Galactic X‐ray source populations

Abstract

The landscape of Galactic X-ray sources made of accreting binaries, isolated objects and active stellar coronae has been significantly modified by the advent of the Chandra, XMM-Newton and INTEGRAL satellites. New types of relatively low X-ray luminosity X-ray binaries have been unveiled in the Galactic disc, while deep observations of the central regions have revealed large numbers of X-ray binaries of so far poorly constrained nature. Because of the high spatial resolution needed and faint X-ray luminosities generally emitted, studying the dependency of the X-ray source composition with parent stellar population, Galactic disc, bulge, nuclear bulge, etc., is only practicable in our Galaxy. The evolutionary links between low LX X-ray binaries and classical X-ray luminous accreting systems are still open in many cases. In addition, the important question of the nature of the compact sources contributing to the Galactic ridge hard X-ray emission remains unresolved. We review the most important results gathered by XMM-Newton over the last years in this domain and show how future observations could be instrumental in addressing several of these issues. The galaxy viewed in X-raysThe discovery of luminous X-ray binaries in the 1960s marked the birth of X-ray astronomy by revealing the source of their power as accretion onto neutron stars or black holes. Among the most salient outcomes of these early studies was a rapid advance in our understanding of accretion disc physics and of emission mechanisms in extreme conditions of gravity and magnetic fields. Sustained improvements in the sensitivity and spatial resolution of X-ray instruments further revealed that many, if not all, classes of astrophysical objects can emit X-rays. In particular, the Einstein observatory and a few years later the ROSAT satellite, with their on-board X-ray telescopes, unveiled the ubiquitous high energy emission of stellar coronae, young stars, early type stars, cataclysmic variables, isolated neutron stars, quiescent stages of low and high mass X-ray binaries.It is now clear that the bright "historical" X-ray sources constitute only the tip of an iceberg harbouring an enormous number of fainter X-ray emitters with typical X-ray luminosities in the range of 10 26−36 erg s −1 . These sources exhibit a wide range of X-ray spectra and timing signatures reflecting very different physical states and emitting mechanisms. Unquestionably X-ray observations have opened a new window for the understanding of such objects.It is far beyond the scope of this short review paper to encompass all astrophysical processes at work in Galactic X-ray sources and we will only recall here the main classes of X-ray emitters. Accretion onto a compact object, a white dwarf, a neutron star or a black hole accounts for the X-ray emission of the most luminous X-ray binaCorresponding author: motch@astro.u-strasbg.fr ries with L X in the range of 10 29−39 erg s −1 depending on accretion rate and depth of the gravitational potential well. Coronal magnetic activity in solar...