scholarly journals The radiative efficiency of neutron stars at low-level accretion

2021 ◽  
Vol 502 (3) ◽  
pp. 3870-3878
Author(s):  
Erlin Qiao ◽  
B F Liu
Keyword(s):  
Black Hole ◽  
Model Parameters ◽  
X Ray ◽  
The Real ◽  
Radiative Efficiency ◽  
Accretion Flow ◽  
Low Level ◽  
Low Mass ◽  
Inner Region ◽  
X Ray Binaries

ABSTRACT When neutron star low-mass X-ray binaries (NS-LMXBs) are in the low-level accretion regime (i.e. $L_{\rm X}\lesssim 10^{36}\ \rm erg\ s^{-1}$), the accretion flow in the inner region around the NS is expected to exist in the form of the hot accretion flow, e.g. the advection-dominated accretion flow (ADAF) as that in black hole X-ray binaries. Following our previous studies in Qiao & Liu (2020a, b) on the ADAF accretion around NSs, in this paper, we investigate the radiative efficiency of NSs with an ADAF accretion in detail, showing that the radiative efficiency of NSs with an ADAF accretion is much lower than that of $\epsilon \sim {\dot{M} GM\over R_{*}}/{\dot{M} c^2}\sim 0.2$ despite the existence of the hard surface. As a result, given an X-ray luminosity LX (e.g. between 0.5 and 10 keV), $\dot{M}$ calculated by $\dot{M}=L_{\rm X}{R_{*}\over {GM}}$ is lower than the real $\dot{M}$ calculated within the framework of the ADAF accretion. The real $\dot{M}$ can be more than two orders of magnitude higher than that calculated by $\dot{M}=L_{\rm X}{R_{*}\over {GM}}$ with appropriate model parameters. Finally, we discuss that if applicable, the model of ADAF accretion around a NS can be applied to explain the observed millisecond X-ray pulsation in some NS-LMXBs (such as PSR J1023+0038, XSS J12270−4859, and IGR J17379−3747) at a lower X-ray luminosity of a few times of $10^{33}\ \rm erg\ s^{-1}$, since at this X-ray luminosity the calculated $\dot{M}$ with the model of ADAF accretion can be high enough to drive a fraction of the matter in the accretion flow to be channelled on to the surface of the NS forming the X-ray pulsation.

scholarly journals 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

2019 ◽  
Vol 492 (1) ◽  
pp. 615-627 ◽  
Author(s):  
Erlin Qiao ◽  
B F Liu
Keyword(s):  
Neutron Stars ◽  
Power Law ◽  
Thermal Emission ◽  
X Ray ◽  
Radiative Efficiency ◽  
Accretion Flow ◽  
Low Level ◽  
Viscosity Parameter ◽  
Low Mass ◽  
X Ray Binaries

ABSTRACT Observationally, the X-ray spectrum (0.5–10 keV) of low-level accreting neutron stars (NSs) ($L_{\rm 0.5{-}10\,\rm kev}\lesssim 10^{36}\ \rm erg \ s^{-1}$) 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 ($\eta \equiv L^{\rm power\ law}_{\rm 0.5{-}10\,\rm kev}/L_{\rm 0.5{-}10\,\rm kev}$) 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 $\eta \gtrsim 10{{\ \rm per\ cent}}$ 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 $\epsilon \sim {\dot{M} GM\over R_{*}}/{\dot{M} c^2}\sim 0.2$ despite the existence of the hard surface.

scholarly journals Radiative efficiency of hot accretion flow and the radio/X-ray correlation in X-ray binaries

2014 ◽  
Vol 10 (S312) ◽  
pp. 139-140
Author(s):  
Fu-Guo Xie
Keyword(s):  
Black Hole ◽  
Active Galactic Nuclei ◽  
Accretion Rate ◽  
Galactic Nuclei ◽  
X Ray ◽  
Radiative Efficiency ◽  
Accretion Flow ◽  
Accretion Rates ◽  
Distinctive Property ◽  
X Ray Binaries

AbstractSignificant progresses have been made since the discovery of hot accretion flow, a theory successfully applied to the low-luminosity active galactic nuclei (LLAGNs) and black hole (BH) X-ray binaries (BHBs) in their hard states. Motivated by these updates, we re-investigate the radiative efficiency of hot accretion flow. We find that, the brightest regime of hot accretion flow shows a distinctive property, i.e. it has a constant efficiency independent of accretion rates, similar to the standard thin disk. For less bright regime, the efficiency has a steep positive correlation with the accretion rate, while for faint regime typical of advection-dominated accretion flow, the correlation is shadower. This result can naturally explain the observed two distinctive correlations between radio and X-ray luminosities in black hole X-ray binaries. The key difference in systems with distinctive correlations could be the viscous parameter, which determines the critical luminosity of different accretion modes.

scholarly journals The advection-dominated accretion flow for the anticorrelation between the X-ray photon index and the X-ray luminosity in neutron star low-mass X-ray binaries

2020 ◽  
Vol 496 (3) ◽  
pp. 2704-2714
Author(s):  
Erlin Qiao ◽  
B F Liu
Keyword(s):  
Neutron Star ◽  
Thermal Emission ◽  
X Ray ◽  
Radiative Efficiency ◽  
Accretion Flow ◽  
Self Similar Solution ◽  
Photon Index ◽  
Low Mass ◽  
Self Similar ◽  
X Ray Binaries

ABSTRACT Observationally, an anticorrelation between the X-ray photon index Γ (obtained by fitting the X-ray spectrum between 0.5 and 10 keV with a single power law) and the X-ray luminosity L0.5-10 keV, i.e. a softening of the X-ray spectrum with decreasing L0.5-10 keV, is found in neutron star low-mass X-ray binaries (NS-LMXBs) in the range of $L_{\rm 0.5\!-\!10\,keV}\sim 10^{34}\!-\!10^{36}\ \rm erg\ s^{-1}$. In this paper, we explain the observed anticorrelation between Γ and L0.5–10 keV within the framework of the self-similar solution of the advection-dominated accretion flow (ADAF) around a weakly magnetized NS. The ADAF model intrinsically predicts an anticorrelation between Γ and L0.5–10 keV. In the ADAF model, there is a key parameter, fth, which describes the fraction of the ADAF energy released at the surface of the NS as thermal emission to be scattered in the ADAF. We test the effect of fth on the anticorrelation between Γ and L0.5–10 keV. It is found that the value of fth can significantly affect the anticorrelation between Γ and L0.5–10 keV. Specifically, the anticorrelation between Γ and L0.5–10 keV becomes flatter with decreasing fth as taking fth = 0.1, 0.03, 0.01, 0.005, 0.003, and 0, respectively. By comparing with a sample of non-pulsating NS-LMXBs with well measured Γ and L0.5–10 keV, we find that indeed only a small value of 0.003 ≲ fth ≲ 0.1 is needed to match the observed anticorrelation between Γ and L0.5–10 keV. Finally, we argue that the small value of fth ≲ 0.1 derived in this paper further confirms our previous conclusion that the radiative efficiency of NSs with an ADAF accretion may not be as high as $\epsilon \sim {\dot{M} GM\over R_{*}}/{\dot{M} c^2}\sim 0.2$.

scholarly journals A Machine Learning Approach For Classifying Low-mass X-ray Binaries Based On Their Compact Object Nature

2020 ◽  
Author(s):  
R Pattnaik ◽  
K Sharma ◽  
K Alabarta ◽  
D Altamirano ◽  
M Chakraborty ◽  
...  
Keyword(s):  
Machine Learning ◽  
Black Hole ◽  
Neutron Star ◽  
Binary Systems ◽  
Compact Object ◽  
X Ray ◽  
Average Accuracy ◽  
Machine Learning Approach ◽  
Low Mass ◽  
X Ray Binaries

Abstract Low Mass X-ray binaries (LMXBs) are binary systems where one of the components is either a black hole or a neutron star and the other is a less massive star. It is challenging to unambiguously determine whether a LMXB hosts a black hole or a neutron star. In the last few decades, multiple observational works have tried, with different levels of success, to address this problem. In this paper, we explore the use of machine learning to tackle this observational challenge. We train a random forest classifier to identify the type of compact object using the energy spectrum in the energy range 5-25 keV obtained from the Rossi X-ray Timing Explorer archive. We report an average accuracy of 87±13% in classifying the spectra of LMXB sources. We further use the trained model for predicting the classes for LMXB systems with unknown or ambiguous classification. With the ever-increasing volume of astronomical data in the X-ray domain from present and upcoming missions (e.g., SWIFT, XMM-Newton, XARM, ATHENA, NICER), such methods can be extremely useful for faster and robust classification of X-ray sources and can also be deployed as part of the data reduction pipeline.

scholarly journals Formation of black hole low-mass X-ray binaries

2015 ◽  
Vol 64-66 ◽  
pp. 1-6 ◽  
Author(s):  
Xiang-Dong Li
Keyword(s):  
Black Hole ◽  
X Ray ◽  
Low Mass ◽  
X Ray Binaries

scholarly journals Magnetically Driven Warping and Precession of Accretion Disks: Implications for Exotic Stellar Variabilities

2002 ◽  
Vol 187 ◽  
pp. 215-220
Author(s):  
Dong Lai
Keyword(s):  
Accretion Disks ◽  
Low Frequency ◽  
Disk Accretion ◽  
X Ray ◽  
T Tauri ◽  
Magnetic Stars ◽  
Low Mass ◽  
Inner Region ◽  
X Ray Binaries

AbstractThe inner region of the accretion disk around a magnetized star is subjected to magnetic torques that induce warping and precession of the disk. These torques arise from interactions between the stellar field and the induced electric currents in the disk. These novel magnetic effects give rise to some “exotic” stellar variabilities, and may play an important role in explaining a number of puzzling behaviors related to disk accretion onto magnetic stars, such as mHz QPOs in X-ray pulsars, long-term periodicities of X-ray binaries (including precession of jets), low-Frequency (10-50 Hz) QPO’s in low-mass X-ray binaries, and photometric variabilities of T Tauri stars.

scholarly journals Alignment and precession of a black hole misaligned with its accretion disc: application to low-mass X-ray binaries

2019 ◽  
Vol 487 (3) ◽  
pp. 3488-3504
Author(s):  
Srimanta Banerjee ◽  
Chandrachur Chakraborty ◽  
Sudip Bhattacharyya
Keyword(s):  
Black Hole ◽  
Accretion Disc ◽  
X Ray ◽  
Low Mass ◽  
X Ray Binaries

scholarly journals Formation of Low-Mass Black Hole X-Ray Transients

1998 ◽  
Vol 11 (2) ◽  
pp. 775-778
Author(s):  
Simon Portegies Zwart ◽  
Frank Verbunt ◽  
Ene Ergma
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Neutron Star ◽  
Formation Rate ◽  
X Ray ◽  
Low Mass ◽  
X Ray Binaries

We study the formation of low-mass X-ray binaries with a black hole as accreting object. The performed semi-analytic analysis reveals that the formation rate of black holes in low-mass X-ray binaries is about two orders of magnitude smaller than that of systems with a neutron star as accretor. This is contradicted by the six observed systems, which are all transients, which suggest that the majority of these systems has not been seen jet. The birthrate for both type of objects are expected to be similar (for reviews see Cowley 1992, Tanaka & Lewin 1995).

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