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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 \textit{Insight}-HXMT mission, we employed X-ray reflection fitting method and analyzed 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 disk to be $28.91_{-1.24}^{+1.82}$ degrees (90\% confidence limits, statistical only), then we discussed the influence of high luminosity. Based on the \texttt{relxill} series models are not suitable for thick disk scenario, and in comparison with findings from other studies, we propose that our estimation of the spin value may be exaggerated., Comment: 11 pages, 10 figures, 5 tables, Accepted for publication in MNRAS

LB-1 is a binary system that has drawn great attention since its discovery in 2019. The nature of the two components of LB-1 is not very clear, which however is suggested very possibly to be a B-type star plus a black hole (BH). In this paper, we first calculate the wind mass-loss rate of the B-type star. We then calculate the mass capture rate by the BH, with which as the initial mass accretion rate, we calculate the truncation radius of the accretion disk and the corresponding emergent spectra of the accretion flow (comprising an inner advection-dominated accretion flow (ADAF) + an outer truncated accretion disk) within the framework of the disk evaporation model. It is found that the predicted truncation radius of the accretion disk with appropriate model parameters is consistent with observations inferred from the observed broad H$_\alpha$ emission line. The predicted X-ray luminosity is definitely below the estimated upper limits with the sensitivity of Chandra X-ray Observatory of the X-ray luminosity $\sim 2\times 10^{31}$ erg/s. Finally, we argue that if the disk evaporation model indeed reflects the intrinsic physics of the accretion flow, the value of the viscosity parameter $\alpha$ is constrained to be $\alpha \gtrsim 0.05$ (with BH mass being $68M_{\rm \odot}$), or $\alpha \gtrsim 0.003$ (with BH mass being $21M_{\rm \odot}$) to match the observed upper limit of the X-ray luminosity of LB-1., Comment: 11 pages, 2 figures. Submitted to The Astrophysical Journal, comments are welcome

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$\alpha$ 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 $\sim 10^{-3}$ 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$\alpha$ 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 2-10 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., Comment: Accepted for publication in MNRAS, 12 pages, 12 figures

We present the results of a detailed timing and spectral analysis of the quasi-regular modulation (QRM) phenomenon in the black hole X-ray binary 4U 1630--47 during its 1998 outburst observed by Rossi X-ray Timing Explore (RXTE). We find that the $\sim$ 50-110 mHz QRM is flux dependent, and the QRM is detected with simultaneous low frequency quasi-periodic oscillations (LFQPOs). According to the behavior of the power density spectrum, we divide the observations into four groups. In the first group, namely behavior A, LFQPOs are detected, but no mHz QRM. The second group, namely behavior B, a QRM with frequency above $\sim$ 88 mHz is detected and the $\sim$ 5 Hz and $\sim$ 7 Hz LFQPOs are almost overlapping. In the third group, namely behavior C, the QRM frequency below $\sim$ 88 mHz is detected and the LFQPOs are significantly separated. In the forth group, namely behavior D, neither QRM nor LFQPOs are detected. We study the energy-dependence of the fractional rms, centroid frequency, and phase-lag of QRM and LFQPOs for behavior B and C. We then study the evolution of QRM and find that the frequency of QRM increases with hardness, while its rms decreases with hardness. We also analyze the spectra of each observation, and find that the QRM rms of behavior B has a positive correlation with $\rm F_{\rm powerlaw}$ / $\rm F_{\rm total}$. Finally, we give our understanding for this mHz QRM phenomena., Comment: 14pages, 15 figures

One key question in tidal disruption events theory is that 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, $15\%$ of the debris can be accreted for a $10^7$ solar mass ($M_\odot$) black hole. While for a $10^6M_\odot$ black hole, the value is $43\%$. We find that wind can be launched in the super-Eddington accretion phase regardless of the black hole mass. The maximum velocity of wind can reach $0.7c$ (with $c$ being speed of light). The kinetic power of wind is well above $10^{44} {\rm erg \ s^{-1}}$. The results can be used to study the interaction of wind and the circumnuclear medium around quiescent super-massive black holes., Comment: 10 pages, 15 figures, accepted by MNRAS

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., Comment: 11 pages, 10 figures, accepted to MNRAS

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 $\beta$ and the viscosity parameter $\alpha$, it is found that only $\alpha$ has significant effects on $\dot m_{\rm crit}-r$ relation. We define the minimum $\dot m_{\rm crit}$ (roughly at the innermost stable circular orbit) as the hard-to-soft transition rate $\dot m_{\rm tr: H\rightarrow S}$, 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 $\alpha$ respectively. By comparing with observations, it is found that the mean value of $\alpha$ are $\alpha \sim 0.85$ and $\alpha \sim 0.33$ for the hard-to-soft transition and the soft-to-hard transition respectively, which indicates that two classes of $\alpha$ are needed for explaining the hysteresis effect during the state transition. Finally, we argue that such a constrained $\alpha$ may provide valuable clues for further exploring the accretion physics in BH-XRBs., Comment: 10 pages, 7 figures, 4 tables, accepted for publication in MNRAS

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 days 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 days 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 days 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., Comment: 20 pages, 10 figures, 1 table. Accepted for publication in MNRAS

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 disk 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 luminosity, as well as the evolution of these quantities with time, providing a strong theoretical basis for understanding the origin of optical/UV TDEs., Comment: 7 pages, 4 figures, submitted

Recently, the red giant V723 Mon is reported to have an unseen companion with a mass of $3.04\pm0.06M_{\odot}$, but question remains about whether it is a single (thus the so-called mass-gap) black hole or an inner binary of two more ordinary compact objects (neutron stars or white dwarfs). In this work, we estimate the X-ray emission by considering the wind-fed accretion from V723 Mon onto the compact companion. We analyze three different scenarios of the dark companion, i.e., a single black hole, binary neutron stars and binary of a neutron star and a white dwarf. We show that the single black hole is the most favored scenario. We also calculate the synchrotron emission from the bow shock caused by the interaction of the compact companion with the wind. We find this emission peaks at $\sim$ 0.1-1 GHz, with a flux density $\sim$ 1 mJy, which is expected to be detected by observations with higher angular resolution in the future., Comment: 8 pages, 2 figures, 1 table, submitted to MNRAS

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., Comment: 39 pages, 6 figures

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., Comment: 11 pages, 5 figures; published in Nature Communications; An artist's impression of Figure 5 is updated here, and the animation is available at http://youbei.work/research/

When neutrons 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 be existed 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 and 2020b 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 a X-ray luminosity $L_{\rm X}$ (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 of 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 onto the surface of the NS forming the X-ray pulsation., Comment: 9 pages, 5 figures, accepted for publication by MNRAS

Observationally, an anti-correlation between the X-ray photon index $\Gamma$ (obtained by fitting the X-ray spectrum between 0.5 and 10 keV with a single power law) and the X-ray luminosity $L_{\rm 0.5-10keV}$, is found in neutron star low-mass X-ray binaries (NS-LMXBs) in the range of $L_{\rm 0.5-10keV}\sim 10^{34}-10^{36}\ \rm erg\ s^{-1}$. In this paper, we explain the observed anti-correlation between $\Gamma$ and $L_{\rm 0.5-10keV}$ 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 anti-correlation between $\Gamma$ and $L_{\rm 0.5-10keV}$. In the ADAF model, there is a key parameter, $f_{\rm th}$, 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 $f_{\rm th}$ on the anti-correlation between $\Gamma$ and $L_{\rm 0.5-10keV}$. It is found that the value of $f_{\rm th}$ can significantly affect the anti-correlation between $\Gamma$ and $L_{\rm 0.5-10keV}$. Specifically, the anti-correlation between $\Gamma$ and $L_{\rm 0.5-10keV}$ becomes flatter with decreasing $f_{\rm th}$ as taking $f_{\rm th}=0.1, 0.03, 0.01, 0.005$, $0.003$ and $0$ respectively. By comparing with a sample of non-pulsating NS-LMXBs with well measured $\Gamma$ and $L_{\rm 0.5-10keV}$, we find that indeed only a small value of $0.003\lesssim f_{\rm th}\lesssim 0.1$ is needed to match the observed anti-correlation between $\Gamma$ and $L_{\rm 0.5-10keV}$. Finally, we argue that the small value of $f_{\rm th}\lesssim 0.1$ 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 $\epsilon \sim {\dot M GM\over R_{*}}/{\dot M c^2}\sim 0.2$., Comment: 12 pages, 12 figures. Accepted for publication by MNRAS

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., Comment: 14 pages, 10 figures, 2 tables, published in MNRAS

We study the evolution of the temporal properties of MAXI 1820+070 during the 2018 outburst in its hard state from MJD 58190 to 58289 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 58257, suggesting a transition occurred at 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; first increase until around MJD 58257 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., Comment: Accepted for publication in ApJ

Numerical simulations of radiative two-temperature hot accretion flows (HAFs) around Neutron stars (NSs) are performed. We assume that all of the energy carried by the HAF around a NS will be thermalized and radiated out at the surface of the NS. The thermal photons will propagate outwards radially and cool the HAF vis Comptonization. We define $\dot m$ as the mass accretion rate at the surface of the central object in unit of Eddington accretion rate ($\dot M_{\rm Edd}=10L_{\rm Edd}/c^2$, with $L_{\rm Edd}$ and $c$ being Eddington luminosity and speed of light, respectively). When $\dot m$ is lower than $\sim 10^{-4}$, the cooling of the HAF is not important and outflows are very strong. When $\dot m > \sim 10^{-3}$, cooling becomes important and outflows are significantly weak. In the range $10^{-4} < \dot m < 10^{-3}$, the HAFs transients from a strong outflow phase to a very weak outflow phase with increase of $\dot m$. The properties of the HAF around a NS are also compared to those of the HAF around a BH. We find that with a similar $\dot m$, the dynamical properties of the HAF around a NS are quite similar as those of the HAF around a BH. However, the emitted spectrum of a HAF around a NS can be quite different from that of a HAF around a BH due to the presence of a thermal soft X-ray component coming from the surface of the NS., Comment: 12 pages, 4 figures, accepted by ApJ

Observationally, the X-ray spectrum ($0.5-10$ keV) of low-level accreting neutron stars (NSs) 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 $\eta$ ($\eta\equiv L^{\rm power\ law}_{\rm 0.5-10\rm keV}/L_{\rm 0.5-10\rm keV}$) 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 $\alpha$ in the ADAF and thermalized parameter $f_{\rm th}$ (describing the fraction of the ADAF energy released at the surface of the NS as thermal emission) on the relation of $\eta$ versus $L_{\rm 0.5-10\rm keV}$. It is found that $\eta$ is nearly a constant ($\sim$ zero) with $L_{\rm 0.5-10\rm keV}$ for different $\alpha$ with $f_{\rm th}=1$, which is inconsistent with observations. Meanwhile, it is found that a change of $f_{\rm th}$ can significantly change the relation of $\eta$ versus $L_{\rm 0.5-10\rm keV}$. By comparing with a sample of non-pulsating NS-LMXBs probably dominated by low-level accretion onto NSs, it is found that a small value of $f_{\rm th} \lesssim 0.1$ is needed to match the observed range of $\eta \gtrsim 10\%$ in the diagram of $\eta$ versus $L_{\rm 0.5-10\rm keV}$. Finally, we argue that the small value of $f_{\rm th} \lesssim 0.1$ 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., Comment: 13 pages, 23 figures. Accepted for publication by MNRAS

We report the discovery of large amplitude X-ray variability in the low luminosity AGN (LLAGN) MGC 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/XRT and XMM-Newton over 17 years. Results of long-term monitoring data in the UV, optical and infrared bands over ~20 years are also presented. The Eddington ratio increased from $10^{-3}$ to $\sim0.13$, suggesting a transition of the accretion flow from an ADAF 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 ($\sim19^{+15}_{-7}$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 $\sim10^{7}\,M_{\mathrm{Sun}}$. These results may indicate that NGC 7589 had experienced accretion mode transition over a timescale of a few years, suggesting the idea that similar accretion processes are at work for massive black hole and black hole X-ray binaries., Comment: 10 pages, 2 figures, resubmitted to MNRAS after revision

Observationally, for neutron star low-mass X-ray binaries, so far, the correlation between the radio luminosity $L_{\rm R}$ and the X-ray luminosity $L_{\rm X}$, 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 $\beta$ of the radio/X-ray correlation of the three sources is different, i.e., $\beta \sim 1.4$ for 4U 1728-34, $\beta \sim 0.4$ for Aql X-1, and $\beta \sim 1.6$ 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 (ADAF)-jet model respectively. We calculate the emergent spectrum of the ADAF-jet model for $L_{\rm X}$ and $L_{\rm R}$ at different $\dot m$ ($\dot m=\dot M/\dot M_{\rm Edd}$), adjusting $\eta$ ($\eta \equiv \dot M_{\rm jet}/\dot M$, describing the fraction of the accreted matter in the ADAF transfered vertically forming the jet) to fit the observed radio/X-ray correlations. Then we derive a fitting formula of $\eta$ as a function of $\dot m$ for 4U 1728-34, Aql X-1 and EXO 1745-248 respectively. If the relation between $\eta$ 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 $\eta$ 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., Comment: 8 pages, 5 figures; Accepted for publication by MNRAS

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, $f_{\rm th}$, of the energy transferred onto 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 $f_{\rm th}$ on the structure, as well as the emergent spectrum of the ADAF. It is found that a change of $f_{\rm th}$ 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 $f_{\rm th}$ can be constrained by fitting the high-quality X-ray data, such as the $\it XMM$-$\it Newton$ spectrum between 0.5-10 keV in the future work., Comment: 10 pages, 15 figures; Accepted for publication by MNRAS

We report on the detection of a pulsating Fe Ka 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 Ka 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 Ka flux increased only by about 10\%, 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 Ka 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., Comment: 8 pages, 5 figures, MNRAS accepted

A new accretion picture based on a small disk surrounding a black hole is developed for the wind-fed source Cyg X-1. The hard and soft spectral states of Cyg X-1 are interpreted in terms of co-spatial two component flows for the innermost region of an accretion disk. The state transitions result from the outward expansion and inward recession of this inner disk for the hard to soft and soft to hard transition respectively. The theoretical framework for state transitions in black hole X-ray binaries with high mass companions involving a change in the inner disk size, thus, differs from systems with low mass companions involving the change in the outer disk size. This fundamental difference stems from the fact that matter captured and supplied to the black hole in wind-fed systems has low specific angular momentum and is hot essentially heated in the bow and spiral shocks, whereas it has high specific angular momentum and is cool in Roche lobe overflow systems. The existence of a weak cool disk around the ISCO region in the hard state allows for the presence of a relativistically broadened Fe K line. The small disk fed by gas condensation forms without an extensive outer disk, precluding thermal instabilities and large outbursts, resulting in the lack of large amplitude outbursts and hysteresis effects in the light curve of high mass black hole X-ray binaries. Their relatively persistent X-ray emission is attributed to their wind-fed nature., Comment: 13 pages, 2 figures. Accepted for publication in ApJ

Flare research is becoming a burgeoning realm in the study of stellar activity due to the launch of {\it Kepler} in 2009. {\it Kepler} provides data with two time resolutions, i.e., the long-cadence (LC) data with a time resolution of 30 minutes and the short-cadence(SC) data with a time resolution of 1 minute, both of which can be used to study stellar flares. In this paper, we search flares in light curves with both LC data and SC data, and compare them in aspects of the true-flare rate, the flare energy, the flare amplitude, and the flare duration. It is found that LC data systematically underestimated the energies of flares by 25\%, and underestimated the amplitudes of flares by 60\% compared with SC flares. The duration are systematically overestimated by 50\% compared with SC flares. However, the above percentages are poorly constrained and there is a lot of scatter. About 60\% SC flares have not been detected by LC data. We investigate the limitation of LC data, and suggest that although LC data cannot reflect the detailed profiles of flares, they also can capture the basic properties of stellar flares., Comment: 9 pages,9 figures, accept, 2018 APJ

In this paper, we explore the physics of the accretion and jet in narrow-line Seyfert 1 galaxies (NLS1). Specifically, we compile a sample composed of 16 nearby NLS1 with $L_{\rm bol}/L_{\rm Edd} \gtrsim 0.1$. We investigate the mutual correlation between their radio luminosity $L_{\rm R}$, X-ray luminosity $L_{\rm X}$, optical luminosity $L_{\rm 5100}$ and black hole mass $M_{\rm BH}$. By adopting partial correlation analysis: (1) we find a positive correlation between $L_{\rm X}$ and $M_{\rm BH}$, and (2) we find a weak positive correlation between $L_{\rm R}$ and $L_{5100}$. However, we don't find significant correlations between $L_{\rm R}$ and $L_{\rm X}$ or between $L_{\rm X}$ and $L_{5100}$ after considering the effect of the black hole mass, which leads to a finding of the independence of $L_{\rm X}/L_{\rm Edd}$ on $L_{5100}/L_{\rm Edd}$. Interestingly, the findings that $L_{\rm X}$ is correlated with $M_{\rm BH}$ and $L_{\rm X}/L_{\rm Edd}$ is not correlated with $L_{5100}/L_{\rm Edd}$ support that the X-ray emission is saturated with increasing $\dot{M}$ for $L_{\rm bol}/L_{\rm Edd} \gtrsim 0.1$ in NLS1s, 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., Comment: 9 pages, accepted for publication in MNRAS

In this paper, we explained the observed $\Gamma_{\rm 2-10keV}-L_{\rm bol}/L_{\rm Edd}$ correlation in luminous active galactic nuclei within the framework of the condensation of the corona around a supermassive black hole (Liu et al. 2015; Qiao & Liu 2017). Specifically, we systemically test the effects of black hole mass $M$, the viscosity parameter $\alpha$, and the magnetic parameter $\beta$ (with magnetic pressure $p_{\rm m}=B^2/{8\pi}=(1-\beta)p_{\rm tot}$, $p_{\rm tot}=p_{\rm gas}+p_{\rm m}$) 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 $\Gamma_{\rm 2-10keV}$ nearly does not change with changing black hole mass $M$, or changing magnetic parameter $\beta$. 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 $\alpha$. 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 $\Gamma_{\rm 2-10keV}-L_{\rm bol}/L_{\rm Edd}$ correlation can be well matched with a relatively bigger value of $\alpha$, i.e., $\alpha \sim 1$, as previously also suggested by Narayan (1996) for luminous accreting black holes., Comment: 10 pages, 13 figures, Accepted for publication by MNRAS

The X-ray emission from bright active galactic nuclei (AGNs) is believed to originate in a hot corona lying above a cold, geometrically thin accretion disk. A highly concentrated corona located within $\sim10$ gravitational radii above the black hole is inferred from observations. Based on the accretion of interstellar medium/wind, a disk corona model has been proposed in which the corona is well coupled to the disk by radiation, thermal conduction, as well as by mass exchange \citep{Liu2015, Qiao2017}. Such a model avoids artificial energy input to the corona and has been used to interpret the spectral features observed in AGN. In this work, it is shown that the bulk emission size of the corona is very small for the extended accretion flow in our model. More than 80\% of the hard X-ray power is emitted from a small region confined within 10 Schwarzschild radii around a non-spinning black hole, which is expected to be even smaller accordingly for a spinning black hole. Here, the corona emission is more extended at higher Eddington ratios. The compactness parameter of the corona, $l={L\over R}{\sigma_{\rm T}\over m_{\rm e} c^3}$, is shown to be in the range of 1-33 for Eddington ratios of 0.02 - 0.1. Combined with the electron temperature in the corona, this indicates that electron--positron pair production is not dominant in this regime. A positive relation between the compactness parameter and photon index is also predicted. By comparing the above model predictions with observational features, we find that the model is in agreement with observations., Comment: 11 pages, 4 figures

Observationally, it is found that there is a strong correlation between the hard X-ray photon index $\Gamma$ and the Compton reflection scaling factor $\Re$ in active galactic nuclei. In this paper, we propose that the $\Gamma-\Re$ correlation can be explained within the framework of the condensation of the hot corona onto 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 $\dot M/ \dot M_{\rm Edd} \gtrsim 0.01$, at a critical radius $r_{\rm d}$, part of the hot gas begins to condense onto the equatorial disc plane of the black hole, forming an inner cold accretion disc. Then the matter is accreted in the form of the 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 $\Re$ and the increase of the hard X-ray photon index $\Gamma$. 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 to apply our model to high mass X-ray binaries, which are believed to be fueled by the hot wind from the companion star., Comment: Accepted for publication by MNRAS, 8 pages, 7 figures

It is believed that the hard X-ray emission in the luminous active galactic nuclei (AGNs) is from the hot corona above the cool accretion disk. However, the formation of the corona is still debated. Liu et al. investigated the spectrum of the corona heated by the reconnection of the magnetic field generated by dynamo action in the thin disk and emerging into the corona as a result of buoyancy instability. In the present paper, we improve this model to interpret the observed relation of the hard X-ray spectrum becoming softer at higher accretion rate in luminous AGNs. The magnetic field is characterized by $\beta_{\rm 0}$, i.e., the ratio of the sum of gas pressure and radiation pressure to magnetic pressure in the disk ($\beta_{\rm 0}=(P_{\rm g,d}+P_{\rm r,d})/P_{\rm B}$). Besides, both the intrinsic disk photons and reprocessed photons by the disk are included as the seed photons for inverse Compton scattering. These improvements are crucial for investigating the effect of magnetic field on the accretion disk-corona when it is not clear whether the radiation pressure or gas pressure dominates in thin disk. We change the value of $\beta_{\rm 0}$ in order to constrain the magnetic field in the accretion disk. We find that the energy fraction released in the corona ($f$) gradually increases with the decrease of $\beta_{\rm 0}$ for the same accretion rate. When $\beta_{\rm 0}$ decreases to less than 50, the structure and spectrum of the disk-corona is independent on accretion rate, which is similar to the hard spectrum found in Liu et al.(2003). Comparing with the observational results of the hard X-ray bolometric correction factor in a sample of luminous AGNs, we suggest that the value of $\beta_{\rm 0}$ is about 100-200 for $\alpha=0.3$ and the energy fraction $f$ should be larger than $30\%$ for hard X-ray emission., Comment: 21 pages, 6 figures. ApJ accepted

We report on the identification of a new $\gamma$-ray-emitting narrow-line Seyfert 1 (NLS1) galaxy, SDSS J122222.55+041315.7, which increases the number of known objects of this remarkable but rare type of active galactic nuclei (AGN) to seven. Its optical spectrum, obtained in the Sloan Digital Sky Survey-Baryon Oscillation Spectroscopic Survey, reveals a broad H $\beta$ emission line with a width (FWHM) of 1734$\pm$104 km s$^{-1}$. This, along with strong optical Fe II multiplets [$R_{4570}=0.9$] and a weak [O III] $\lambda 5007$ emission line, makes the object a typical NLS1. On the other hand, the source exhibits a high radio brightness temperature, rapid infrared variability, and a flat X-ray spectrum extending up to $\sim$200 keV. It is associated with a luminous $\gamma$-ray source detected significantly with {\it Fermi}/LAT. Correlated variability with other wavebands has not yet been tested. The spectral energy distribution can be well modelled by a one-zone leptonic jet model. This new member is by far the most distant $\gamma$-ray-emitting NLS1, at a redshift of $z=0.966$., Comment: 5 pages, published on MNRAS

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 $L_{\rm R} \propto L_{\rm X}^{\sim 0.5-0.7}$ 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., $L_{\rm R} \propto L_{\rm X}^{\sim 1.4}$ for $L_{\rm X}/L_{\rm Edd} \gtrsim 10^{-3}$, 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 $L_{\rm X}/L_{\rm Edd} \gtrsim 10^{-3}$ 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, $\eta\equiv \dot M_{\rm jet}/\dot M$, is ejected to form the jet, we can calculate the emergent spectrum of the disc corona-jet system. We calculate $L_{\rm R}$ and $L_{\rm X}$ at different $\dot M$, adjusting $\eta$ to fit the observed radio/X-ray correlation of the black hole X-ray transient H1743-322 for $L_{\rm X}/L_{\rm Edd}> 10^{-3}$. 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 $\eta$ for the deviated radio/X-ray correlation for $L_{\rm X}/L_{\rm Edd} > 10^{-3}$, is systematically less than that of the case for $L_{\rm X}/L_{\rm Edd} < 10^{-3}$, 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., Comment: Accepted by MNRAS

Observations show that the accretion flows in low-luminosity active galactic nuclei (LLAGNs) probably have a two-component structure with an inner ADAF and an outer truncated accretion disk. As shown by Taam et al. (2012), 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 of 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. (2012). It is found that an increase of the magnetic field from $\beta=0.8$ to $\beta=0.5$ (with magnetic pressure $p_{\rm m}=B^2/{8\pi}=(1-\beta)p_{\rm tot}$, $p_{\rm tot}=p_{\rm gas}+p_{\rm m}$) results in an increase of $\sim 8.7$ times of the luminosity from the truncated accretion disk. We found that the equipartition of gas pressure to magnetic pressure, i.e., $\beta=0.5$, failed to explain the observed anti-correlation between $L_{\rm 2-10 keV}/L_{\rm Edd}$ and the bolometric correction $\kappa_{\rm 2-10 keV}$ (with $\kappa_{\rm 2-10 keV} = L_{\rm bol}/L_{\rm 2-10 keV}$). The emergent spectra for larger value $\beta=0.8$ or $\beta=0.95$ can well explain the observed $L_{\rm 2-10 keV}/L_{\rm Edd}$-$\kappa_{\rm 2-10 keV}$ 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 the 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., Comment: Accepted for publication by ApJ

Observations show that there is a positive correlation between Eddington ratio $\lambda$ and hard X-ray index $\Gamma$ for $\lambda \gtrsim 0.01$, and there is an anti-correlation between $\lambda$ and $\Gamma$ for $\lambda \lesssim 0.01$ in black hole X-ray binaries (with $\lambda=L_{\rm bol}/L_{\rm Edd}$). In this work, we theoretically investigate the correlation between $\Gamma$ and $\lambda$ within the framework of disk-corona model. We improve the model by taking into account all cooling processes including synchrotron and self-Compton radiations in the corona, Comptonization of the soft photons from the underlying accretion disk, and the Bremsstrahlung radiations. Presuming that the coronal flow above the disk can reach up to 0.1 Eddington rate at the outer region, we calculate the structure of the two-phase accretion flows and the emergent spectra for accretion rates from 0.003 to 0.1. It is found that at accretion rates larger than $\backsim 0.01$ Eddington rate a fraction of coronal gas condenses into the disk and an inner disk can be sustained by condensation. In this case, the X-ray emission is dominated by the scattering the soft photon from the underlying disk in the corona. The emission from the inner disk and corona can produce the positive correlation between $\lambda$ and $\Gamma$. While at accretion rates lower than $\backsim 0.01$ Eddington accretion rate, the inner disk vanishes completely by evaporation, the accretion is dominated by ADAF, in which the X-ray emission is produced by the Comptonization of the synchrotron and bremsstrahlung photons of ADAF itself. The emission from ADAF can produce the anti-correlation between $\lambda$ and $\Gamma$., Comment: Accepted for publication by ApJ

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 canonical low/hard state. The spectrum is characterized by a power law with a photon index $\Gamma \sim 1.5-2.1$ in the range of 2-10 keV and a weak disk component with temperature of $\sim 0.2$ keV. In this work, We revisit the formation of such a cool, optically thick, geometrically thin disk in the most inner region of black hole X-ray binaries at the low/hard state within the context of disk accretion fed by condensation of 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 $\alpha=0.2$, it's found that the inner disk can exist for accretion rate ranging from $\dot M \sim 0.006-0.03 \dot M_{\rm Edd}$, over which the electron temperatures of the corona are in the range of $1-5\times 10^9\ \rm K$ 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 $\alpha$ 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) to the emergent spectra are 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 ISCO region in the low/hard state at luminosity around a few percent of $L_{\rm Edd}$. It's found that the observed maximal effective temperature of the thermal component and the hard X-ray photon index $\Gamma$ can be matched well by our model., Comment: Accepted for publication by ApJ

We investigate the spectral features of accretion flows composed of an outer cool, optically thick disk and inner hot, optically thin, advection dominated accretion flows (ADAF) within the framework of disk and corona with mass evaporation (Liu et al. 2002a). In this work, both the magnetic field and Compton scattering of soft photons from the disk by electrons in the corona are included to calculate the evaporation rates at different distances. The disk is truncated at the distance where the evaporation rate equals to the accretion rate ($\dot m_{\rm evap}(r_{\rm tr})=\dot m$). For a series of accretion rates, the corresponding truncation radii are calculated out, with which we are able to calculate the emergent spectra from the inner ADAF + outer disk + corona. At very low accretion rates, the spectra are similar to that of a pure ADAF because the disk is truncated at large distances. The disk component becomes important at high accretion rates since the truncation occurs at small distances. An anti-correlation between the Eddington ratio $\xi \equiv L_{\rm 0.5-25 \,keV}/L_{\rm Edd}$ and the hard X-ray photon index $\Gamma_{\rm 3-25 \,keV}$ at low/hard states is predicted by the model. Comparing the theoretical results with observations, we find that our model can reproduce the anti-correlation between the Eddington ratio $\xi$ and the hard X-ray photon index observed for the X-ray binary XTE J1118+480., Comment: 11 pages and 7 figures, published by PASJ

A wealth of Galactic accreting X-ray binaries have been observed both in low/hard state and high/soft state. The transition between these two states was often detected. Observation shows that the transition luminosity between these two states is different for different sources, ranging from 1% to 4% of the Eddington luminosity. Even for the same source the transition luminosity at different outbursts is also different. The transition can occur from 0.0069 to 0.15 Eddington luminosity. To investigate the underlying physics, we study the influence of viscosity parameter $\alpha$ on the transition luminosity on the basis of the disk-corona model for black holes. We calculate the mass evaporation rate for a wide range of viscosity parameter, $0.1\le \alpha\le 0.9$. By fitting the numerical results, we obtain fitting formulae for both the transition accretion rate and the corresponding radius as a function of $\alpha$. We find that the transition luminosity is very sensitive to the value of $\alpha$, $L/L_{\rm Edd}\propto\alpha^{2.34}$. For $0.1\le\alpha\le 0.6$, the transition luminosity varies by two orders of magnitude, from 0.001 to 0.2 Eddington luminosity. Comparing with observations we find that the transition luminosity can be fitted by adjusting the value of $\alpha$, and the model determined values of $\alpha$ are mostly in the range of observationally inferred value. Meanwhile we investigate the truncation of the disk in the low/hard state for some luminous sources. Our results are roughly in agreement with the observations., Comment: 8 pages, 3 figures,accepted by PASJ