arXiv subjects

Xiao-Hong Yang

Publications and source records attributed to Xiao-Hong Yang.

29 records · Page 2Linked to original sources

arXiv2019

Magnetohydrodynamic numerical simulation of the outflows driven by magnetic field and radiation force from the corona above a thin disk

A hot corona is suggested to be above the standard thin disk. The anisotropy of hard X-ray emission in radio-quiet active galactic nuclei implies that the corona is not static and probably moves outwards like winds. We perform two-dimensional magnetohydrodynamical simulations to study the outflowing corona driven by magnetic field and radiation force. In our simulations, as the initial state and the boundary condition at the disk surface, the corona temperature is set to $10^9$ K inside 10 Schwarzschild radius ($r_{\rm s}$) while the corona temperature is set to $10^7$ K a 10 $r_{\rm s}$. We employ a weak poloidal magnetic field as the initial magnetic field. A collimated outflow and a wide-angle ordered outflow are observed in our simulations. The collimated outflow is around the rotational axis and has a bulk velocity of $\sim$0.03--0.3 $c$ ( $c$ is speed of light) at 90 $r_{\rm s}$, while their mass outflow rate is very low. The collimated outflow is a weak jet. The wide-angle ordered outflow is distributed at the middle and high latitudes and moves outwards with a velocity of $10^2$--$10^4$ km s$^{-1}$. The outflow velocity depends on the disk luminosity. The gas around the disk surface is turbulent, especially outside of 10 $r_{\rm s}$. The other properties of outflows are discussed in detail.

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arXiv2019

Active galactic nuclei feedback at parsec scale

We perform simulations to study the effects of active galactic nuclei (AGNs) radiation and wind feedback on the properties of slowly rotating accretion flow at parsec scale. We find that when only radiative feedback is considered, outflows can be produced by the radiation pressure due to Thomson scattering. The mass flux of outflow is comparable to that of inflow. Although strong outflow is present, the luminosity of AGN can be easily super-Eddington. When wind feedback is also taken into account, the mass flux of outflow does not change much. Consequently, the luminosity of the central AGN can still be super-Eddington. However, observations show that the luminosity of most AGNs is sub-Eddington. Some other mechanisms are needed to reduce the AGNs luminosity. Although the mass outflow rate does not change much by wind feedback, other properties of outflow (density, temperature, velocity and kinetic power) can be significantly changed by wind feedback. In the presence of wind feedback, the density of outflow becomes significantly lower, temperature of outflow becomes significantly higher, velocity of outflow is increased by one order of magnitude, the kinetic power of outflow is increased by a factor of $40-100$.

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arXiv2019

Hot accretion flow around neutron stars

We perform as the first time hydrodynamic simulations to study the properties of hot accretion flow (HAF) around a neutron star (NS). The energy carried by the HAF will eventually be radiated out at the surface of the NS. The emitted photons can propagate inside the HAF and cool the HAF via Comptonization. We find that the Compton cooling can affect the properties of HAF around a NS significantly. We define the Eddington accretion rate as $\dot M_{\rm Edd}=10L_{\rm Edd}/c^2$, with $L_{\rm Edd}$ and $c$ being the Eddington luminosity and the speed of light, respectively. We define $\dot m$ as the mass accretion rate at the NS surface in unit of $\dot M_{\rm Edd}$. When $\dot m > 10^{-4}$, Compton cooling can effectively cool the HAF and suppress wind. Therefore, the mass accretion rate is almost a constant with radius. The density profile is $ρ\propto r^{-1.4}$. When $\dot m < 10^{-4}$, the Compton cooling effects become weaker, wind becomes stronger, accretion rate is proportional to $r^{0.3-0.5}$. Consequently, the density profile becomes flatter, $ρ\propto r^{-1 \sim -0.8}$. When $\dot m < 10^{-6}$, the Compton cooling effects can be neglected. We find that with a same accretion rate, the temperature of HAF around a NS is significantly lower than that of HAF around a black hole (BH). Also, the Compton $y-$parameter of HAF around a NS is significantly smaller than that of HAF around a BH. This result predicts that HAF around a NS will produce a softer spectrum compared to HAF around a BH, which is consistent with observations.

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arXiv2019

Quenching black hole accretion by active galactic nuclei feedback

Observations of many dim galactic nuclei in local universe give good estimation of gas density and temperature at Bondi radius. If we assume the black hole accretes at Bondi accretion rate and radiates at efficiency of low-luminosity hot accretion flow, the predicted nuclei luminosity can be significantly higher than observations. Therefore, the real black hole mass accretion rate in these sources may be significantly smaller than the Bondi value. Active galactic nuclei (AGN) feedback may be responsible for decreasing black hole accretion rate to values much smaller than Bondi rate. We perform two-dimensional simulations of low angular momentum accretion flow at parsec and sub-parsec scales around low-luminosity active galactic nuclei (LLAGNs). We take into account the radiation and wind feedbacks of the LLAGN. The cross section of particle-particle interaction can be several orders of magnitude larger than that of photon-particle interaction. Therefore, we find that for the LLAGNs, effects of radiation feedback in decreasing black hole accretion rate are small. However, wind feedback can effectively decrease the black hole mass accretion rate. Due to the decrease of accretion rate, the black hole luminosity can be decreased by a factor of $\sim 33-400$. The results may be useful to explain why many galactic nuclei in the local universe are so dim.

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arXiv2019

What is the real accretion rate onto a black hole for low angular momentum accretion?

Mass accretion rate is a key parameter in accretion disk theory. It determines black hole accretion mode. In large scale cosmological simulations studying galaxy formation and evolution, Bondi radius can at most be marginally resolved. In those simulations, Bondi accretion formula is always used to estimate black hole accretion rate. Bondi solution is too simple, which cannot represent the real accretion process. We perform simulations of hot accretion flow at parsec scale irradiated by a low luminosity active galactic nucleus (AGN). We perform 77 simulations with varying density and temperature at outer boundary (10 parsec). Our purpose is to find a formula to calculate real black hole accretion rate based on the gas density and temperature at parsec scale. We define Eddington accretion rate to be $\dot M_{\rm Edd}=10L_{\rm Edd}/c^2$, with $L_{\rm Edd}$ and $c$ been Eddington luminosity and speed of light respectively. We set black hole mass to be $10^8M_{\odot}$, $M_{\odot}$ is solar mass. We find that black hole accretion rate can be expressed as $\frac{\dot M_{\rm BH} }{\dot M_{\rm Edd}}=10^{-3.11} (ρ_0/10^{-22}{\rm g cm}^{-3})^{1.36} (T_0/10^7 {\rm K})^{-1.9}$, with $ρ_0$ and $T_0$ being density and temperature at parsec scale, respectively. We find the formula can accurately predict the luminosity of observed low-luminosity AGNs (with black hole mass $\sim 10^8M_{\odot}$). This formula can be used in the sub-grid models in large scale cosmological simulations with a black hole mass of $\sim 10^8M_{\odot}$.

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arXiv2018

Numerical simulations of winds driven by radiation force from the corona above a thin disk

Observations show that winds can be driven from the innermost region (inside a 50 Schwarschild radius) of a thin disk. It is interesting to study the winds launched from the innermost region. A hot corona above the black hole (BH) thin disk is irradiated by the disk. We perform two-dimensional hydrodynamical simulations to study the winds driven by radiation force from the corona in the innermost regions. The hard X-ray spectrum from active galactic nuclei (AGNs) suggests that the corona temperature is about $10^9$ K, so that we mainly analyze the properties of winds (or outflows) from the $10^9$ K corona. The disk luminosity plays an important role in driving the outflows. The more luminous the disk, the stronger the outflows. Mass outflow rate ($\dot{M}_{\rm out}$) at a 90 Schwarschild radius depends on disk luminosity, which can be described as $\dot{M}_{\rm out}\propto 10^{3.3 Γ}$ ($Γ$ is the ratio of the disk luminosity to the Eddington luminosity). In the case of high luminosity (e.g. $Γ=0.75$), the supersonic outflows with maximum speed $1.0 \times 10^4$ Km s$^{-1}$ are launched at $\sim17^{o}$ --$30^{o}$ and $\sim50^{o}$ --$80^{o}$ away from the pole axis. The Bernoulli parameter keeps increasing with the outward propagation of outflows. The radiation force keeps accelerating the outflows when outflows move outward. Therefore, we can expect the outflows to escape from the BH gravity and go to the galactic scale. The interaction between outflows and interstellar medium may be an important AGN feedback process.

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arXiv2018

Effect of nuclear stars gravity on quasar radiation feedback on the parsec-scale

It is often suggested that a super massive black hole is embedded in a nuclear bulge of size of a few $10^2$ $\text{ parsec}$ . The nuclear stars gravity is not negligible near $\sim 10 \text{ parsec}$. In order to study the effect of nuclear stars gravity on quasar radiation feedback on the parsec scale, we have simulated the parsec scale flows irradiated by a quasar by taking into account the gravitational potential of both the black hole and the nuclear star cluster. We find that the effect of nuclear stars gravity on the parsec-scale flows is related to the fraction of X-ray photons in quasar radiation. For the models in which the fraction of X-ray photons is not small (e.g. the X-ray photons contribute to $20\%$ of the quasar radiation), the nuclear stars gravity is very helpful to collimate the outflows driven by UV photons, significantly weakens the outflow power at the outer boundary and significantly enhances the net accretion rate onto the black hole. For the models in which X-ray photons are significantly decreased (e.g. the X-ray photons contribute to $5\%$ of the quasar radiation), the nuclear stars gravity can just slightly change properties of outflow and slightly enhance the net accretion rate onto the black hole.

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arXiv2018

The Effect of Accretion Environment at Large Radius on Hot Accretion Flows

We study the effects of accretion environment (gas density, temperature and angular momentum) at large radii ($\sim 10$pc) on luminosity of hot accretion flows. The radiative feedback effects from the accretion flow on the accretion environment are also self-consistently taken into account. We find that the slowly rotating flows at large radii can significantly deviate from Bondi accretion when radiation heating and cooling are considered. We further find that when the temperature of environment gas is low (e.g. $T=2\times 10^7$K), the luminosity of hot accretion flows is high. When the temperature of gas is high (e.g. $T\geq4\times 10^7$K), the luminosity of hot accretion flow significantly deceases. The environment gas density can also significantly influence the luminosity of accretion flows. When density is higher than $\sim 4\times 10^{-22}\text{g} \text{cm}^{-3}$ and temperature is lower than $2\times 10^7$K, hot accretion flow with luminosity lower than $2\%L_{\text{Edd}}$ is not present. Therefore, the pc-scale environment density and temperature are two important parameters to determine the luminosity. The results are also useful for the sub-grid models adopted by the cosmological simulations.

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arXiv2018

Thermal wind from hot accretion flows at large radii

We study slowly rotating accretion flow at parsec and sub-parsec scale irradiated by a low luminosity active galactic nuclei. We take into account the Compton heating, photoionization heating by the central X-rays. The bremsstrahlung cooling, recombination and line cooling are also included. We find that due to the Compton heating, wind can be thermally driven. The power of wind is in the range $(10^{-6}-10^{-3}) L_{\rm Edd}$, with $L_{\rm Edd}$ being the Eddington luminosity. The mass flux of wind is in the range $(0.01-1) \dot M_{\rm Edd}$ ($\dot M_{\rm Edd}= L_{\rm Edd}/0.1c^2$ is the Eddington accretion rate, $c$ is speed of light). We define the wind generation efficiency as $ε= P_W/\dot {M}_{\rm BH}c^2$, with $P_W$ being wind power, $\dot M_{\rm BH}$ being the mass accretion rate onto the black hole. $ε$ lies in the rage $10^{-4}-1.18$. Wind production efficiency decreases with increasing mass accretion rate. The possible role of the thermally driven wind in the active galactic feedback is briefly discussed.

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arXiv2016

Magneto-hydrodynamical Numerical simulation of wind production from black hole hot accretion flows at very large radii

Numerical simulations of black hole hot accretion flows have shown the existence of strong wind. Those works focus only on the region close to black hole thus it is unknown whether or where the wind production stops at large radii. To address this question, Bu et al. (2016) have performed hydrodynamic (HD) simulations by taking into account the gravitational potential of both the black hole and the nuclear star clusters. The latter is assumed to be $\propto σ^2 \ln(r)$, with $σ$ being the velocity dispersion of stars and $r$ be the distance from the center of the galaxy. It was found that when the gravity is dominated by nuclear stars, i.e., outside of radius $R_A\equiv GM_{\rm BH}/σ^2$, winds can no longer be produced. That work, however, neglects the magnetic field, which is believed to play a crucial dynamical role in the accretion and thus must be taken into account. In this paper, we revisit this problem by performing magneto-hydrodynamical (MHD) simulations. We confirm the result of Bu et al. (2016), namely wind can't be produced at the region of $R>R_A$. Our result, combined with the results of Yuan et al. (2015), indicates that the formula describing the mass flux of wind $\dot{M}_{\rm wind}=\dot{M}_{\rm BH}(r/20r_s)$ can only be applied to the region where the black hole potential is dominant. Here $\dot{M}_{\rm BH}$ is the mass accretion rate at the black hole horizon and the value of $R_A$ is similar to the Bondi radius.

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arXiv2014

Two dimensional numerical simulations of Supercritical Accretion Flows revisited

We study the dynamics of super-Eddington accretion flows by performing two-dimensional radiation-hydrodynamic simulations. Compared with previous works, in this paper we include the $T_{θϕ}$ component of the viscous stress and consider various values of the viscous parameter $α$. We find that when $T_{θϕ}$ is included, the rotational speed of the high-latitude flow decreases, while the density increases and decreases at the high and low latitudes, respectively. We calculate the radial profiles of inflow and outflow rates. We find that the inflow rate decreases inward, following a power law form of $\dot{M}_{\rm in}\propto r^s$. The value of $s$ depends on the magnitude of $α$ and is within the range of $\sim 0.4-1.0$. Correspondingly, the radial profile of density becomes flatter compared with the case of a constant $\dot{M}(r)$. We find that the density profile can be described by $ρ(r)\propto r^{-p}$, and the value of $p$ is almost same for a wide range of $α$ ranging from $α=0.1$ to $0.005$. The inward decrease of inflow accretion rate is very similar to hot accretion flows, which is attributed to the mass loss in outflows. To study the origin of outflow, we analyze the convective stability of slim disk. We find that depending on the value of $α$, the flow is marginally stable (when $α$ is small) or unstable (when $α$ is large). This is different from the case of hydrodynamical hot accretion flow where radiation is dynamically unimportant and the flow is always convectively unstable. We speculate that the reason for the difference is because radiation can stabilize convection. The origin of outflow is thus likely because of the joint function of convection and radiation, but further investigation is required.

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