points listed above and, accordingto us, deserve to be mentioned.First, it is stressed (Masetti et al., 2012) that the use ofmedium-sized and large telescopes (above 4 metres in diameter)allows the study of the faint end of the distribution of putativeoptical counterparts of the extragalactic share of these high-energy sources. Indeed, a Kolmogorov-Smirnov test showedthat the probability that the redshift distributions of the newly-identified hard X–ray AGNs and of the ones already classifiedin the
INTEGRAL surveys are the same is less than 0.001; thatis, the former ones are are drawn from a di ff erent distributionof more distant objects. Therefore, the deeper INTEGRAL ob-servations available with the latest surveys allow one to explorethe hard X-ray emitting sources in the far universe, at an av-erage distance ∼ / softX–rays plus optical suprisingly boosted the number of mag-netic CVs, suggesting this subclass as an important member ofGalactic X–ray sources: these systematic studies, according tothe review of de Martino et al. (2020), allowed increasing thesample the magnetic CV subclass by a factor of two, permittingextensive and dedicated explorations on specific cases, on theseobjects as a group, as well as comparative studies as a functionof the magnetic field strength of the white dwarf accretor.Finally, we briefly mention that the combination of the in-formation extracted from the INTEGRAL surveys and the opti-cal follow-up work allowed the discovery and / or the character-ization of a number of new classes of Galactic X-ray binaries,such as Supergiant Fast X-ray Transients (Sguera et al., 2005;Negueruela et al., 2006), Transitional Millisecond X–ray Pul-sars (e.g., de Martino et al., 2013; Bassa et al., 2014) and Sym-biotic X-ray Binaries (e.g., Masetti et al., 2007; Smith et al.,2012).The timely multiwavelength exploration of the INTEGRAL sources remaining to be identified and classified is thus of highimportance.12 igure 9: Graphic representation of the followup process described in Sect. 3.2, going from the accurate positioning through soft X–ray observations, to the opticalspectroscopy allowing the identification of the nature and main features of the source, and ending with its multiwavelength characterization thanks to the buildingof a spectral energy distribution for the object (adapted from Masetti et al., 2008).Figure 10: AGNs detected by INTEGRAL / IBIS surveys (Malizia et al., 2016). The stars represent the 107 new active galaxies studied in Malizia et al. (2016) andfirst reported in the INTEGRAL / IBIS survey (Bird et al., 2016). The circles show AGNs detected in previous INTEGRAL / IBIS surveys. Adapted from Maliziaet al. (2016). . Total CXB spectrum measurements by INTEGRAL inthe 4-200 keV band
INTEGRAL / IBIS (Ubertini et al., 2003) together withSwift / BAT (Barthelmy et al., 2005), having both good sensi-tivity and wide-field sky coverage, allowed to make a signifi-cant progress in the study of the high energy domain in the lastdecay. In particular they have provided a great improvementin our knowledge of the extragalactic sky by detecting morethan 1000 (mostly local) AGN at high energies. In Fig. 10 allthe AGN detected by
INTEGRAL / IBIS until 2016, and conse-quently classified and spectrally characterised, have been plot-ted (Malizia et al., 2016). The high energy band (20–200 keV)is extremely important for the study of the extragalactic sky andin particular for AGN; it is also the most appropriate for pop-ulation and survey studies, since it is almost unbiased againstobscuration, a severe bias which a ff ects surveys at other fre-quencies. The great improvement achieved in this field thanksto the INTEGRAL surveys has been extensively discussed in thededicated review on INTEGRAL view of AGN by Malizia etal. in this special issue. Here we want to stress the importanceof the survey work for the determination of the Cosmic X-RayBackground (CXB).As described above, in the deepest extra-galactic fields
IN-TEGRAL can reliably detect sources at the limiting 20–60 keVflux ∼ few 10 − erg s − cm − (e.g. Mereminskiy et al., 2016).At this sensitivity level only a small fraction ( ∼ few %) of the to-tal CXB in this energy band is resolved (Krivonos et al., 2007).Detection of fainter objects comprising the bulk of the CXBrequires a prohibitively long exposure time. This stems fromthe intrinsic limitation of coded mask telescopes, when photonsfrom a single source are distributed across the entire detector.At the same time, coded mask telescopes often have large FOVsand the total count rate associated with the CXB can be large.Therefore, the detector spectra of the telescopes on-board IN-TEGRAL contain information on the total flux from all resolvedand unresolved (no matter how faint they are) sources within theFOV.Separating the contribution of the CXB from the particlebackground in the detector spectra is a non-trivial exercise. One(di ffi cult) route is to build a comprehensive model of the detec-tor non-astrophysical background, which usually dominates thecount rate, so that this model can be subtracted from recordedspectra. Another possibility is to modulate the astrophysicalflux so that it can be singled out from the particle background.Here we discuss the results of total CXB flux measurements by INTEGRAL using the latter possibility.The modulation of the CXB flux has already been employedin early space X-ray experiments. In particular the HEAO-1 ob-servatory used a movable 5 cm thick CsI crystal to partly blockthe instrument field of view and to modulate the CXB signal(Kinzer et al., 1997; Gruber et al., 1999). For
INTEGRAL , theproblem of modulation was solved by ”placing” the Earth intothe telescopes FOVs (Churazov et al., 2007; T¨urler et al., 2010).The same technique was also used by Frontera et al. (2007) forthe BeppoSAX mission and by Ajello et al. (2008) for the dataof the Burst Alert Telescope (BAT) aboard the Swift spacecraft.
Figure 11: Illustration of the
INTEGRAL
Earth observing mode in 2006 (seeChurazov et al., 2007, for the original version of this figure). FOVs of JEM-X, IBIS and SPI are schematically shown with a circle, a box and an hexagonrespectively superposed on to the RXTE 3-20 keV slew map in Galactic coordi-nates. In the course of this observation the pointing direction of the telescopesremains the same, while the Earth moves across the instruments FOVs. Theday side of the Earth is shown by cyan color. As the distance from the Earthincreases during this portion of the 3-day INTEGRAL orbit, the angular sizeof the Earth disk decreases and the the solid angle of the obscured CXB goesdown.
INTEGRAL was launched onto a 3-day elongated orbit witha perigee of ∼ ∼
150 000 km.The orbit crosses the Earth radiation belts at the distance of ∼
50 000 km, so that useful observations are possible at largerdistances. During
INTEGRAL observations of the Earth in2006, the satellite was kept in a controlled 3-axis stabiliza-tion with telescopes’ axes staring at a point where the Earthwas predicted to be some 6 hours after the perigee passage (seeFig. 11). Four such observations were performed in 2006, eachlasting about 30 ks ( ∼ ∼ . ◦ , i.e. the CXB signal from some 90 sq.deg. wassubtended. In terms of the flux near 30 keV, such solid anglecorresponds to ∼
200 mCrab, i.e. a very significant modula-tion. Moreover, as the distance from the Earth changes and theEarth moves through the FOV, the modulation amplitude can bereadily predicted and, therefore, used to separate it from othercontaminating signals.Apart from the genuine CXB signal, modulated by the ob-scuration by the Earth disk, there is a number of other variablecomponents that have to be accounted for. Namely: • Individual compact X-ray sources (mostly in the Galaxy)which induce sharp edges in the recorder light-curveswhen the disk edge goes over them. • Unresolved foreground emission of the Galactic Ridge. • Emission of the Earth atmosphere due to scattering ofthe CXB photons (Churazov et al., 2008) and induced bycosmic rays impinging the atmosphere (Sazonov et al.,2007a).14 igure 12: Model lightcurves of each component for a ∼ ∼
27 keV during the first
INTEGRAL observation of theEarth. The drop of the detector count rate due to the shadowing of the CXBis shown with red circles. The other components are the e ff ective contributionof the point sources (orange stars), the GRXE (blue squares), the Earth CXBreflection (cyan triangles, long-dashed), the CR-induced emission (magenta tri-angles, short-dashed), and the estimated instrumental background (black dia-monds). Adapted from T¨urler et al. (2010). • Earth Auroral emission that was strong and variable in sev-eral
INTEGRAL observations. • Variability of the intrinsic detector background.Individual compact X-ray sources are relatively easy to dealwith, as long as they are not variable. Their spectra can bemeasured using a portion of the observation when they are notobscured by the Earth disk. Their modulation pattern can be de-scribed as a simple mask function (either 0 or 1 at any moment)set by their position with respect to the Earth disk (see Fig. 12).The Galactic Ridge emission is more di ffi cult to accountproperly due to its di ff use nature. The portions of the data whenthe Earth disk was moving over the bright regions of the Ridgecan be either ignored (Churazov et al., 2007), or a spatial modelof the Ridge can be used (T¨urler et al., 2010).The idea of having the Earth shadowing the CXB is basedon the assumption that its disk is dark in X-rays. This is onlypartly true, since the CXB photons can be partly reflected by theEarth atmosphere, while cosmic rays can generate secondarygamma-radiation. The CXB radiation reflected by the atmo-sphere (Fig. 13) can be straightforwardly calculated (Churazovet al., 2008), although it depends on the CXB spectrum itself.Fortunately, the albedo, i.e. the ratio of the reflected and theincident spectra is weakly sensitive to the shape of the inci-dent spectrum in the relevant energy band, implying that thereflected component can be readily evaluated if the albedo iscalculated using reasonable guess on the CXB shape. Figure 13: The CXB spectrum (top) and the spectrum reflected by the Earthatmosphere (bottom). The reflected spectrum was integrated over all angles.The features in the reflected spectrum near 3 keV are the fluorescent lines ofArgon. Adapted from Churazov et al. (2008).
Cosmic rays, in particular protons impinging the Earth atmo-sphere, undergo a series of hadronic interactions and electro-magnetic cascades to induce a glow of the atmosphere in hardX-rays. Examples of expected spectra from the Monte Carlosimulations (Sazonov et al., 2007a) based on the GEANT4 soft-ware package (Agostinelli et al., 2003) are shown in Fig. 14.Finally, the emission of the Earth Aurora can be bright andhighly variable. Currently, there are no good recipes for prop-erly modeling the contribution of the Aurora. Therefore all ob-servations severely a ff ected by the Aurora were excluded fromthe analysis. For instance, among four 30 ks INTEGRAL obser-vations of the Earth done in 2006, two have signatures of theAurora emission.With all the components mentioned above, the spectrum ob-served by the
INTEGRAL instruments S ( E , T ) at any given mo-ment can be represented as S ( E , t ) = B ( E , t ) + CXB ( E ) − CXB ( E ) × Ω ( t ) × [1 − A ( E )] + CR ( E ) × Ω ( t ) +Σ i I i ( E , α, δ ) × M ( α, δ, t ) , (1)where B ( E , t ) is the intrinsic background; CXB ( E ) is the spec-trum of the CXB, Ω ( t ) is the solid angle subtended by the Earthdisk, A is the Earth albedo for the CXB-like spectrum (Fig. 13), CR ( E ) is the particle-induced emission of the Earth atmosphere(Fig. 14), I i ( E , α, δ ) is the spectrum of individual bright sourcelocated at coordinates ( α, δ ) and M ( α, δ, t ) is the time dependentmask due to the Earth occultation of the source position.The examples of the light curves predicted for the IS-GRI / IBIS instrument in a narrow energy band near 27 keV are15 igure 14: Examples of simulated spectra (solid lines) of atmospheric emissionproduced by cosmic protons of given energy: E p =
1, 10 and 100 GeV. It canbe seen that in the photon energy range 25-300 keV the shape of the emergentspectrum is almost invariant (the dashed lines). Adapted from Sazonov et al.(2007a). shown in Fig. 12. As is clear from this Figure and also fromeq. (1), the CXB obscuration, the CXB and CR albedos sharethe same time dependence associated with Ω ( t ). Therefore,these components have to be modeled simultaneously using theinformation about their spectral shapes (see Figs. 13 and 14).Actual light curves for all three instruments on board the IN-TEGRAL are shown in Fig. 15. It is clear that the detector lightcurves are indeed modulated and the time variations are con-sistent with expectations (red curves). Repeating the analysisin many di ff erent bands and measuring the amplitude of themodulation allows the reconstruction of the CXB spectrum (seeChurazov et al., 2007; T¨urler et al., 2010, for details of the anal-ysis).The derived CXB spectrum is shown in Fig. 16 along withthe data from other experiments, including HEAO1 A4 in the100–300 keV band (Gruber et al., 1999) and Swift / BAT in the14–195 keV band (Ajello et al., 2008). The obtained normaliza-tion of CXB spectrum is ∼
10% higher than suggested by Gruberet al. (1999) and consistent with recent CXB measurement per-formed with the
NuSTAR telescope in 3–20 keV band (Krivonoset al., 2020). The observed CXB spectrum is well described bythe standard population synthesis model of AGNs, including thefraction of Compton-thick AGNs and the reflection strengthsfrom the accretion disk and torus based on the luminosity- andredshift- dependent unified scheme (Ueda et al., 2014).
5. Conclusions
One of the many areas where the
INTEGRAL observatoryprovides a significant scientific outcome to the astrophysical
Figure 15: The light curves (crosses) of JEM-X, IBIS / ISGRI and SPI instru-ments in units of counts per second. The red curves show the model light curvethat includes the shadowing by the Earth disk. In the middle panel the bluecurve shows schematically (with arbitrary normalization) the time dependenceof the Galactic Ridge emission, modulated by the Earth occultation. In orderto avoid contamination of the CXB measurements due to Galactic plane contri-bution, the first few ksec of data (on the left of the dotted vertical lines) weredropped from the analysis. Note that for JEM-X a less strict cut was appliedsince its field of view is smaller than that of the other instruments. Adaptedfrom Churazov et al. (2007).Figure 16: Cosmic X-ray Background (CXB) spectrum calculated from AGNpopulation synthesis models (upper solid curve, red; Ueda et al., 2014) com-pared with the observed data by di ff erent X-ray missions (Ajello et al., 2008).Middle solid curve (black): the integrated spectrum of Compton-thin AGNs(log NH < = = = <
22. Data points in the 0.8–5 keV (blue), 4–215 keV (cyan), 14–195 keV (magenta), and 100–300 keV (green) bands referto the CXB spectra observed with ASCA / SIS (Gendreau et al., 1995),
INTE-GRAL (Churazov et al., 2007), Swift / BAT (Ajello et al., 2008), and HEAO A4(Gruber et al., 1999), respectively. Adapted from Ueda et al. (2014).
IN-TEGRAL surveys of the Galactic Plane and extragalactic fieldstriggered a large number of new studies and observational cam-paigns in other wavelengths.Thanks to its coded-aperture design, the IBIS telescope, themain instrument for
INTEGRAL hard X-ray surveys, incorpo-rates a very large fully-coded FOV of 28 ◦ × ◦ , which allowsto conduct cartography of the sky in reasonable time. In par-ticular, INTEGRAL is able to take hard X-ray snapshots of thewhole Milky Way over a time scale of a year, which is far fromthe capabilities of narrow-FOV grazing X-ray telescopes.Apart from providing the census of hard X-ray emitters overthe whole sky,
INTEGRAL conducted a unique observation ofthe large-scale cosmic X-ray background via Earth-occultationmanoeuvre, which will undoubtedly be included in the legacyof the
INTEGRAL observatory.
List of abbreviations
List of definitions of abbreviations used in the paper.FOV: Field of View;AGN: Active Galactic Nuclei;HMXB: High Mass X-ray Binary;LMXB: Low Mass X-ray Binary;CV: Cataclysmic Variable;PSR: Pulsar;PWN: Pulsar Wind Nebula;CXB: Cosmic X-Ray Background;GRXE: Galactic Ridge X-ray Emission;PoWR: the Potsdam Wolf-Rayet Models.
Acknowledgements
We would like to thank all our colleagues who contributedover the years to INTEGRAL data analysis and the interpreta-tion. This review is based on observations with INTEGRAL,an ESA project with instruments and the science data cen-tre funded by ESA member states (especially the PI coun-tries: Denmark, France, Germany, Italy, Switzerland, Spain),and Poland, and with the participation of Russia and the USA.RK, EC and RS acknowledge support from the Russian Sci-ence Foundation grant 19-12-00369 in working on this review.The Italian co-authors acknowledge support from the ItalianSpace Agency (ASI) via di ff erent agreements including the lastones, 2017-14-H.0 and 2019.HH-35-HH.0. JAT acknowledgespartial support from NASA through Chandra
Award NumberGO8-19030X issued by the
Chandra
X-ray Observatory Cen-ter, which is operated by the Smithsonian Astrophysical Obser-vatory.
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