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Fig. 1:
X-ray image of the inner bulge of the Andromeda galaxy in the 0.5-2
keV band obtained by Chandra. The majority of sources inside the dark
blue circle (radius of 60") are formed via tidal captures of low-mass
stars by black holes and neutron stars and via collisions of the
latter with red giants.
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Fig. 2:
The radial distribution of compact X-ray sources in M31. Two smooth
solid lines show projected distributions of stellar mass ρ∗ and
of its square ρ∗².
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It has been long known that the ratio of the number of low mass
X-ray binaries (LMXBs) to stellar mass is ~ two orders of
magnitude higher in globular clusters than in the Galactic
disc. With the advent of Chandra and XMM-Newton, studies of X-ray
point sources in external galaxies have become possible, and have
shown that also there globular clusters are especially abundant in LMXBs.
This is attributed to dynamical processes, through which LMXBs are
formed in close encounters of compact objects with normal stars.
The most well-known of these is tidal capture of a main sequence
star by a neutron star or a black hole. Collisions of compact objects
with red giants and exchange reactions in the course of binary-single
interactions are also important.
Due to the ρ∗² dependence of two-body encounters on the stellar
density (ρ∗) they are frequent in globular clusters and are
negligible in the field. For example, in massive ellipticals which
are usually characterized by rich globular cluster systems, as much as
∼ 2/3 of X-ray binaries may be located in globular clusters.
In the central parts of massive galaxies, the stellar densities can
reach ∼ 103 - 104 pc-3. This is still somewhat lower
than the densities found in the most luminous globular clusters, where
the LMXBs are preferentially found.
However, the large volume compensates for the smaller density
and LMXBs can be created near the galactic centers
in two-body encounters in significant numbers.
Due to the large stellar mass contained in the central region of a
galaxy, a number of primordial LMXBs formed through the standard
evolutionary path exist there too. Although these can not be easily
distinguished from binaries resulting from two-body encounters,
a statistical argument can be employed which has been used previously
for the discovery of dynamical formation of binaries in globular
clusters. The volume density of the primordial LMXBs follows the
distribution of the stellar mass in a galaxy whereas
the spatial distribution of the dynamically formed binaries
is expected to obey the ρ∗²/v law.
Hence the latter should be expected to be much more concentrated
towards the center of the host galaxy and reveal themselves as a
population of “surplus” sources in its nucleus.
M31 is the closest “full-size” spiral galaxy. At a distance
of 780 kpc X-ray sources can be easily resolved with
Chandra, even near the center of the galaxy (Fig.1).
It has been extensively explored with
Chandra. Using these data we find a significant increase of the
specific frequency of X-ray sources, per unit stellar mass, within 1
arcmin from the center of the galaxy. The radial distribution of
surplus sources in this region follows the ρ∗² law
(Fig.2), suggesting that they are low-mass X-ray
binaries formed dynamically in the dense stellar environment of the
inner bulge.
While dynamical interactions in globular clusters have been
investigated in the 70-ies and 80-ies, the parameter range
typical of galactic centers remains unexplored. Velocities of stars
are an order of magnitude higher in the bulge, changing the character
of the dynamical interactions and, hence, the role of
different formation channels.
We have investigated dynamical formation of binaries and their
further evolution to the X-ray active phase in the high
stellar velocity regime.
We find that X-ray binaries are formed in the bulge of a
typical spiral galaxy at a rate of ∼ 50 - 100 per Gyr.
The calculations suggest that the majority of the surplus sources
result from tidal captures of black holes by main sequence stars of
low mass, M∗ < 0.3Msun, with some contribution of neutron star
systems of same type. Because of large stellar velocities in the bulge,
very compact and bright binary systems will be created, with X-ray
luminosity log LX > 37 and orbital periods ∼ 1 - 2 hours and
shorter. Due to the small size of the accretion discs in such systems
a large fraction of black holes may be persistent X-ray sources, in
contrast to the population of mostly transient primordial black
hole binaries in the Galactic disk. Some of sources will be
ultra-compact X-ray binaries with helium star/white
dwarf companions formed in the collisions of compact objects with red
giants. We also predict a large number of faint
transients within ∼ 1 arcmin from the M31 galactic center, both
BH and NS systems. The latter may be progenitors of
the accreting millisecond pulsars, similar to the famous
SAX J1808.4-3658 discovered in our Galaxy ten
years ago.
Marat Gilfanov and Rasmus Voss
Publications
R.Voss and M.Gilfanov, 2007, A&A, 468, 49
R.Voss and M.Gilfanov, 2007, MNRAS, 380, 1685
R.Voss, M.Gilfanov, R.Kraft et al., 2009, ApJ, submitted
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