arXiv subjects

Xiao-Hong Yang

Publications and source records attributed to Xiao-Hong Yang.

At least 19 recordsLinked to original sources

arXiv2026

Hydrodynamical simulation of wind production from hot accretion flows in tidal disruption events

Wind is a key mechanism for supermassive black hole (SMBH) feedback to their host galaxies. In tidal disruption events (TDEs), black holes spend most of their time accreting at highly sub-Eddington rates, implying that feedback from persistent sub-Eddington winds could be significant. We investigate the effects of black hole mass, viscosity parameter and stellar debris temperature on the properties of winds from hot accretion flows in TDEs. We find that more massive black holes yield a higher accreted fraction and launch faster winds, while the debris temperature has a negligible influence on the accretion flow. For $α=0.1$, the mildly-relativistic unbound winds ($\sim 0.1c$) are launched predominantly from the outside of the accretion flows along the equatorial plane, with a kinetic energy of $\sim10^{-4}L_\mathrm{Edd}$. In contrast, convective bound outflows dominate for $α=0.01$, which differs from the true winds typically seen in active galactic nuclei and X-ray binaries. Potential applications for explaining delayed radio brightening in TDEs at $\sim10^3$ days and for searching for intermediate-mass black holes through radio and X-ray surveys are also discussed.

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arXiv2026

Radiative cooling effects on black hole hot accretion flows around the sub-Bondi radius

It is difficult to implement numerical simulations on a region extending from the vicinity of a black hole to the Bondi radius. Most previous numerical simulations have primarily concentrated on the region close to the black hole. They found that strong winds can be generated in the hot accretion flows near the black hole, and that radiative cooling significantly affects the strength of these winds. However, the effects of radiative cooling on the production and properties of winds around the Bondi radius remain unclear. In this paper, we perform two-dimensional magnetohydrodynamic simulations to study the impact of radiative cooling on the dynamics and wind production in hot accretion flows around the sub-Bondi radius. As the increase of mass accretion rate, radiative cooling gradually becomes strong, resulting in a reduction in the thickness of the accretion disk (defined as the accretion flows within the density scale height). Based on the Høiland criterion, we find that within the accretion disk, the region of convective stability accounts for $\sim$ 55 - 62 %, and therefore the accretion flows are marginally stable in convective stability. In the runs with weak radiative cooling, the winds play a significant role in the inward decrease of the mass inflow rate. In the runs with strong radiative cooling, the mass outflow rate of winds is significantly reduced and then the inward decrease of the mass inflow rate is mainly attributed to turbulence driven by magnetorotational instability and convection. Radiative cooling has the potential to suppress accretion processes and reduce the power of winds.

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arXiv2026

Numerical Simulations of the Circularized Accretion Flow in Population III Star Tidal Disruption Events. II. Radiative Properties

Tidal Disruption Events (TDEs) release enormous amounts of energy, offering a promising avenue for detecting Population III (Pop III) stars. However, the radiative properties of TDEs of Pop III stars have so far been studied only analytically, relying on many assumptions. Based on our radiative hydrodynamic simulations that follow the evolution of the accretion system for Pop III star TDEs where a $300\ M_{\odot}$ ($M_{\odot}$ is the solar mass) star is disrupted by a $10^{6}\ M_{\odot}$ black hole (BH), we compute the emission properties of the event in rest frame and find that the spectrum peaks in the optical/UV waveband. After accounting for redshift ($z \sim 10$) and extinction effects, we find the observed spectral peak shifts to the infrared, with fluxes exceeding $10^{2}\mathrm{nJy}$-making such events detectable with both the James Webb Space Telescope (JWST) and the Nancy Grace Roman Space Telescope (Roman). The dependence of the observed spectrum on viewing angle is suppressed due to dust extinction. Using our simulation results, we also calculate the radio emission generated by the interaction between the wind and the circumnuclear medium (CNM) and find that a Pop III star TDE can produce an unusually long-lasting, continuously increasing radio flare with a duration greater than $10^4$ days and thus has the potential to be detected in radio wavebands. These results may be helpful to the detection of Pop III stars.

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arXiv2026

Numerical simulations of cold clumps in the hot accretion flows around black holes

Previous numerical simulations have shown that cold clumps can form within hot accretion flows, offering insights into the detailed processes of the state transition in black hole X-ray binaries. However, the evolution of the cold clumps has not been investigated in detail yet. In this paper, we conduct hydrodynamic simulations to investigate the evolution of the cold clumps. In addition to previous result that when the accretion rate is high enough the cold clumps emerge within the hot accretion flow, we found that instead of directly moving toward to the black hole, the clumps moves outward when they initially form. The reason should be the combination of viscous torque and the condensation of hot gas from larger radii, which lead to the slightly super-Keplerian angular momentum of the clumps. After reaching the equilibrium position, the clumps begin to fragment at the inner edge with each fragment moving inward sequentially. Generally, the azimuthal movement of the clumps are quasi-Keplerian, being closer to the outer detached Keplerian cold disk rather than the surrounding sub-Keplerian hot accretion flow, which agrees well with the semi-analytical results for weak coupling case in Wang et al. (2012).

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arXiv2025

Numerical Simulations of the Circularized Accretion Flow in Population III Star Tidal Disruption Events. I. The Accretion Flow and the Wind

Tidal Disruption Events (TDEs) have recently been proposed as potential probes for Population III stars. However, the properties of the accretion flow and the wind from the Pop III star TDE system are not clear. By performing radiative hydrodynamic simulations, we study the 'circularized' accretion flow of the Pop III star TDE system. The masses of the black hole (BH) and the disrupted star are $10^6$ and $300$ solar masses, respectively. We focus on the properties of the wind. We find that the black hole accretion rate is highly super-Eddington. A strong wind is driven by radiation pressure. Due to the presence of a strong wind, only $25\%$--$35\%$ of the fallback debris is accreted by the BH. The remaining part is taken away by the wind. The kinetic power of the wind can be as high as $10^{46} {\rm \ erg \ s^{-1}}$. The properties of the wind obtained in this paper may be useful for understanding the radiation properties of Pop III star TDEs in the context of the wind 'reprocessing' model.

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arXiv2025

Line-force driven wind from a thin disk in tidal disruption event

Winds from the accretion disk in tidal disruption events (TDEs) play a key role in determining the radiation of TDEs. The winds from the super-Eddington accretion phase in TDEs have recently been studied. However, properties of the winds from the sub-Eddington accretion disk in TDEs are not clear. We aim to investigate properties of winds from the circularized sub-Eddington accretion disk in TDEs. We study the line force driven accretion disk wind. We perform two-dimensional hydrodynamic simulations using the PLUTO code to study the line force driven wind from the circularized accretion disk around a $10^6$ solar mass black hole in TDEs. We find that although the disk has a very small size in TDEs, strong wind can be driven by line force when the disk have luminosity higher than $20\%$ of the Eddington luminosity. The maximum velocity of wind can be as high as $0.3$ times the speed of light. The kinematic power of wind is in the range of $1\%-6\%$ times the Eddington luminosity. Strong wind can be driven by line force from the thin disk around a $10^6$ solar mass black hole in TDEs. We briefly discuss the possible radio emission from the shock when the wind collides with the surrounding medium.

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arXiv2023

Time-dependent global simulations of a thin accretion disc: the effects of magnetically-driven winds on thermal instability

According to the standard thin disc theory, it is predicted that the radiation-pressure-dominated inner region of a thin disc is thermally unstable, while observations suggest that it is common for a thin disc of more than 0.01 Eddington luminosity to be in a thermally stable state. Previous studies have suggested that magnetically driven winds have the potential to suppress instability. In this work, we implement one-dimensional global simulations of the thin accretion disc to study the effects of magnetically driven winds on thermal instability. The winds play a role in transferring the angular momentum of the disc and cooling the disc. When the mass outflow rate of winds is low, the important role of winds is to transfer the angular momentum and then shorten the outburst period. When the winds have a high mass outflow rate, they can calm down the thermal instability. We also explore the parameter space of the magnetic field strength and the mass loading parameter.

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arXiv2023

Radiative hydrodynamical simulations of super-Eddington accretion flow in tidal disruption event: the accretion flow and wind

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.

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arXiv2023

Radio emission of tidal disruption events from wind-cloud interaction

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.

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arXiv2022

Effect of radiation drag on the line-force-driven winds

Ultra-fast outflows (UFOs) with mildly relativistic velocities are measured using the X-ray spectra of radio-quiet and -loud active galactic nuclei (AGNs). In general, UFOs are believed to be generated from the accretion disk around a black hole (BH). A line-force driving model is suggested to be the mechanism to drive UFOs from the accretion disk. In this paper, we use the non-hydrodynamic approach to examine the influences of radiation-drag effects on the line-force-driven winds generated from the accretion disk. We find that the radiation-drag effects can significantly weaken the line-force-driven winds. Compared with the case without the radiation-drag effects, when the radiation-drag effects are considered, the maximum speed of winds is reduced by $\sim$60\%--70\%, the mass outflow rate is reduced by $\sim$50\%--80\%, and the kinetic power is reduced by about an order of magnitude. The radiation-drag effects narrow the area where the winds are generated.

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arXiv2022

The origin of the optical/ultraviolet emission of optical/ultraviolet tidal disruption events

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.

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arXiv2022

Large-scale Dynamics of Winds Driven by Line Force from a Thin Accretion Disk

Winds play a significant role in active galactic nuclei feedback process. Previous simulations studying winds only focus on a small dynamical range. Therefore, it is unknown how far the winds can go and what the properties of the winds will be if they can move to large radii. We perform simulations to study the large scale dynamics of winds driven by line force. We find that the properties of the winds depend on both black hole mass ($M_{BH}$) and accretion disk luminosity. When the accretion disk luminosity is $0.6L_{edd}$ ($L_{edd}$ being Eddington luminosity), independent of $M_{BH}$, the winds have kinetic energy flux exceeding $1\% L_{edd}$ and can escape from the black hole potential. For the case with the accretion disk luminosity equaling 0.3$L_{edd}$, the strength of the winds decreases with the decrease of $M_{BH}$. If $M_{BH}$ decreases from $10^9$ to $10^6$ solar mass ($M_\odot$), the winds kinetic energy flux decreases from $\sim 0.01 L_{edd}$ to $ \sim 10^{-6} L_{edd}$. In case of $M_{BH}\geq 10^7 M_\odot$, winds can escape from black hole potential. In the case of $M_{BH}=10^6 M_\odot$, the winds can not escape. We find that for the ultra-fast winds observed in hard X-ray bands (\citealt{Gofford et al. 2015}), the observed dependence of the mass flux and the kinetic energy flux on accretion disk luminosity can be well produced by line force driven winds model. We also find that the properties of the ultra-fast winds observed in soft X-ray bands can be explained by the line force driven winds model.

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arXiv2021

Magnetohydrodynamic winds driven by line force from the standard thin disk around supermassive black holes: II. a possible model for ultra-fast outflows in radio-loud AGNs

In radio-loud active galactic nuclei (AGNs), ultra-fast outflows (UFOs) were detected at the inclination angle of $\sim10^{\rm o}$--$70^{\rm o}$ away from jets. Except for the inclination angle of UFOs, the UFOs in radio-loud AGNs have similar properties to that in radio-quiet AGNs. The UFOs with such low inclination cannot be explained in the line-force mechanism. The magnetic-driving mechanism is suggested to explain the UFOs based on a self-similar solution with radiative transfer calculations. However, the energetics of self-similar solution need to be further confirmed based on numerical simulations. To understand the formation and acceleration of UFOs in radio-loud AGNs, this paper presents a model of the disk winds driven by both line force and magnetic field and implements numerical simulations. Initially, a magnetic field is set to 10 times stronger than the gas pressures at the disk surface. Simulation results imply that the disk winds driven by both line force and magnetic field could describe the properties of UFOs in radio-loud AGNs. Pure magnetohydrodynamics (MHDs) simulation is also implemented. When the initial conditions are the same, the hybrid models of magnetic fields and line force are more helpful to form UFOs than the pure MHD models. It is worth studying the case of a stronger magnetic field to confirm this result.

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arXiv2021

Can warm absorbers be driven by ultra-fast outflows?

Warm absorbers (WAs) located approximately in the region of $1-1000$ parsecs are common phenomena in many active galactic nuclei (AGNs). The driving mechanism of WAs is still under debate. Ultra-fast outflows (UFOs) which are launched very close to the central black hole are also frequently observed in AGNs. When UFOs move outwards, they will collide with the interstellar medius (ISM) gas. In this paper, we study the possibility that whether WAs can be generated by the interaction between ISM gas and the UFOs. We find that under some ISM gas conditions, WAs can be generated. However, the covering factor of WAs is much smaller than that given by observations. This indicates that other mechanisms should also be at work. We also find that the properties of the WAs mainly depend on the density of the ISM injected into the computational domain from the outer radial boundary (1000 parsec). The higher the density of the ISM is, the higher the mass flux and kinetic power of the WAs will be. The kinetic power of the UFO driven WAs is much less than $1\%$ of the bolometric luminosity of its host AGNs. Therefore, the UFO driven WAs might not contribute sufficient feedback to its host galaxy.

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arXiv2021

Dynamical and thermal properties of the parsec-scale gases spherically accreted onto low luminous active galactic nuclei

We analytically study the dynamical and thermal properties of the optically-thin gases at the parsec-scale when they are spherically accreted onto low luminous active galactic nuclei (LLAGNs). The falling gases are irradiated by the central X-ray radiation with the Compton temperature of 5--15$\times10^7$ K. The radiative heating/cooling and the bulge stellar potential in galaxies are taken into account. We analyze the effect of accretion rate, luminosity, gas temperature, and Compton temperature on steady solutions of dynamical and thermal properties. The steady solutions are obviously different from Bondi solution. Compared to our models, the Bondi model underestimates the accretion rate. We give the boundary between thermal stability and instability. The boundary is significantly affected by Compton temperature. When Compton temperature is higher, the falling gases tend to become thermally unstable. When thermal instability takes place in the irradiated gases, the gases become two phases (i.e. hot gases and cool gases) and the hot gases may become outflows. This effect may reduce the accretion rates.

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arXiv2021

Magnetohydrodynamic Winds Driven by the Line Force from the Standard Thin Disk around Supermassive Black Holes. I. The Case of Weak Magnetic Field

Absorption lines with high blue-shifted velocities are frequently found in the ultraviolet (UV) and X-ray spectra of luminous active galactic nuclei (AGNs). This implies that high-velocity winds/outflows are common in AGNs. In order to study the formation of high-velocity winds, especially ultrafast outflows (UFOs), we perform two-dimensional magnetohydrodynamic (MHD) simulations. Initially, a magnetic field is set to be weaker than the gas pressure at the disk surface. In our simulations, line force operates on the region like filaments because the X-ray radiation from corona is shielded by dense gas in the inner region at some angle. The location of filaments changes with time and then the line-driven winds are exposed to X-ray and become highly ionized. The line force at the UV bands does not directly drive the highly ionized winds. In the sense of time average, the properties of high-velocity winds meet the formation condition of UFOs. Compared with line force, the function of magnetic field is negligible in directly driving winds. In the MHD model, the region around the rotational axis becomes magnetic-pressure dominated, which prevents gases from spreading to higher latitudes and then enhances the gas column density at middle and low latitudes (20$^{\rm o}$--70$^{\rm o}$). Higher column density is helpful to shield X-ray photons, which causes the line force to be more effective in the MHD model than in the hydrodynamic model. Higher-velocity winds with a broader opening angle are produced in the MHD model.

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arXiv2020

Gas flows in an Active galactic nuclei. I. Two-phase gas inflow

We perform two-dimensional hydrodynamic simulations to study the non-rotating gas flow in the region from 1-1000 parsec. This region connects the galaxy scale and the accretion disk scale. The gas is irradiated by the photons generated by the accretion disk system near the central black hole with $10^8$ solar mass. We assume that the luminosity of the central accretion system is constant and equals to $0.3$ Eddington luminosity. Gas with density higher than $10^{-24} {\rm g \ cm^{-3}}$ is found to be thermally unstable. Two phase, cold and hot, gas flow will form due to thermal instability. We calculated the ratio of cold gas accretion rate to hot gas accretion rate. This ratio is a function of gas density and generally increases with the increase of gas density. The maximum value of this ratio is $16$, when gas density is $10^{-21} {\rm g \ cm^{-3}} $. Gas with density lower than $10^{-24} {\rm g \ cm^{-3}}$ is found to be thermally stable and the gas flow is one-phase. The applications of the results are briefly discussed.

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arXiv2020

Two-temperature radiative hot accretion flow around neutron stars

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.

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