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

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 $\eta$ ($\eta \equiv L_{0.5-10\mathrm{keV}}^{\mathrm{power}\mathrm{law}}/L_{0.5-10\mathrm{keV}}$) varies with the X-ray luminosity $L_{0.5-10\mathrm{keV}}$. 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 $f_{\mathrm{th}}$ (describing the fraction of the ADAF energy released at the surface of the NS as thermal emission) on the relation of $\eta$ versus $L_{0.5-10\mathrm{keV}}$. It is found that $\eta$ is nearly a constant ($\sim$ zero) with $L_{0.5-10\mathrm{keV}}$ for different $\alpha$ with $f_{\mathrm{th}}=1$, which is inconsistent with observations. Meanwhile, it is found that a change of $f_{\mathrm{th}}$ can significantly change the relation of $\eta$ versus $L_{0.5-10\mathrm{keV}}$. 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 $f_{\mathrm{th}}\lesssim 0.1$ is needed to match the observed range of $\eta \gtrsim 10\%$ in the diagram of $\eta$ versus $L_{0.5-10\mathrm{keV}}$. Finally, we argue that the small value of $f_{\mathrm{th}}\lesssim 0.1$ implies that the radiative efficiency of NSs with an ADAF accretion may not be as high as the predicted result previously of $\epsilon \sim \frac{\dot{M}GM}{R_{\ast }}/\dot{M}{c}^2\sim 0.2$ despite the existence of the hard surface.

arXiv comment13 pages, 23 figures. Accepted for publication by MNRAS

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