Low-mass X-ray binaries in the bulge of the Milky Way

¹ Max-Planck-Institute für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany e-mail: mikej@mpa-garching.mpg.de
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
³ Excellence Cluster Universe, Technische Universität München, Garching, Germany

Received: 2 May 2008
Accepted: 21 July 2008

Abstract

We study the population of low-mass X-ray binaries (LMXBs) in the Galactic bulge using the deep survey of this region by the IBIS telescope aboard the INTEGRAL observatory. Thanks to the increased sensitivity with respect to previous surveys of this field, we succeeded in probing the luminosity function (LF) of LMXBs down to ~710³⁴ erg/s in the 17-60 keV energy band. The slope of the 17-60 keV LF in the erg/s range is d log Nd log L = -0.96±0.20. At these low luminosities, the spectra of LMXBs between 2 and 60 keV are similar to that of the Crab, and therefore nearly equal amounts of energy are released in the standard (2-10 keV) and hard (17-60 keV) X-ray bands. This implies that the LMXB LF is flat both in the 17-60 keV and 2-10 keV energy bands over the  erg/s luminosity range. This further suggests that the faint end of the 2-10 keV LF is significantly flatter than its bright end (above 10³⁷ erg/s), where a slope of d log N/d log L ~ -1.8 was previously measured. We discuss the origin of this flattening. We also demonstrate that the spatial distribution of persistent LMXBs in the Galactic center/Galactic bulge region is consistent with a model of stellar mass distribution that includes the nuclear stellar disk component in the innermost degree of the Galaxy. The spatial distribution of transient LMXBs detected in the Galactic center region indicates an increased fraction of transient sources in the innermost degree of the Galaxy with respect to outer regions.