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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]

A black hole X-ray binary produces hard X-ray radiation from its corona and disk when the accreting matter heats up. During an outburst, the disk and corona co-evolves with each other. However, such an evolution is still unclear in both its geometry and dynamics. Here we report the unusual decrease of the reflection fraction in MAXI J1820+070, which is the ratio of the coronal intensity illuminating the disk to the coronal intensity reaching the observer, as the corona is observed to contrast during the decay phase. We postulate a jet-like corona model, in which the corona can be understood as a standing shock where the material flowing through. In this dynamical scenario, the decrease of the reflection fraction is a signature of the corona's bulk velocity. Our findings suggest that as the corona is observed to get closer to the black hole, the coronal material might be outflowing faster. Evolution of accretion disk and corona during outbursts in black hole binary systems is still unclear. Here, the authors show spectral analysis of MAXI J1820+070 and propose a scenario of a dynamical corona to explain the evolution of the reflection fraction observed by Insight-HXMT. [ABSTRACT FROM AUTHOR]

We study the evolution of the temporal properties of MAXI J1820+070 during the 2018 outburst in its hard state from MJD 58,190 to 58,289 with Insight-HXMT in a broad energy band 1–150 keV. We find different behaviors of the hardness ratio, the fractional rms and time lag before and after MJD 58,257, suggesting a transition occurred around this point. The observed time lags between the soft photons in the 1–5 keV band and the hard photons in higher energy bands, up to 150 keV, are frequency-dependent: the time lags in the low-frequency range, 2–10 mHz, are both soft and hard lags with a timescale of dozens of seconds but without a clear trend along the outburst; the time lags in the high-frequency range, 1–10 Hz, are only hard lags with a timescale of tens of milliseconds; they first increase until around MJD 58,257 and decrease after this date. The high-frequency time lags are significantly correlated to the photon index derived from the fit to the quasi-simultaneous NICER spectrum in the 1–10 keV band. This result is qualitatively consistent with a model in which the high-frequency time lags are produced by Comptonization in a jet. [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]

The observed tight radio/X-ray correlation in the low spectral state of some black hole X-ray binaries implies the strong coupling of the accretion and jet. The correlation of Lr a LR ∝ Lx~0.5-0.7 was well explained by the coupling of a radiatively inefficient accretion flow and a jet. Recently, however, a growing number of sources show more complicated radio/X-ray correlations, e.g. LR ∝ Lx~1.4for LX/LEdd ≳ 10-3, which is suggested to be explained by the coupling of a radiatively efficient accretion flow and a jet. In this work, we interpret the deviation from the initial radio/X-ray correlation for LX/LEdd ≳ 10-3 with a detailed disc corona-jet model. In this model, the disc and corona are radiatively and dynamically coupled. Assuming a fraction of the matter in the accretion flow, η = Mjet/M, is ejected to form the jet, we can calculate the emergent spectrum of the disc corona-jet system. We calculate LR and LX at different M, adjusting η to fit the observed radio/X-ray correlation of the black hole X-ray transient H1743--322 for LX/LEdd > 10-3. It is found that always the X-ray emission is dominated by the disc corona and the radio emission is dominated by the jet. We noted that the value of η for the deviated radio/X-ray correlation for LX/LEdd > 10-3 is systematically less than that of the case for LX/LEdd < 10-3, which is consistent with the general idea that the jet is often relatively suppressed at the high-luminosity phase in black hole X-ray binaries. [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]