2019/12/13 by Erlin Qiao, B F Liu, B. F. Liu
Earth and Planetary Sciences · Physics and Astronomy · #Astrophysical Phenomena and Observations #High-pressure geophysics and materials #Pulsars and Gravitational Waves Research
paper · doi:10.1093/mnras/stz3510
openalex publication_date 2019/12/13 · openalex created_date 2019/12/26 · openalex updated_date 2026/07/30
ABSTRACT Observationally, the X-ray spectrum (0.5–10 keV) of low-level accreting neutron stars (NSs) (L_\rm 0.5-10 \rm kev\lesssim 1036 \rm 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\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 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_\rm 0.5-10 \rm kev. It is found that η is nearly a constant (∼zero) with L_\rm 0.5-10 \rm 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_\rm 0.5-10 \rm 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 \gtrsim 10 \rm per cent in the diagram of η versus L_\rm 0.5-10 \rm 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 ε ∼ M GM\over R*/M c2∼ 0.2 despite the existence of the hard surface.