The Luminosity Function of X-ray Sources in Spiral Galaxies

TL;DR

This study constructs and analyzes the X-ray luminosity functions of hard and soft sources in spiral galaxies, revealing different populations and properties consistent with low and high mass X-ray binaries.

Contribution

It provides the first combined XLF analysis segregated by X-ray color in spiral galaxies, supporting the identification of source types and their properties.

Findings

Hard sources' XLF fits a power law similar to elliptical galaxies' LMXBs.

Soft sources' XLF resembles the universal HMXB XLF.

Some soft sources may be super-soft ultra-luminous X-ray sources.

Abstract

X-ray sources in spiral galaxies can be approximately classified into bulge and disk populations. The bulge (or hard) sources have X-ray colors which are consistent with Low Mass X-ray Binaries (LMXB) but the the disk sources have softer colors suggesting a different type of source. In this paper, we further study the properties of hard and soft sources by constructing color segregated X-ray Luminosity Functions (XLF) for these two populations. Since the number of sources in any given galaxy is small, we coadded sources from a sample of nearby, face-on spiral galaxies observed by Chandra as a Large Project in Cycle 2. We use simulations to carefully correct the XLF for completeness. The composite hard source XLF is not consistent with a single power-law fit. At luminosities Lx>3E38 ergs/s it is well fit by a power law with a slope that is consistent with that found for sources in elliptical galaxies by Kim and Fabbiano 2004. This is supports the suggestion that the hard sources are dominated by LMXBs. In contrast, the high luminosity XLF of soft sources has a slope similar to the ``universal'' High Mass X-ray Binary XLF. Some of these sources are stellar mass black-hole binaries accreting at high rates in a thermal/steep power law state. The softest sources have inferred disk temperatures that are considerably lower than found in galactic black holes binaries. These sources are not well understood, but some may be super-soft ultra-luminous X-ray sources in a quiescent state as suggested by Soria et al 2009.