A systematic study of the advection-dominated accretion flow for the origin of the X-ray emission in weakly magnetized low-level accreting neutron stars

Abstract

Observationally, the X-ray spectrum (0.5 − 10 keV) of low-level accreting neutron stars (NSs) (L 0.5−10keV 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 0.5−10keV /L 0.5−10keV ) varies with the X-ray luminosity L 0.5−10keV . 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 f th (describing the fraction of the ADAF energy released at the surface of the NS as thermal emission) on the relation of η versus L 0.5−10keV . It is found that η is nearly a constant (∼ zero) with L 0.5−10keV for different α with f th = 1, which is inconsistent with observations. Meanwhile, it is found that a change of f th can significantly change the relation of η versus L 0.5−10keV . By comparing with a sample of non-pulsating NS-low mass X-ray binaries probably dominated by low-level accretion onto NSs, it is found that a small value of f th 0.1 is needed to match the observed range of η 10% in the diagram of η versus L 0.5−10keV . Finally, we argue that the small value of f th 0.1 implies that the radiative efficiency of NSs with an ADAF accretion may not be as high as the predicted result previously of ǫ ∼Ṁ GM R * /Ṁc 2 ∼ 0.2 despite the existence of the hard surface.

Possible mechanism for multiple changing-look phenomena in active galactic nuclei

“…Since this is a simple toy model, we do not use any advanced description of this process which would require the knowledge of the ADAF density or ion and electron temperature (e.g., Różańska & Czerny 2000;Liu et al 2007;Qiao & Liu 2020). Instead, we postulate that the efficiency of the process should be proportional to the zone height since the hot inner ADAF is geometrically thick so the interacting surface is set by the cold disk state.…”

Accretion around black holes: The geometry and spectra

“…The radiation spectra from such interacting disk and corona were calculated and compared with observations in BHXRBs (e.g., Qiao and Liu, 2012 , 2013 ; Meyer-Hofmeister and Meyer, 2014 ; Qiao and Liu, 2015 ) and AGNs (e.g., Liu and Taam, 2009 ; Liu et al., 2012 ; Taam et al., 2012 ; Qiao et al., 2013 ; Liu and Taam, 2013 ; Liu, 2013 ). The condensation model was extended to the case of wind accretion and applied to HMXBs and AGN ( Liu et al., 2015 , 2017 ; Meyer-Hofmeister et al., 2017 , 2020 ; Qiao and Liu, 2017 , 2018 , 2019 , 2020 ; Taam et al., 2018 ).…”

The radiative efficiency of neutron stars at low-level accretion

“…The last parameter is, f th , describing the fraction of the ADAF energy released at the surface of the NS as thermal emission to be scattered in the ADAF to cool the ADAF itself, which consequently controls the feedback between the ADAF and the NS. We always take m = 1.4, and R * in the range of 10-12.5 km (Degenaar & Suleimanov 2018) [R * = 12.5 km in Qiao & Liu (2020a), and R * = 10 km in Qiao & Liu (2020b)]. In general, it has been proven that the effect of the NS spin frequency ν NS on the structure and the emergent spectra of the ADAF around a NS is very little, and nearly can be neglected [see Figure 8 of Qiao & Liu (2020a) for taking ν NS = 0, 200, 500, 700 Hz respectively].…”

“…We always take m = 1.4, and R * in the range of 10-12.5 km (Degenaar & Suleimanov 2018) [R * = 12.5 km in Qiao & Liu (2020a), and R * = 10 km in Qiao & Liu (2020b)]. In general, it has been proven that the effect of the NS spin frequency ν NS on the structure and the emergent spectra of the ADAF around a NS is very little, and nearly can be neglected [see Figure 8 of Qiao & Liu (2020a) for taking ν NS = 0, 200, 500, 700 Hz respectively]. So we fix ν NS = 0 Hz in Qiao & Liu (2020a,b).…”

“…The physics of the boundary layer can significantly affect the global dynamics and radiation of the ADAF (Medvedev & Narayan 2001;Medvedev 2004;D'Angelo et al 2015). Recently, in a series of papers, i.e., Qiao & Liu (2018), Qiao & Liu (2020a), and Qiao & Liu (2020b), we study the dynamics and the radiation of the ADAF around a weakly magnetized NS in the framework of the selfsimilar solution of the ADAF by simplifying the physics of the boundary layer. Specifically, we introduce a parameter, f th , which describes the fraction of the ADAF energy released at the surface of the NS as thermal emission to be scattered in the ADAF.…”