Low-mass X-ray binaries (LMXBs) which either contain a black hole (BH) or a neutron star (NS), accreting matter from its low-mass companion star (≲1M⊙) are ideal natural laboratories for studying the physics of accretion and jet around a BH or an NS (e.g. Migliari & Fender 2006). According to the timing and spectral features in the X-ray band, LMXBs are generally divided into two main spectral states, i.e. the high/soft state and the low/hard state (Gilfanov 2010, for review). For BH-LMXBs, when they are in the high/soft state, the accretion flow is widely believed to be dominated by the optically thick, geometrically thin, cool accretion disc (Shakura & Sunyaev 1973), and the X-ray spectrum can be well described by a multicolour blackbody spectrum (e.g. Mitsuda et al. 1984; Makishima et al. 1986; Merloni, Fabian & Ross 2000). Whereas, for NS-LMXBs, besides the emission from the disc, there is a significant thermal emission from the boundary layer between the accretion disc and the surface of the NS (Popham & Narayan 1992; Inogamov & Sunyaev 1999; Popham & Sunyaev 2001; Gilfanov & Sunyaev 2014, for review). Observationally, for both BH-LMXBs and NS-LMXBs, when they are in the low/hard state, generally, the accretion flow is suggested to be dominated by the optically thin, geometrically thick, hot, advection-dominated accretion flow (ADAF) (Done, Gierliński & Kubota 2007, for review). Theoretically, the ADAF solution has been studied in detail by several researchers since it was discovered in 1970’s (Ichimaru 1977; Rees et al. 1982; Narayan & Yi 1994, 1995a,b; Abramowicz et al. 1995; Chen et al. 1995; Yuan & Narayan 2014, for review). In the BH case, the ADAF solution is a kind of radiatively inefficient accretion flow, in which a fraction of the viscously dissipated energy will be advected into the event horizon of the BH. While in the NS case, the viscously dissipated energy advected onto the surface of the NS will eventually be radiated out, so the ADAF solution is radiatively efficient (Narayan & Yi 1995b). Qiao & Liu (2018b) calculated the structure and the corresponding emergent spectrum of the ADAF around a weakly magnetized NS within the framework of the self-similar solution of the ADAF. The authors compared the electron temperature of the ADAF around a NS and a BH, it is found that the electron temperature of the ADAF around an NS is systemically lower than that of a BH, which is consistent with observations (Burke, Gilfanov & Sunyaev 2017; Qiao & Liu 2018b). Meanwhile, the authors compared the Compton y-parameter (defined as $y={{4kT_{\rm e}}\over {m_{\rm e}c^2}} \rm {Max}(\tau _{\rm es}, \tau ^2_{\rm es})$, with Te being the election temperature, me being the electron mass, c being the speed of light, and τes being the Compton scattering optical depth) of the ADAF around an NS and a BH, it is found that the Compton y-parameter of the ADAF around an NS is systemically lower than that of a BH, producing a softer X-ray spectrum, which is also consistent with observations (Wijnands et al. 2015; Parikh et al. 2017; Sonbas, Dhuga & Göğüş 2018; Qiao & Liu 2018b).
Citation: Merloni, Fabian & Ross 2000
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