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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]

We report the soft X-ray excess feature and an opposite tendency between the optical/UV and X-ray bands during the flare that occurred in 2014 in flat-spectrum radio quasar (FSRQ) 4C+21.35 with the observations by Swift - XRT and Swift-UVOT. These unusual phenomena are rarely reported in FSRQs. The flare was covered from the optical to the X-ray while no prominent activity in the γ-ray was observed, which could be an indication of weaker jet activity during the flare. We carry out the spectral energy distribution ranging from the optical to X-ray based on four models which are the warm corona model (Model-1), the relativistic reflection model (Model-2), the corona with the jet model (Model-3), and the relativistic reflection with the jet model (Model-4). We find some extreme variation of the parameters in Model-2 and Model-4 and they are unable to explain the correlations between the optical/UV and X-ray bands. Model-1 and Model-3 can obtain reasonable parameters and provide an explanation for the observed opposite tendency between the optical/UV and X-ray bands. However, considering the weak jet activity during the X-ray flare, Model-1 is much preferred for describing the soft X-ray excess, hard X-ray emission, as well as the opposite tendency between the optical/UV and X-rays. Based on Model-1, it is suggested that the flare in 2014 is probably due to the variation of the geometry of the corona, which is expected to be more strictly tested with further observations in the future. [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]

A long-standing question in active galactic nucleus (AGN) research is how the corona is heated up to produce X-ray radiation much stronger than that arising from the viscous heating within the corona. In this paper, we carry out detailed investigations of magnetic-reconnection heating to the corona, specifically, studying how the disc and corona are self-consistently coupled with the magnetic field, and how the emergent spectra depend on the fundamental parameters of AGN. It is shown that diverse spectral shapes and luminosities over a broad bandpass from optical to X-ray can be produced from the coupled disc and corona within a limited range of the black hole mass, accretion rate, and magnetic field strength. The relative strength of X-ray emission with respect to optical/ultraviolet (UV) depends on the strength of the magnetic field in the disc, which, together with accretion rate, determines the fraction of accretion energy transported and released in the corona. This refined disc–corona model is then applied to reproduce the broad-band spectral energy distributions (SEDs) of a sample of 20 bright local AGNs observed simultaneously in X-ray and optical/UV. We find that, in general, the overall observed broad-band SEDs can be reasonably reproduced, except for rather hard X-ray spectral shapes in some objects. The radiation pressure-dominant region, as previously predicted for the standard accretion disc in AGN, disappears for strong X-ray sources, revealing that AGN accretion discs are indeed commonly stable as observed. Our study suggests the disc–corona coupling model involving magnetic fields to be a promising approach for understanding the broad-band spectra of bright AGNs. [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]

We report the discovery of large amplitude X-ray variability in the low-luminosity active galactic nucleus NGC 7589, and present possible observational evidence for accretion mode transition in this source. Long-term X-ray flux variations by a factor of more than 50 are found using X-ray data obtained by Swift /X-Ray Telescope and XMM–Newton over 17 yr. Results of long-term monitoring data in the UV, optical, and infrared bands over ∼20 yr are also presented. The Eddington ratio λEdd increased from 10−3 to ∼0.13, suggesting a transition of the accretion flow from an advection dominated accretion flow to a standard thin accretion disc. Further evidence supporting the thin disc in the high-luminosity state is found by the detection of a significant soft X-ray component in the X-ray spectrum. The temperature of this component (⁠|$\sim 19^{+15}_{-7}$|  eV, fitted with a blackbody model) is in agreement with the predicted temperature of the inner region for a thin disc around a black hole (BH) with mass of ∼107M⊙. These results may indicate that NGC 7589 had experienced accretion mode transition over a time-scale of a few years, suggesting the idea that similar accretion processes are at work for massive BH and BH X-ray binaries. [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]

Characterizing the long-term variability of active galactic nuclei (AGNs) is a key legacy of the Rossi X-ray Timing Explorer (RXTE). We carry out a spectral analysis on a sample of 20 Seyfert 1 galaxies, which had been observed by the RXTE for at least 100 times. All 18 335 spectra are fitted in a uniform way using a power-law component plus an additional Gaussian line when necessary. For any source in our sample, we confirm that the spectrum softens or the photon index, Γ, increases with increasing the 2–10 keV luminosity, L X. However, different source holds distinct Γ– L X/ L Edd relation, rather than a common one. We also fit the correlation with a function of Γ = C + β × log (flux), where C is a constant and parameter β is the slope of the correlation. In this way, the increase speed of the Γ– L X/ L Edd relation can be depicted with the parameter β. Since our sample contains a large sample of long-term monitored sources, it allows us to explore the second-order effect of spectral evolution. We find that there is an anticorrelation between β and the X-ray Eddington ratio, L X/ L Edd. That is, the increasing tendency of Γ with increasing X-ray luminosity becomes slower when the source has a larger X-ray Eddington ratio. Our results may indicate changes in the geometry and/or the viscosity parameter of the accretion disc at different Eddington ratios. [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]

We report on the detection of a pulsating Fe K α line in the High Mass X-ray Binary (HMXB) GX 301-2, from a 40-ks Chandra observation near periastron. The pulsations in the Fe K α emission appeared only in the first 7 ks of the observation, with a period and phase profile similar to those of the continuum. The presence of pulsed fluorescent lines is an unusual property in HMXBs. After 7 ks, the continuum flux increased by a factor of three, the Fe K α flux increased only by about 10 per cent, and the pulsating signal in the line disappeared. Finally, in the second half of the observation, both the continuum and the line flux dropped by a similar factor of 2. We suggest that the pulsating component of the Fe K α line is coming from a transient non-isotropic distribution of dense gas around the neutron star, for example, an accretion stream induced by periastron passage, or from the illuminated surface of the donor star. [ABSTRACT FROM AUTHOR]

In this paper, we explore the physics of the accretion and jet in narrow-line Seyfert 1 (NLS1) galaxy. Specifically, we compile a sample composed of 16 nearby NLS1 with Lbol/LEdd ≳ 0.1. We investigate the mutual correlation between their radio luminosity LR, X-ray luminosity LX, optical luminosity L5100, and black hole mass MBH. By adopting partial correlation analysis, we find (1) a positive correlation between LX and MBH and (2) a weak positive correlation between LR and L5100. However, we don't find significant correlations between LR and LX or between LX and L5100 after considering the effect of the black hole mass, which leads to a finding that LX/LEdd is independent of L5100/LEdd. Interestingly, the findings that LX is correlated with MBH and LX/LEdd is not correlated with L5100/LEdd support that the X-ray emission is saturated with increasing M for Lbol/LEdd ≳ 0.1 in NLS1, which may be understood in the framework of slim disc scenario. Finally, we suggest that a larger NLS1 sample with high-quality radio and X-ray data is needed to further confirm this result in the future. [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]

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]

Observations show that the accretion flows in low-luminosity active galactic nuclei probably have a two-component structure with an inner, hot, optically thin, advection-dominated accretion flow (ADAF) and an outer, truncated, cool, optically thick accretion disk. As shown by Taam et al., the truncation radius as a function of mass accretion rate is strongly affected by including the magnetic field within the framework of disk evaporation model, i.e., an increase in the magnetic field results in a smaller truncation radius of the accretion disk. In this work, we calculate the emergent spectrum of an inner ADAF + an outer truncated accretion disk around a supermassive black hole based on the prediction by Taam et al.. It is found that an increase in the magnetic field from β = 0.8 to β = 0.5 (with magnetic pressure pm = B2/8π = (1 – β)ptot, ptot = pgas + pm) results in a factor of ∼8.7 increase in the luminosity from the truncated accretion disk. Meanwhile, results of the peak emission of the truncated accretion disk shift toward a a factor of ∼5 higher frequency. We found that the equipartition of gas pressure to magnetic pressure, i.e., β = 0.5, failed to explain the observed anti-correlation between L2-10 keV/LEdd and the bolometric correction κ2-10 keV (with κ2-10 keV = Lbol/L2-10 keV). The emergent spectra for larger values of β = 0.8 or β = 0.95 can explain the observed L2-10 keV/LEdd-κ2-10 keV correlation. We argue that in the disk evaporation model, the electrons in the corona are assumed to be heated only by a transfer of energy from the ions to electrons via Coulomb collisions, which is reasonable for accretion with a lower mass accretion rate. Coulomb heating is the dominated heating mechanism for the electrons only if the magnetic field is strongly sub-equipartition, which is roughly consistent with observations. [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]

We interpret the radio/X-ray correlation of LR ∝ LX~1.4 for LX/LEdd ≳ 10−3 with a detailed disk corona-jet model, in which the accretion flow and the jet are connected by a parameter, η, describing the fraction of the matter in the accretion flow ejected outward to form the jet. We calculate LR and LX at different Ṁ, 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 the value of η for this 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 PUBLISHER]

Recent observations reveal that a cool disk may survive in the innermost stable circular orbit (ISCO) for some black hole X-ray binaries in the 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 temperature of ~ 0.2 keV. The formation of such a cool disk in the most inner region of black hole X-ray binaries at the low/hard state is investigated within the framework of disk accretion fed by condensation of hot corona. We also calculate the emergent spectra of the inner disk and corona. It's found that our model can very well explain the spectral features of GX 339-4 and Cyg X-1, in which the thin disk may exist at ISCO in the low/hard state. [ABSTRACT FROM PUBLISHER]

Observations of black hole X-ray binaries and active galactic nuclei indicate that the accretion flows around black holes are composed of hot and cold gas, which have been theoretically described in terms of either a hot geometrically thick corona lying above and below a cold geometrically thin disk or an inner advection dominated accretion flow connected to an outer thin disk. This article reviews the accretion flows around black holes, with an emphasis on the physics that determines the configuration of hot and cold accreting gas, and how the configuration varies with the accretion rate and thereby produces various luminosity and spectra., (© 2021 The Author(s).)