15 years of galactic surveys and hard X-ray background measurements

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Abstract

The INTEGRAL hard X-ray surveys have proven to be of fundamental importance. INTEGRAL has mapped the Galactic plane with its large field of view and excellent sensitivity. Such hard X-ray snapshots of the whole Milky Way on a time scale of a year are beyond the capabilities of past and current narrow-FOV grazing incidence X-ray telescopes. By expanding the INTEGRAL X-ray survey into shorter timescales, a productive search for transient X-ray emitters was made possible. In more than fifteen years of operation, 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 manoeuvres, 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 our understanding of the hard X-ray sky.

Introduction

A wide variety of astrophysical phenomena cannot be sufficiently well investigated via observations of individual sources, but requires instead a systematic approach based on large statistical samples. The last few decades of X-ray astronomy have provided us with great opportunities for studies of the populations of compact X-ray sources (white dwarfs, neutron stars, black holes) in our Galaxy and beyond, with the use of new long-lasting facilities.

Two powerful and currently active hard X-ray missions, ESA’s INTEGRAL observatory (Winkler et al., 2003a) and NASA’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 contrast to Swift, with a nearly uniform all-sky survey, which is especially useful for studies of active galactic nuclei (AGN; Tueller et al., 2010, Cusumano et al., 2010, Ajello et al., 2012, Baumgartner et al., 2013, Oh et al., 2018), the INTEGRAL observatory provides a sky survey with exposures that are deeper in the Galactic plane (GP) and Galactic Centre (GC) regions and with higher angular resolution, which is essential in these crowded regions. It allowed to study in depth different populations of galactic binary systems, such as low- and high-mass X-ray binaries, cataclysmic variables, symbiotic systems, etc. (see, e.g., Revnivtsev et al., 2008, Bodaghee et al., 2012, Lutovinov et al., 2005a, Lutovinov et al., 2013b, Kretschmar et al., 2019). This makes the Swift and INTEGRAL surveys complementary to each other. In this review we concentrate on the valuable contribution of the INTEGRAL observatory to the surveying of the hard X-ray sky over the last 15 years.

The INTEGRAL observatory, selected as the M2 mission within ESA’s Horizon 2000 program, has been successfully operating in orbit since its launch in 2002. Due to the high sensitivity and relatively good angular resolution of its instruments, in particular the coded-mask telescope IBIS (Ubertini et al., 2003), surveying the sky in hard X-rays is one of the mission primary goals.

Section snippets

Observations before INTEGRAL

Since the beginning of X-ray astronomy many X-ray surveys have been successfully carried out with the aim of both discovering new types of X-ray emitters and to investigate the nature of the Cosmic X-ray Background. A brief review of the hard X-ray surveys before the INTEGRAL era is presented hereafter. A more detailed overview of the Hard X-ray/Soft gamma-ray experiments and missions can be found in Cavallari and Frontera (2017).

Markert et al. (1979) described observations of the cosmic X-ray

Follow-up campaigns of INTEGRAL surveys

Multiwavelength followup of serendipitously detected X-ray sources is crucial to understand the properties of the objects observed, resulting in large imaging campaigns from radio frequencies to gamma-rays for specific areas of the sky. INTEGRAL provides input for many follow-up X-ray and optical campaigns.

Total CXB spectrum measurements by INTEGRAL in the 4–200 keV band

INTEGRAL/IBIS (Ubertini et al., 2003) together with Swift/BAT (Barthelmy et al., 2005), having both good sensitivity and wide-field sky coverage, allowed to make a significant progress in the study of the high energy domain in the last decay. In particular they have provided a great improvement in our knowledge of the extragalactic sky by detecting more than 1000 (mostly local) AGN at high energies. In Fig. 10 all the AGN detected by INTEGRAL/IBIS until 2016, and consequently classified and

Conclusions

One of the many areas where the INTEGRAL observatory provides a significant scientific outcome to the astrophysical community is surveying the sky at energies above 20 keV. INTEGRAL surveys of the Galactic Plane and extragalactic fields triggered a large number of new studies and observational campaigns in other wavelengths.

Thanks to its coded-aperture design, the IBIS telescope, the main instrument for INTEGRAL hard X-ray surveys, incorporates a very large fully-coded FOV of 28°×28°, which

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 contributed over the years to INTEGRAL data analysis and the interpretation. This review is based on observations with INTEGRAL, an ESA project with instruments and the science data centre funded by ESA member states (especially the PI countries: 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 Science Foundation grant 19-12-00369 in

Roman Krivonos has been working in the field of X-ray astronomy including all aspects of experiments: instrumentation, observation, and data interpretation since 2001, when he came to the High Energy Astrophysics Department of the Space Research Institute (IKI), Moscow. He graduated with a degree in Nuclear Physics from the National Research Nuclear University MEPHI (Moscow Engineering Physics Institute) in 2003. From the beginning of the INTEGRAL mission he worked on the IBIS data analysis, in

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  • Roman Krivonos has been working in the field of X-ray astronomy including all aspects of experiments: instrumentation, observation, and data interpretation since 2001, when he came to the High Energy Astrophysics Department of the Space Research Institute (IKI), Moscow. He graduated with a degree in Nuclear Physics from the National Research Nuclear University MEPHI (Moscow Engineering Physics Institute) in 2003. From the beginning of the INTEGRAL mission he worked on the IBIS data analysis, in particular on the hard X-ray surveys and accretion processes in the Galactic X-ray binaries and nearby supermassive black holes. Roman obtained a Ph.D. in 2007 from IKI with a thesis on the X-ray background observed at large angular scales: the Galactic Ridge X-ray Emission and Cosmic X-ray Background. In 2008, Roman continued to work with the INTEGRAL data as a Postdoctoral researcher at Max Planck Institute for Astrophysics, Garching, Germany. He has served in the Target Allocation Committee of INTEGRAL in 2007–2015. In 2012, Roman accepted an Assistant Project Scientist Position in the Space Sciences Laboratory at the University of California (USA), where he got involved with NuSTAR – the first focussing hard X-ray satellite in orbit, as part of the science team during the primary phase of the mission. Roman returned back to IKI in 2015, where he continues to work on data from INTEGRAL, NuSTAR and other X-ray telescopes, in particular, he takes an active part in the work of the Spectrum-Roentgen-Gamma project launched in 2019. By the end of 2020, the list of peer-reviewed publications by Roman, or with his active participation, contains more than 180 refereed papers, cited more than 3000 times in the scientific literature (H-index 25).

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