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

TL;DR

This study analyzes the luminosity function and spatial distribution of low-mass X-ray binaries in the Milky Way's bulge using deep INTEGRAL survey data, revealing a flattening of the LF at lower luminosities and a concentration of transients near the Galactic center.

Contribution

It provides the first detailed measurement of the LMXB luminosity function down to low luminosities and models their spatial distribution including the nuclear stellar disk.

Findings

LF flattens below 1e37 erg/sec

Spatial distribution aligns with stellar mass including nuclear disk

Higher transient fraction in the innermost Galaxy

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 to probe the luminosity function (LF) of LMXBs down to ~7e34 erg/sec in the 17-60keV energy band. The slope dlog N/dlog L=-0.96+/0.20 measured in the 1e35-1e37 erg/sec range confirms that the LMXB LF flattens below L_x<1e37 erg/sec with respect to higher luminosities. We discuss the origin of the observed LF flattening. We 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.