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 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 us 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 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 we used
results of the All Sky Survey recently performed by INTEGRAL (see
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 (see e.g.
INTEGRAL Picture of the Month June 2006).
Details of these discoveries are presented in two papers (M. Revnivtsev et al.)
, which are accepted for publication in Astronomy and Astrophysics
(preprints
arXiv:0805.0259,
arXiv:0805.2699)
