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 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 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 Quiescent thermal emission from neutron stars in low-mass X-ray binaries

2015 ◽  
Vol 577 ◽  
pp. A5 ◽  
Author(s):  
A. Turlione ◽  
D. N. Aguilera ◽  
J. A. Pons
Keyword(s):  
Neutron Stars ◽  
Thermal Emission ◽  
X Ray ◽  
Low Mass ◽  
X Ray Binaries

scholarly journals New Constraints on the Nuclear Equation of State from the Thermal Emission of Neutron Stars in Quiescent Low-mass X-Ray Binaries

2019 ◽  
Vol 887 (1) ◽  
pp. 48 ◽  
Author(s):  
Nicolas Baillot d’Etivaux ◽  
Sebastien Guillot ◽  
Jérôme Margueron ◽  
Natalie Webb ◽  
Márcio Catelan ◽  
...  
Keyword(s):  
Equation Of State ◽  
Neutron Stars ◽  
Thermal Emission ◽  
X Ray ◽  
Nuclear Equation Of State ◽  
Low Mass ◽  
X Ray Binaries

scholarly journals Search for intermittent X-ray pulsations from neutron stars in low-mass X-ray binaries

2021 ◽  
Vol 38 ◽  
Author(s):  
Yunus Emre Bahar ◽  
Manoneeta Chakraborty ◽  
Ersin Göğüş
Keyword(s):  
Neutron Stars ◽  
Orbital Period ◽  
Oscillation Frequency ◽  
Orbital Motion ◽  
X Rays ◽  
X Ray ◽  
Second Stage ◽  
Corrected Data ◽  
Low Mass ◽  
X Ray Binaries

Abstract We present the results of our extensive binary orbital motion corrected pulsation search for 13 low-mass X-ray binaries. These selected sources exhibit burst oscillations in X-rays with frequencies ranging from 45 to 1 122 Hz and have a binary orbital period varying from 2.1 to 18.9 h. We first determined episodes that contain weak pulsations around the burst oscillation frequency by searching all archival Rossi X-ray Timing Explorer data of these sources. Then, we applied Doppler corrections to these pulsation episodes to discard the smearing effect of the binary orbital motion and searched for recovered pulsations at the second stage. Here we report 75 pulsation episodes that contain weak but coherent pulsations around the burst oscillation frequency. Furthermore, we report eight new episodes that show relatively strong pulsations in the binary orbital motion corrected data.

scholarly journals UV and X-ray observations of the neutron star LMXB EXO 0748–676 in its quiescent state

2020 ◽  
Vol 501 (1) ◽  
pp. 1453-1462
Author(s):  
A S Parikh ◽  
N Degenaar ◽  
J V Hernández Santisteban ◽  
R Wijnands ◽  
I Psaradaki ◽  
...  
Keyword(s):  
Neutron Star ◽  
Thermal Emission ◽  
Hubble Space Telescope ◽  
Dense Matter ◽  
Continuum Emission ◽  
Quiescent State ◽  
X Ray ◽  
Low Level ◽  
Low Mass ◽  
Different Origins

ABSTRACT The accretion behaviour in low-mass X-ray binaries (LMXBs) at low luminosities, especially at <1034 erg s−1, is not well known. This is an important regime to study to obtain a complete understanding of the accretion process in LMXBs, and to determine if systems that host neutron stars with accretion-heated crusts can be used probe the physics of dense matter (which requires their quiescent thermal emission to be uncontaminated by residual accretion). Here, we examine ultraviolet (UV) and X-ray data obtained when EXO 0748–676, a crust-cooling source, was in quiescence. Our Hubble Space Telescope spectroscopy observations do not detect the far-UV continuum emission, but do reveal one strong emission line, C iv. The line is relatively broad (≳3500 km s−1), which could indicate that it results from an outflow such as a pulsar wind. By studying several epochs of X-ray and near-UV data obtained with XMM–Newton, we find no clear indication that the emission in the two wavebands is connected. Moreover, the luminosity ratio of LX/LUV ≳ 100 is much higher than that observed from neutron star LMXBs that exhibit low-level accretion in quiescence. Taken together, this suggests that the UV and X-ray emission of EXO 0748–676 may have different origins, and that thermal emission from crust-cooling of the neutron star, rather than ongoing low-level accretion, may be dominating the observed quiescent X-ray flux evolution of this LMXB.

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 Non-Linear Resonances in Accretion Disks and QPOs

2004 ◽  
Vol 194 ◽  
pp. 128-129
Author(s):  
Włodek Kluźniak
Keyword(s):  
Black Holes ◽  
Neutron Stars ◽  
Accretion Disk ◽  
Accretion Disks ◽  
High Frequency ◽  
Light Curves ◽  
X Ray ◽  
Non Linear ◽  
Low Mass ◽  
X Ray Binaries

AbstractNon-linear oscillations in the accretion disk are favored as an explanation of high-frequency QPOs observed in the light curves of low-mass X-ray binaries containing neutron stars, black holes, or white dwarfs.

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