The advection-dominated accretion flow for the origin of the thermal soft X-ray component in low-level accreting neutron stars

TL;DR

This paper models the origin of the thermal soft X-ray component in low-level accreting neutron stars using ADAF theory, highlighting the role of surface energy thermalization and its observational implications.

Contribution

It introduces a self-consistent ADAF model incorporating surface energy thermalization to explain the thermal soft X-ray component in neutron stars, contrasting with black holes.

Findings

The Compton y-parameter in NSs is lower than in BHs.

The soft X-ray temperature decreases with lower accretion rates.

The thermal component's temperature is sensitive to the fraction of energy thermalized at the NS surface.

Abstract

A thermal soft X-ray component is often detected in low-level accreting neutron stars (NSs), but is not detected in low-level accreting stellar-mass black holes (BHs). In this paper, we investigate the origin of such a thermal soft X-ray component in the framework of the self-similar solution of the advection-dominated accretion flow (ADAF) around NSs. It is assumed that a fraction, $f_{\rm th}$, of the energy transferred onto the surface of the NS is thermalized at the surface of the NS as the soft photons to be scattered in the ADAF. We self-consistently calculate the structure and the corresponding emergent spectrum of the ADAF by considering the radiative coupling between the soft photons from the surface of the NS and the ADAF itself. We show that the Compton $y$-parameter of the ADAF for NSs is systematically lower than that of BHs. Meanwhile, we find that the temperature of the thermal soft X-ray component in NSs decreases with decreasing mass accretion rate, which is qualitatively consistent with observations. We test the effect of $f_{\rm th}$ on the structure, as well as the emergent spectrum of the ADAF. It is found that a change of $f_{\rm th}$ can significantly change the temperature of the thermal soft X-ray component as well as the spectral slope in hard X-rays. Finally, it is suggested that the value of $f_{\rm th}$ can be constrained by fitting the high-quality X-ray data, such as the $\it XMM$-$\it Newton$ spectrum between 0.5-10 keV in the future work.