Contribution of cataclysmic variables and low-mass X-ray binaries to galactic hard X-ray emission
Top: Map of the Galactic Center region in the 17-60 keV energy band obtained by INTEGRAL/IBIS. The ellipse encloses the region of a study done by M. Revnivtsev and colleagues. Sources used in the analysis of the low-mass X-ray binary (LMXB) luminosity function are marked by circles. Contours are isophotes of the 4.9 μm surface brightness of the Galaxy (COBE/DIRBE) demonstrating the bulge/disk structure of the inner Galaxy.
Bottom: Combined luminosity function of cataclysmic variables (Lhx < 1034.5 erg s-1, red) and LMXBs (Lhx > 1034.5 erg s−1, blue).
The INTEGRAL observatory has devoted a major part of its observing time to investigate populations of X-ray and hard X-ray sources in our Galaxy and achieved a few breakthroughs in our understanding of these populations.
The most luminous X-ray sources in our Galaxy - binary stars with compact objects and with low mass companion stars (low mass X-ray binaries, LMXBs) - are strongly concentrated towards the Galactic bulge, where INTEGRAL has now accumulated very deep images (see top panel). A very deep census of LMXBs in this region allowed for the first time to detect sources with low luminosities, to construct their luminosity function and to study their spatial distribution. It was found that these low luminosity LMXBs might be compact binaries with very small accretion disks and with hydrogen poor or even degenerated companion stars and the mass overflow from their companion stars is governed by emission of gravitational waves. The spatial distribution of LMXBs in the Galactic bulge region matches the distribution of stellar mass and even allows us to marginally detect the presence of the Nuclear stellar disk at the heart of our Galaxy.
At fainter hard X-ray luminosities (see bottom panel), binary stars with accreting white dwarfs (also known as cataclysmic variables, CV) become the dominant population. To make a census of these sources M. Revnivtsev and colleagues used results of the All Sky Survey recently performed by INTEGRAL (see related publication in right-hand menu by Krivonos et al. 2007, or http://hea.iki.rssi.ru/rsdc/catalog/ index.php). It turned out that their cumulative contribution over the entire Galaxy can well explain the unresolved hard X-ray (17-100 keV) emission of the Galactic ridge.
Details of these discoveries are presented in two papers (M. Revnivtsev et al. [2008a] and [2008b]), which are accepted for publication in Astronomy and Astrophysics (preprints arXiv:0805.0259, arXiv:0805.2699, see related publications in the right-hand menu).