2020MNRAS.492..615Q

Observationally, the X-ray spectrum (0.5-10 keV) of low-level accreting neutron stars (NSs) (L_0.5{-10 kev}<= 10^{36} erg s^{-1}) 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 η (η ≡ L^{power law}_0.5{-10 kev}/L_0.5{-10 kev}) varies with the X-ray luminosity L_0.5{-10 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 α in the ADAF and thermalized parameter fth (describing the fraction of the ADAF energy released at the surface of the NS as thermal emission) on the relation of η versus L_0.5{-10 kev}. It is found that η is nearly a constant (∼zero) with L_0.5{-10 kev} for different α with fth = 1, which is inconsistent with observations. Meanwhile, it is found that a change of fth can significantly change the relation of η versus L_0.5{-10 kev}. By comparing with a sample of non-pulsating NS-low mass X-ray binaries probably dominated by low-level accretion on to NSs, it is found that a small value of fth <= 0.1 is needed to match the observed range of η >= 10{{ per cent}} in the diagram of η versus L_0.5{-10 kev}. Finally, we argue that the small value of fth <= 0.1 implies that the radiative efficiency of NSs with an ADAF accretion may not be as high as the predicted result previously of [?] ∼ {dot}M GM / R_*/{dot}M c^2∼ 0.2 despite the existence of the hard surface.