A Study of Low-Mass X-Ray Binaries in the Low-Luminosity Regime

TL;DR

This study investigates the spectral softening of low-luminosity neutron star and black hole X-ray binaries, revealing a significant anticorrelation in neutron stars at very low luminosities and exploring the physical mechanisms behind the dichotomy.

Contribution

It extends the analysis of spectral behavior to lower luminosities and compares neutron star and black hole binaries using Comptonization models, highlighting differences in corona properties.

Findings

Neutron star binaries show a strong anticorrelation between spectral index and luminosity at 10^{32}-10^{33} erg/s.

Black hole binaries do not exhibit the same spectral softening behavior.

Neutron star corona temperatures are significantly lower than those of black holes.

Abstract

A recent study of a small sample of X-ray binaries (XRBs) suggests a significant softening of spectra of neutron star (NS) binaries as compared to black hole (BH) binaries in the luminosity range 10$^{34}$ - 10$^{37}$ erg/s. This softening is quantified as an anticorrelation between the spectral index and the 0.5 - 10 keV X-ray luminosity. We extend the study to significantly lower luminosities (i.e., $\sim$ a few $\times$ $10^{30}$ erg/s) for a larger sample of XRBs. We find evidence for a significant anticorrelation between the spectral index and the luminosity for a group of NS binaries in the luminosity range 10$^{32}$ to 10$^{33}$ erg/s. Our analysis suggests a steep slope for the correlation i.e., -2.12 $\pm$ 0.63. In contrast, BH binaries do not exhibit the same behavior. We examine the possible dichotomy between NS and BH binaries in terms of a Comptonization model that assumes a feedback mechanism between an optically thin hot corona and an optically thick cool source of soft photons. We gauge the NS-BH dichotomy by comparing the extracted corona temperatures, Compton-y parameters and the Comptonization amplification factors: The mean temperature of the NS group is found to be significantly lower than the equivalent temperature for the BH group. The extracted Compton-y parameters and the amplification factors follow the theoretically predicted relation with the spectral index.