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.

13 pages, 23 figures. Accepted for publication by MNRAS

References in corpus (14)

• Advection-Dominated Accretion and the Black Hole Event Horizon
• Discovery of coherent millisecond X-ray pulsations in Aql X-1
• Cooling of Accretion-Heated Neutron Stars
• The Variable Quiescent X-ray Emission of the Neutron Star Transient XTE J1701-462
• Rapid Cooling of the Neutron Star in the Quiescent Super-Eddington Transient XTE J1701-462
• A strongly heated neutron star in the transient Z source MAXI J0556-332
• The return to quiescence of Aql X-1 following the 2010 outburst
• A Hard X-Ray Power-Law Spectral Cutoff in Centaurus X-4
• Swift observations of SAX J1808.4-3658: monitoring the return to quiescence
• The Radiative Efficiency of a Radiatively Inefficient Accretion Flow
• Chandra and Swift observations of the quasi-persistent neutron star transient EXO 0748-676 back to quiescence
• XMM-Newton observations of IGR J00291+5934: signs of a thermal spectral component during quiescence
• A model for the radio/X-ray correlation in three neutron star low-mass X-ray binaries 4U 1728-34, Aql X-1 and EXO 1745-248
• Hot accretion flow around neutron stars

Cited by in corpus (3)

• A possible mechanism for multiple changing look phenomenon in Active Galactic Nuclei
• The radiative efficiency of neutron stars at low-level accretion
• The advection-dominated accretion flow for the anti-correlation between the X-ray photon index and the X-ray luminosity in neutron star low-mass X-ray binaries