There is a broad consensus that accretion onto supermassive black holes and consequent jet formation power the observed emission from active galactic nuclei (AGNs). However, there has been less ...agreement about how jets form in accretion flows, their possible relationship to black hole spin, and how they interact with the surrounding medium. There have also been theoretical concerns about instabilities in standard accretion disk models and lingering discrepancies with observational constraints. Despite seemingly successful applications to X-ray binaries, the standard accretion disk model faces a growing list of observational constraints that challenge its application to AGNs. Theoretical exploration of these questions has become increasingly reliant on numerical simulations owing to the dynamic nature of these flows and the complex interplay between hydrodynamics, magnetic fields, radiation transfer, and curved spacetime. We conclude the following:
The advent of general relativistic magnetohydrodynamics (MHD) simulations has greatly improved our understanding of jet production and its dependence on black hole spin.
Simulation results show both disks and jets are sensitive to the magnetic flux threading the accretion flow as well as possible misalignment between the angular momentum of the accretion flow and the black hole spin.
Radiation MHD simulations are providing new insights into the stability of luminous accretion flows and highlighting the potential importance of radiation viscosity, UV opacity from atoms, and spiral density waves in AGNs.
By fitting synthetic spectral models computed via the TLUSTY code, we examine how the spectra from thin accretion disks are expected to vary in accreting black hole systems. We fit color-corrected ...blackbody models to our synthetic spectra to estimate the spectral hardening factor f, which parameterizes the departure from blackbody and is commonly used to help interpret multitemperature blackbody fitting results. We find we can define a reasonably robust f value to spectra when the effects of Compton scattering dominate radiation transfer. We examine the evolution of f with black hole mass and accretion rate, typically finding a moderate variation (f ∼ 1.4-2) for accretion rates between 1% and 100% of the Eddington rate. Consistent with most previous work, we find that f tends to increase with accretion rate, but we also infer a weaker correlation of f with black hole mass. We find that f is rarely much larger than 2 unless the disk becomes photon starved, in contention with some previous calculations. Significant spectral hardening (f > 2) is only found when the disk mass surface density is lower than expected for -disk models unless is near unity or larger.
The radiative efficiency of active galactic nucleus (AGN) is commonly estimated based on the total mass accreted and the total AGN light emitted per unit volume in the universe integrated over time ...(the Soltan argument). In individual AGNs, thin accretion disk model spectral fits can be used to deduce the absolute accretion rate , if the black hole mass M is known. The radiative efficiency Delta *h is then set by the ratio of the bolometric luminosity L bol to . We apply this method to determine Delta *h in a sample of 80 Palomar-Green quasars with well-determined L bol, where is set by thin accretion disk model fits to the optical luminosity density, and the M determination based on the bulge stellar velocity dispersion (13 objects) or the broad line region. We derive a mean log Delta *h = --1.05 ? 0.52 consistent with the Soltan-argument-based estimates. We find a strong correlation of Delta *h with M, rising from Delta *h ~ 0.03 at M = 107 M and L/L Edd ~ 1 to Delta *h ~ 0.4 at M = 109 M and L/L Edd ~ 0.3. This trend is related to the overall uniformity of L opt/L bol in our sample, particularly the lack of the expected increase in L opt/L bol with increasing M (and decreasing L/L Edd), which is a generic property of thermal disk emission at fixed Delta *h. The significant uncertainty in the M determination is not large enough to remove the correlation. The rising Delta *h with M may imply a rise in the black hole spin with M, as proposed based on other indirect arguments.
We use global three-dimensional radiation magnetohydrodynamical simulations to study accretion disks onto a black hole with accretion rates varying from to . We initialize the disks with a weakly ...magnetized torus centered at either 50 or 80 gravitational radii, leading to self-consistent turbulence generated by the magnetorotational instability (MRI). The inner regions of all disks have radiation pressure ∼104-106 times the gas pressure. Nonaxisymmetric density waves that steepen into spiral shocks form as gas flows toward the black hole. Maxwell stress from MRI turbulence can be larger than the Reynolds stress only when the net vertical magnetic flux is sufficiently large. Outflows are formed with a speed of ∼0.1-0.4c. When the accretion rate is smaller than , outflows are launched from ∼10 gravitational radii, and the radiative efficiency is ∼5%-7%. For an accretion rate reaching , most of the funnel region near the rotation axis becomes optically thick, and the outflow is launched from beyond 50 gravitational radii. The radiative efficiency is reduced to 1%. We always find that the kinetic energy luminosity associated with the outflow is at most ∼15%-30% of the radiative luminosity. The mass flux in the outflow is ∼15%-50% of the net mass accretion rates. We discuss the implications of our simulation results on the observational properties of these disks.
AGN SEDs generally show a turnover at λ ∼ 1000 Å, implying a maximal accretion disc (AD) temperature of T
max ∼ 50 000 K. Massive O stars display a similar T
max, associated with a sharp rise in a ...line-driven mass-loss
with increasing surface temperature. AGN AD are also characterized by similar surface gravity to massive O stars. The
of O stars reaches ∼10−5 M yr−1. Since the surface area of AGN AD can be 106 larger, the implied
in AGN AD can reach the accretion rate
. A rise to
towards the AD centre may therefore set a similar cap of T
max ∼ 50 000 K. To explore this idea, we solve the radial structure of an AD with a mass-loss term, and calculate the implied AD emission using the mass-loss term derived from observations of O stars. We find that
becomes comparable to
typically at a few tens of GM/c
2. Thus, the standard thin AD solution is effectively truncated well outside the innermost stable orbit. The calculated AD SED shows the observed turnover at λ ∼ 1000 Å, which is weakly dependent on the AGN luminosity and black hole mass. The AD SED is generally independent of the black hole spin, due to the large truncation radius. However, a cold AD (low
, high black hole mass) is predicted to be windless, and thus its SED should be sensitive to the black hole spin. The accreted gas may form a hot thick disc with a low radiative efficiency inside the truncation radius, or a strong line-driven outflow, depending on its ionization state.
We study dusty winds driven by radiation pressure in the atmosphere of a rapidly star-forming environment. We apply the variable Eddington tensor algorithm to re-examine the two-dimensional radiation ...hydrodynamic problem of a column of gas that is accelerated by a constant infrared radiation flux. In the absence of gravity, the system is primarily characterized by the initial optical depth of the gas. We perform several runs with different initial optical depths and resolutions. We find that the gas spreads out along the vertical direction, as its mean velocity and velocity dispersion increase. In contrast to previous work using the flux-limited diffusion algorithm, we find little evolution in the trapping factor. The momentum coupling between radiation and gas in the absence of gravity is similar to that with gravity. For Eddington ratio increasing with the height in the system, the momentum transfer from the radiation to the gas is not merely , but amplified by a factor of , where is the integrated infrared optical depth through the system, and , decreasing with the optical depth. We apply our results to the atmosphere of galaxies and conclude that radiation pressure may be an important mechanism for driving winds in the most rapidly star-forming galaxies and starbursts.
The launching of cosmic ray-driven outflows Huang, Xiaoshan; Davis, Shane W
Monthly Notices of the Royal Astronomical Society,
03/2022, Letnik:
511, Številka:
4
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
Cosmic rays (CRs) are thought to be an important feedback mechanism in star-forming galaxies. They can provide an important source of pressure support and possibly drive outflows. We perform ...multidimensional CR magnetohydrodynamic simulations including transport by streaming and diffusion to investigate wind launching from an initially hydrostatic atmosphere by CRs. We estimate a characteristic Eddington limit on the CR flux for which the CR force exceeds gravity and compare it to simulated systems. Scaling our results to conditions in star-forming galaxies, we find that CRs are likely to contribute to driving outflows for a broad range of star formation environments. We quantify the momentum and energy transfer between CRs and gas, along with the associated mass outflow rates under different assumptions about the relative importance of streaming and diffusion for transport. In simulations with streaming, we observe the growth and saturation of the CR acoustic instability, but the CRs and gas remain well coupled, with CR momentum transferred efficiently to the gas even when this instability is present. Higher CR fluxes transfer more energy to the gas and drive stronger outflows. When streaming is present, most of the transferred energy takes the form of Alfvén wave heating of the gas, raising its pressure and internal energy, with a lower fractional contribution to the kinetic energy of the outflow. We also consider runs with radiative cooling, which modifies gas temperature and pressure profiles but does not seem to have a large impact on the mass outflow for super-Eddington CR fluxes.
We study super-Eddington accretion flows onto black holes using a global three-dimensional radiation magneto-hydrodynamic al simulation. We solve the time-dependent radiative transfer equation for ...the specific intensities to accurately calculate the angular distribution of the emitted radiation. Turbulence generated by the magneto-rotational instability provides self-consistent angular momentum transfer. The simulation reaches inflow equilibrium with an accretion rate ~220 L sub(Edd)/c super(2) and forms a radiation-driven outflow along the rotation axis. The mechanical energy flux carried by the outflow is ~20% of the radiative energy flux. The total mass flux lost in the outflow is about 29% of the net accretion rate. The radiative luminosity of this flow is ~10 L sub(Edd). This yields a radiative efficiency ~4.5%, which is comparable to the value in a standard thin disk model. In our simulation, vertical advection of radiation caused by magnetic buoyancy transports energy faster than photon diffusion, allowing a significant fraction of the photons to escape from the surface of the disk before being advected into the black hole. We contrast our results with the lower radiative efficiencies inferred in most models, such as the slim disk model, which neglect vertical advection. Our inferred radiative efficiencies also exceed published results from previous global numerical simulations, which did not attribute a significant role to vertical advection. We briefly discuss the implications for the growth of supermassive black holes in the early universe and describe how these results provided a basis for explaining the spectrum and population statistics of ultraluminous X-ray sources.
ABSTRACT We study active galactic nucleus (AGN) line-driven disc winds using time-dependent radiation hydrodynamics. The key criterion for determining wind launching is the coupling strength of the ...ultraviolet radiation field via the spectral lines of the gas. The strength of these lines in turn relies crucially on the gas ionization state, determined by the local X-ray intensity. We consider a suite of models where the central ionizing radiation is affected by scattering, absorption, and re-emission by the intervening gas. In a pure attenuation model, the disc launches an episodic wind, as previous studies have shown. Including scattering or re-emission tends to weaken the wind, lowering the mass flux and outflow velocity and, if sufficiently dominant, suppressing the outflow entirely. However, the exponential nature of radiative attenuation means that only a modest, factor of a few, increase in the absorption cross-section can overcome the wind suppression due to scattering and re-emission. We find mass outflow rates of ∼20 per cent or more of the assumed inflow rate through the disc, indicating that radiation-driven winds may significantly alter the structure of the accretion flow. The winds also supply a large, time-varying column of material above the nominal constant disc scale height, which will determine the geometry of reprocessed emission from the central source. Our results suggest the need for accurate photoionization modelling, radiation transport, and accretion disc physics, to study their effects on the AGN disc winds.
We conduct global three-dimensional radiation magnetohydrodynamic simulations of the inner regions of accretion flows around a 5 × 108M black hole, with mass accretion rates reaching 7% and 20% of ...the Eddington value. We choose initial field topologies that result in an inner disk supported by magnetic pressure, with surface density significantly smaller than the values predicted by the standard thin-disk model as well as a much larger disk scale height. The disks do not show any sign of thermal instability over many thermal timescales. More than half of the accretion is driven by radiation viscosity in the optically thin coronal region for the case of the lower accretion rate, while accretion in the optically thick part of the disk is driven by the Maxwell and Reynolds stresses from turbulence caused by magnetorotational instability. Optically thin plasma with gas temperatures 108 K is generated only in the inner 10 gravitational radii in both simulations, and is more compact in the case of the higher accretion rate. Such plasma does not form at larger radii because the surface density increases outward with radius, causing less dissipation outside the photosphere. In contrast to standard thin-disk models, the surface density in our simulations increases with increasing mass accretion rate at each radius. This causes a relatively weaker hot plasma component for the simulation with a higher accretion rate. We suggest that these results may provide a physical mechanism for understanding some of the observed properties of coronae and spectra of active galactic nuclei.
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