| Issue |
A&A
Volume 468, Number 1, June II 2007
|
|
|---|---|---|
| Page(s) | 49 - 59 | |
| Section | Extragalactic astronomy | |
| DOI | https://doi.org/10.1051/0004-6361:20066614 | |
| Published online | 26 March 2007 | |
A study of the population of LMXBs in the bulge of M 31*
1
Max Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany e-mail: [voss;gilfanov]@mpa-garching.mpg.de
2
Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
Received:
21
October
2006
Accepted:
27
February
2007
Aims. We explore the population of X-ray point sources in the bulge of M 31 to contrast properties of various subpopulations, such as persistent and transient sources and primordial LMXBs and dynamically formed ones.
Methods.Based on the data from 26 archival Chandra observations we study the source content and properties of various subpopulations of X-ray sources to a maximum distance of 12´ from the centre of M 31.
Results.
To a limiting luminosity of 
erg s-1 we find
263 X-ray point sources, with 
of these being background
galaxies.
A study of the spatial distribution and the
luminosity function of the X-ray sources shows that the distribution
of primordial LMXBs is consistent with the distribution of the
K-band light and that their luminosity function flattens
below 
erg s-1 to the 
law
in agreement with the behaviour found earlier for LMXBs in the Milky
Way and in Cen A. Within a radius of 
, the
luminosity function is independent of distance to the centre
of M 31, in contrast to earlier Chandra studies.
The LMXBs located in globular clusters and within 
from
the centre of M 31 are presumably created via dynamical
interactions. The dynamical origin of the 
sources is
strongly suggested by their radial distribution which follows the
profile
rather than the K-band light distribution.
Their luminosity function shows a prominent
fall-off below 
. Although the statistics
is insufficient to claim a genuine low-luminosity cut-off in the luminosity
function,
the best fit powerlaw with a slope of 
is
significantly flatter than the law.
We also searched for transients and found 28 sources
that varied by a factor larger than 20. Their spatial distribution
follows the distribution of the persistent LMXBs within the accuracy
allowed by the limited number of transients.
Key words: galaxies: individual: M 31 / X-rays: binaries / X-rays: galaxies
© ESO, 2007
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