A systematic study of the advection-dominated accretion flow for the origin of the X-ray emission in weakly magnetized low-level accreting neutron star

TL;DR

This study explores the origin of X-ray emission in weakly magnetized neutron stars using ADAF models, revealing the importance of the thermalized energy fraction at the star's surface for matching observations.

Contribution

It systematically investigates how ADAF parameters influence X-ray spectral components and constrains the thermalized energy fraction to match observed data.

Findings

A nearly constant power-law contribution for certain parameters is inconsistent with observations.

A small thermalized energy fraction (f_th ≤ 0.1) matches observed spectral properties.

The radiative efficiency of neutron stars with ADAF may be lower than previously predicted.

Abstract

Observationally, the X-ray spectrum ($0.5-10$ keV) of low-level accreting neutron stars (NSs) can generally be well fitted by the model with two components, i.e, a thermal soft X-ray component plus a power-law component. Meanwhile, the fractional contribution of the power-law luminosity $\eta$ ($\eta\equiv L^{\rm power\ law}_{\rm 0.5-10\rm keV}/L_{\rm 0.5-10\rm keV}$) varies with the X-ray luminosity $L_{\rm 0.5-10\rm keV}$. In this paper, we systematically investigate the origin of such X-ray emission within the framework of the advection-dominated accretion flow (ADAF) around a weakly magnetized NS, in which the thermal soft X-ray component arises from the surface of the NS and the power-law component arises from the ADAF itself. We test the effects of the viscosity parameter $\alpha$ in the ADAF and thermalized parameter $f_{\rm th}$ (describing the fraction of the ADAF energy released at the surface of the NS as thermal emission) on the relation of $\eta$ versus $L_{\rm 0.5-10\rm keV}$. It is found that $\eta$ is nearly a constant ($\sim$ zero) with $L_{\rm 0.5-10\rm keV}$ for different $\alpha$ with $f_{\rm th}=1$, which is inconsistent with observations. Meanwhile, it is found that a change of $f_{\rm th}$ can significantly change the relation of $\eta$ versus $L_{\rm 0.5-10\rm keV}$. By comparing with a sample of non-pulsating NS-LMXBs probably dominated by low-level accretion onto NSs, it is found that a small value of $f_{\rm th} \lesssim 0.1$ is needed to match the observed range of $\eta \gtrsim 10\%$ in the diagram of $\eta$ versus $L_{\rm 0.5-10\rm keV}$. Finally, we argue that the small value of $f_{\rm th} \lesssim 0.1$ implies that the radiative efficiency of NSs with an ADAF accretion may not be as high as the predicted result previously of $\epsilon \sim {\dot M GM\over R_{*}}/{\dot M c^2}\sim 0.2$ despite the existence of the hard surface.