An XMM-Newton View of the ANdromeda Galaxy as Explored in a Legacy Survey (New-ANGELS) II: Luminosity Function of X-ray Sources

TL;DR

This study analyzes the X-ray luminosity functions of over 4,500 sources in M31, revealing the dominance of low-mass X-ray binaries and variations in their luminosity related to the galaxy's star formation history.

Contribution

It provides a detailed decomposition of X-ray source populations in M31 and highlights regional differences in luminosity functions, offering insights into the galaxy's stellar evolution.

Findings

Low-mass X-ray binaries dominate X-ray sources in M31.

M31's LMXB luminosity per stellar mass is lower than in other galaxies.

Inner-arm region shows rapid fading of LMXBs around 1 Gyr.

Abstract

As part of the New-ANGELS program, we systematically investigate the X-ray luminosity functions (XLFs) of 4506 X-ray sources projected within a radius of 2.5 deg centering on M31. We construct XLFs for different regions in the disk and halo of M31, accounting for the incompleteness with an effective sensitivity map. Assuming that the halo regions contain (mostly) foreground stars and background active galactic nuclei, they are taken as "background" for deriving the XLFs of the sources in the disk. Through modeling XLFs, we decompose the X-ray sources into distinct populations for each region. We find that low-mass X-ray binaries are the dominant X-ray population throughout the disk of M31. The XLFs of M31 reveal a consistently lower integrated LMXB luminosity per stellar mass ($\alpha_\mathrm{LMXB}$) compared to other galaxies, likely due to M31's prolonged period of quiescent star formation. Variations in the XLF shape and $\alpha_\mathrm{LMXB}$ across different regions of M31 suggest that the relationship between integrated luminosity and stellar mass may vary within the galaxy. Additionally, the relatively low integrated luminosity observed in the inner-arm region provides crucial evidence for a rapid fading of M31's LMXBs around 1 Gyr, a finding consistent with recent observations of other nearby galaxies.