Search

We provided a comprehensive study of the properties of the black hole in the low-mass X-ray binary system 4U 1543-47, specifically focusing on the 2021 outburst (MJD 59380–59470). Using observations from the Insight - HXMT mission, we employed X-ray reflection fitting method and analysed spectral data to estimate key black hole parameters. Through our investigation redbased on 6 out of the 52 available observations, we estimated the spin parameter of the black hole to be |$0.902_{-0.053}^{+0.054}$| and the inclination angle of the accretion disc to be |$28.91_{-1.24}^{+1.82}$| ° (90 per cent confidence limits, statistical only), then we discussed the influence of high luminosity. Based on the relxill series models are not suitable for thick disc scenario, and in comparison with findings from other studies, we propose that our estimation of the spin value may be exaggerated. [ABSTRACT FROM AUTHOR]

It is commonly believed that accretion discs are truncated and their inner regions are described by advection dominated accretion flows (ADAFs) in the hard spectral state of black hole X-ray binaries. However, the increasing occurrence of a relativistically blurred Fe K α line together with a hard continuum points to the existence of a thin disc located near the innermost stable circular orbit (ISCO). Assuming the accretion in the hard state is via an ADAF extending to near 100 Schwarzschild radii, which is supplied by either a stellar wind from a companion star or resulting from an evaporated disc, we study the possible condensation of the hot gas during its accretion towards the black hole. It is found that a small fraction of the ADAF condenses into a cold disc as a consequence of efficient radiative cooling at small distances, forming a disc-corona configuration near the ISCO. This takes place for low accretion rates corresponding to luminosities ranging from ∼10−3 to a few per cent of the Eddington luminosity. The coexistence of the weak inner disc and the dominant hot accretion flow provides a natural explanation of the broad K α line in the hard state. Detailed computations demonstrate that such accretion flows produce a hard X-ray spectrum accompanied by a weak disc component with a negative correlation between the 2 and 10 keV photon index and the Eddington ratio. The predicted spectrum of Cygnus X-1 and the correlation between the photon index and the Eddington ratio are in good agreement with observations. [ABSTRACT FROM AUTHOR]

One key question in tidal disruption events theory is how much of the fallback debris can be accreted to the black hole. Based on radiative hydrodynamic simulations, we study this issue for efficiently 'circularized' debris accretion flow. We find that for a black hole disrupting a solar-type star, |$15{{\, \rm per\, cent}}$| of the debris can be accreted for a 107 M⊙ black hole. While for a 106 M⊙ black hole, the value is |$43{{\, \rm per\, cent}}$|⁠. We find that wind can be launched in the super-Eddington accretion phase regardless of the black hole mass. The maximum velocity of the wind can reach 0.7 c (with c being the speed of light). The kinetic power of wind is well above 1044 erg s−1. The results can be used to study the interaction of wind and the circumnuclear medium around quiescent supermassive black holes. [ABSTRACT FROM AUTHOR]

Observationally, two main spectral states, i.e. the low/hard state and the high/soft state, are identified in black hole X-ray binaries (BH-XRBs). Meanwhile, the transitions between the two states are often observed. In this paper, we re-investigate the transition luminosities in the framework of the self-similar solution of the advection-dominated accretion flow (ADAF). Specifically, we search for the critical mass accretion rate |$\dot{m}_{\rm crit}$| of ADAF for different radii r , respectively. It is found that |$\dot{m}_{\rm crit}$| decreases with decreasing r. By testing the effects of BH mass m , the magnetic parameter β and the viscosity parameter α, it is found that only α has significant effects on |$\dot{m}_{\rm crit}\text{--}r$| relation. We define the minimum |$\dot{m}_{\rm crit}$| (roughly at the innermost stable circular orbit) as the hard-to-soft transition rate |$\dot{m}_{\rm tr:H\text{--} S}$|⁠ , above which BH will gradually transit from the low/hard state to the high/soft state, and |$\dot{m}_{\rm crit}$| at 30 Schwarzschild radii as the soft-to-hard transition rate |$\dot{m}_{\rm tr:S\rightarrow H}$|⁠ , below which BH will gradually transit from the high/soft state to the low/hard state. We derive fitting formulae of |$\dot{m}_{\rm tr:H\rightarrow S}$| and |$\dot{m}_{\rm tr:S\rightarrow H}$| as functions of α, respectively. By comparing with observations, it is found that the mean value of α are α ∼ 0.85 and α ∼ 0.33 for the hard-to-soft transition and the soft-to-hard transition, respectively, which indicates that two classes of α are needed for explaining the hysteresis effect during the state transition. Finally, we argue that such a constrained α may provide valuable clues for further exploring the accretion physics in BH-XRBs. [ABSTRACT FROM AUTHOR]

Winds can be launched in tidal disruption event (TDE). It has been proposed that the winds can interact with the cloud surrounding the black hole, produce bow shocks, accelerate electrons, and produce radio emission. We restudy the wind–cloud interaction model. We employ the properties of winds found by the radiation hydrodynamic simulations of super-Eddington circularized accretion flow in TDEs. We can calculate the peak radio emission frequency, the luminosity at the peak frequency, and their time-evolution based on the TDEs wind–cloud interaction model. We find that the model predicted peak radio emission frequency, the luminosity at peak frequency, and their time evolution can be well consistent with those in TDEs AT2019dsg and ASASSN-14li. This indicates that in these two radio TDEs, the wind–cloud interaction mechanism may be responsible for the radio emission. [ABSTRACT FROM AUTHOR]

AT 2019avd is a nuclear transient detected from infrared to soft X-rays, though its nature is yet unclear. The source has shown two consecutive flaring episodes in the optical and the infrared bands, and its second flare was covered by X-ray monitoring programs. During this flare, the UVOT/ Swift photometries revealed two plateaus: one observed after the peak and the other one appeared ∼240 d later. Meanwhile, our NICER and XRT / Swift campaigns show two declines in the X-ray emission, one during the first optical plateau and one 70–90 d after the optical/UV decline. The evidence suggests that the optical/UV could not have been primarily originated from X-ray reprocessing. Furthermore, we detected a timelag of ∼16–34 d between the optical and UV emission, which indicates the optical likely comes from UV reprocessing by a gas at a distance of 0.01–0.03 pc. We also report the first VLA and VLBA detection of this source at different frequencies and different stages of the second flare. The information obtained in the radio band – namely a steep and a late-time inverted radio spectrum, a high brightness temperature and a radio-loud state at late times – together with the multiwavelength properties of AT 2019avd suggests the launching and evolution of outflows such as disc winds or jets. In conclusion, we propose that after the ignition of black hole activity in the first flare, a super-Eddington flaring accretion disc formed and settled to a sub-Eddington state by the end of the second flare, associated with a compact radio outflow. [ABSTRACT FROM AUTHOR]

One of the most prominent problems of optical/ultraviolet (UV) tidal disruption events (TDEs) is the origin of their optical/UV emission. It has been proposed that the soft X-rays produced by the stellar debris accretion disc can be reprocessed into optical/UV photons by a surrounding optically thick envelope or outflow. However, there is still no detailed models for this mechanism. In this paper, by performing hydrodynamic simulations with radiative transfer, we calculate the optical/UV emission of the circularized stellar debris accretion flow/outflow system. We find that the optical/UV photons can be generated by reprocessing the emission of the accretion flow in the optically thick outflows. The model can well interpret the observed emission properties of optical/UV TDEs, including the emission radius, the radiation temperature, and the blackbody luminosity, as well as the evolution of these quantities with time, providing a strong theoretical basis for understanding the origin of optical/UV TDEs. [ABSTRACT FROM AUTHOR]

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 L X (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. [ABSTRACT FROM AUTHOR]

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 L 0.5-10 keV, i.e. a softening of the X-ray spectrum with decreasing L 0.5-10 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 L 0.5–10 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 L 0.5–10 keV. In the ADAF model, there is a key 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. We test the effect of f th on the anticorrelation between Γ and L 0.5–10 keV. It is found that the value of f th can significantly affect the anticorrelation between Γ and L 0.5–10 keV. Specifically, the anticorrelation between Γ and L 0.5–10 keV becomes flatter with decreasing f th as taking f th = 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 L 0.5–10 keV, we find that indeed only a small value of 0.003 ≲ f th ≲ 0.1 is needed to match the observed anticorrelation between Γ and L 0.5–10 keV. Finally, we argue that the small value of f th ≲ 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 |$\epsilon \sim {\dot{M} GM\over R_{*}}/{\dot{M} c^2}\sim 0.2$|⁠. [ABSTRACT FROM AUTHOR]

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 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 |$L_{\rm 0.5{-}10\,\rm kev}$|⁠. It is found that η is nearly a constant (∼zero) with |$L_{\rm 0.5{-}10\,\rm kev}$| for different α with 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 |$L_{\rm 0.5{-}10\,\rm kev}$|⁠. 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 f th ≲ 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 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 |$\epsilon \sim {\dot{M} GM\over R_{*}}/{\dot{M} c^2}\sim 0.2$| despite the existence of the hard surface. [ABSTRACT FROM AUTHOR]

Observationally, for neutron star low-mass X-ray binaries, so far, the correlation between the radio luminosity L R and the X-ray luminosity L X, i.e. |$L_{\rm R}\propto L_{\rm X}^{\beta }$|⁠ , has been reasonably well-established only in three sources 4U 1728-34, Aql X-1, and EXO 1745-248 in their hard state. The slope β of the radio/X-ray correlation of the three sources is different, i.e. β ∼ 1.4 for 4U 1728-34, β ∼ 0.4 for Aql X-1, and β ∼ 1.6 for EXO 1745-248. In this paper, for the first time we explain the different radio/X-ray correlation of 4U 1728-34, Aql X-1, and EXO 1745-248 with the coupled advection-dominated accretion flow (ADAF)-jet model, respectively. We calculate the emergent spectrum of the ADAF-jet model for L X and L R at different |$\dot{m}$| (⁠|$\dot{m}=\dot{M}/\dot{M}_{\rm Edd}$|⁠), adjusting η (⁠|$\eta \equiv \dot{M}_{\rm jet}/\dot{M}$|⁠ , describing the fraction of the accreted matter in the ADAF transferred vertically forming the jet) to fit the observed radio/X-ray correlations. Then, we derive a fitting formula of η as a function of |$\dot{m}$| for 4U 1728-34, Aql X-1, and EXO 1745-248, respectively. If the relation between η and |$\dot{m}$| can be extrapolated down to a lower value of |$\dot{m}$|⁠ , we find that in a wide range of |$\dot{m}$|⁠ , the value of η in Aql X-1 is greater than that of in 4U 1728-34 and EXO 1745-248, implying that Aql X-1 may have a relatively stronger large-scale magnetic field, which is supported by the discovery of the coherent millisecond X-ray pulsation in Aql X-1. [ABSTRACT FROM AUTHOR]

A thermal soft X-ray component is often detected in low-level accreting neutron stars (NSs), but is not detected in low-level accreting stellar-mass black holes (BHs). In this paper, we investigate the origin of such a thermal soft X-ray component in the framework of the self-similar solution of the advection-dominated accretion flow (ADAF) around NSs. It is assumed that a fraction, f th, of the energy transferred on to the surface of the NS is thermalized at the surface of the NS as the soft photons to be scattered in the ADAF. We self-consistently calculate the structure and the corresponding emergent spectrum of the ADAF by considering the radiative coupling between the soft photons from the surface of the NS and the ADAF itself. We show that the Compton y -parameter of the ADAF for NSs is systematically lower than that of BHs. Meanwhile, we find that the temperature of the thermal soft X-ray component in NSs decreases with decreasing mass accretion rate, which is qualitatively consistent with observations. We test the effect of f th on the structure, as well as the emergent spectrum of the ADAF. It is found that a change of f th can significantly change the temperature of the thermal soft X-ray component as well as the spectral slope in hard X-rays. Finally, it is suggested that the value of f th can be constrained by fitting the high-quality X-ray data, such as the XMM–Newton spectrum between 0.5 and 10 keV in a future work. [ABSTRACT FROM AUTHOR]

In this paper, we explained the observed Γ2-10 keV-Lbol/LEdd correlation in luminous active galactic nuclei within the framework of the condensation of the corona around a supermassive black hole (Liu et al.; Qiao & Liu). Specifically, we systemically test the effects of black holemassM, the viscosity parameter α, and the magnetic parameter β (with magnetic pressure pm = B2/8π = (1 - β)ptot, ptot = pgas + pm) on the structure of the accretion disc and the corona, as well as the corresponding emergent spectra. It is found that the hard X-ray photon index Γ2-10 keV nearly does not change with changing black hole massM, or changing magnetic parameter β. Meanwhile, it is found that the geometry of the accretion flow, i.e. the relative configuration of the disc and corona, as well as the emergent spectra can be strongly affected by changing the value of α. By comparing with a sample composed of 29 luminous active galactic nuclei with well constrained X-ray spectra and Eddington ratios, it is found that the observed Γ2-10 keV-Lbol/LEdd correlation can be well matched with a relatively bigger value of α, i.e. α ~ 1, as previously also suggested by Narayan for luminous accreting black holes. [ABSTRACT FROM AUTHOR]

Observationally, it is found that there is a strong correlation between the hard X-ray photon index Γ and the Compton reflection scaling factor ℜ in active galactic nuclei. In this paper, we propose that the Γ - ℜ correlation can be explained within the framework of the condensation of the hot corona on to the cold accretion disc around a supermassive black hole. In the model, it is presumed that, initially, a vertically extended hot gas (corona) is supplied to the central supermassive black hole by capturing the interstellar medium and stellar wind. In this scenario, when the initial mass accretion rate Ṁ/ṀEdd ≳ 0.01, at a critical radius rd, part of the hot gas begins to condense on to the equatorial disc plane of the black hole, forming an inner cold accretion disc. Then, the matter is accreted in the form of a disc-corona structure extending down to the innermost stable circular orbits of the black hole. The size of the inner disc is determined by the initial mass accretion rate. With the increase of the initial mass accretion rate, the size of the inner disc increases, which results in both the increase of the Compton reflection scaling factor ℜ and the increase of the hard X-ray photon index Γ. By comparing with a sample of Seyfert galaxies with well-fitted X-ray spectra, it is found that our model can roughly explain the observations. Finally, we discuss the possibility of applying our model to high-mass X-ray binaries, which are believed to be fuelled by hot winds from the companion stars. [ABSTRACT FROM AUTHOR]

Recent observations reveal that a cool disk may survive in the innermost stable circular orbit (ISCO) for some black hole X-ray binaries (BHXRBs) in the canonical low/hard state. The spectrum is characterized by a power law with a photon index Γ ~ 1.5-2.1 in the range of 2-10 keV and a weak disk component with a temperature of ~0.2 keV. In this work, we revisit the formation of such a cool, optically thick, geometrically thin disk in the innermost region of BHXRBs at the low/hard state within the context of disk accretion fed by condensation of the hot corona. By taking into account the cooling process associated with both Compton and conductive processes in a corona and the irradiation of the hot corona to the disk, we calculate the structure of the corona. For viscosity parameter α = 0.2, it is found that the inner disk can exist for an accretion rate ranging from M ~ 0.006 to 0.03 MEdd, over which the electron temperatures of the corona are in the range of 1-5 x 109 K producing the hard X-ray emission. We calculate the emergent spectra of the inner disk and corona for different mass accretion rates. The effect of viscosity parameter α and albedo a (a is defined as the energy ratio of reflected radiation from the surface of the thin disk to incident radiation upon it from the corona) on the emergent spectra is also presented. Our model is used to explain the recent observations of GX 339-4 and Cyg X-1, in which the thin disk may exist at the ISCO region in the low/hard state at a luminosity around a few percent of LEdd. It is found that the observed maximal effective temperature of the thermal component and the hard X-ray photon index Γ can be matched well by our model. [ABSTRACT FROM AUTHOR]