X-ray emission from star-forming galaxies - I. High-mass X-ray binaries

TL;DR

This study analyzes high-mass X-ray binaries in 29 star-forming galaxies, establishing their correlation with star formation rate, constructing an improved X-ray luminosity function, and discussing implications for binary evolution.

Contribution

It provides a comprehensive catalog of HMXBs, refines the XLF with better statistical accuracy, and explores the evolution and population characteristics of HMXBs in star-forming galaxies.

Findings

HMXB luminosity scales with SFR, but with large dispersion.

XLF follows a power law with a slope of 1.6, with possible cutoff at 10^{40} erg/s.

High fraction of compact objects experience an X-ray active phase, much more frequent than in LMXBs.

Abstract

Based on a homogeneous set of X-ray, infrared and ultraviolet observations from Chandra, Spitzer, GALEX and 2MASS archives, we study populations of high-mass X-ray binaries (HMXBs) in a sample of 29 nearby star-forming galaxies and their relation with the star formation rate (SFR). In agreement with previous results, we find that HMXBs are a good tracer of the recent star formation activity in the host galaxy and their collective luminosity and number scale with the SFR, in particular, Lx~2.6 10^{39} SFR. However, the scaling relations still bear a rather large dispersion of ~0.4 dex, which we believe is of a physical origin. We present the catalog of 1057 X-ray sources detected within the $D25$ ellipse for galaxies of our sample and construct the average X-ray luminosity function (XLF) of HMXBs with substantially improved statistical accuracy and better control of systematic effects than achieved in previous studies. The XLF follows a power law with slope of 1.6 in the logLx~35-40 luminosity range with a moderately significant evidence for a break or cut-off at Lx~10^{40} erg/s. As before, we did not find any features at the Eddington limit for a neutron star or a stellar mass black hole. We discuss implications of our results for the theory of binary evolution. In particular we estimate the fraction of compact objects that once upon their lifetime experienced an X-ray active phase powered by accretion from a high mass companion and obtain a rather large number, fx~0.2 (0.1 Myr/tau_x) (tau_x is the life time of the X-ray active phase). This is ~4 orders of magnitude more frequent than in LMXBs. We also derive constrains on the mass distribution of the secondary star in HMXBs.