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Dynamical formation of X-ray binaries in the nucleus of Andromeda galaxy.

Scientists at the Max-Planck-Institute for Astrophysics have identified an unusual population of bright compact X-ray sources in the nucleus of Andromeda galaxy. These sources are powered by accretion onto relativistic objects and have been formed in close encounters of black holes and neutron stars with ordinary stars, occurring frequently in the high stellar density environment of the nucleus.

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.

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 ρ².

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 ρ² 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 ρ²/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 ρ² 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


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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