Effects of variability of X-ray binaries on the X-ray luminosity functions of Milky Way

TL;DR

This study investigates how the variability of X-ray binaries affects the shape and spread of the Milky Way's X-ray luminosity functions, highlighting the importance of long-term light-curve data in population analysis.

Contribution

It introduces a method to incorporate X-ray binary variability into luminosity function construction, revealing its impact on the observed spread and shape of the functions.

Findings

Variability causes significant spread in the XLF slopes.

HMXB XLF follows a power-law with index -0.48.

LMXB XLF follows a cut-off power-law with index -0.31.

Abstract

The X-ray luminosity functions of galaxies have become a useful tool for population studies of X-ray binaries in them. The availability of long term light-curves of X-ray binaries with the All Sky X-ray Monitors opens up the possibility of constructing X-ray luminosity functions, by also including the intensity variation effects of the galactic X-ray binaries. We have constructed multiple realizations of the X-ray luminosity functions (XLFs) of Milky Way, using the long term light-curves of sources obtained in the 2-10 keV energy band with the RXTE-ASM. The observed spread seen in the value of slope of both HMXB and LMXB XLFs are due to inclusion of variable luminosities of X-ray binaries in construction of these XLFs as well as finite sample effects. XLFs constructed for galactic HMXBs in the luminosity range 10^{36} - 10^{39} erg/sec is described by a power-law model with a mean power-law index of -0.48 and a spread due to variability of HMXBs as 0.19. XLFs constructed for galactic LMXBs in the luminosity range 10^{36} - 10^{39} erg/sec has a shape of cut-off power-law with mean power-law index of -0.31 and a spread due to variability of LMXBs as 0.07.