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Register a new userNumerical simulation of hot accretion flows (IV): effects of black hole spin and magnetic field strength on the wind and the comparison between wind and jet properties
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Hai Yang
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This is the fourth paper of our series of works studying winds from hot accretion flows around black holes. In the first two papers, we have shown the existence of strong winds in hot accretion flows using hydrodynamical and magnetohydrodynamical (MHD) simulations. In the third paper, by using three dimensional general relativity MHD numerical simulation data of hot accretion flows and adopting a virtual particle trajectory data analysis approach, we have calculated the properties of wind, such as its mass flux and velocity. However, that paper focuses only on a non-spinning black hole and SANE (standard and normal accretion). In the present paper, we extend the third paper by including cases of a rapidly rotating black hole and MAD (magnetically arrested disk). We focus on investigating the effect of spin and magnetic field on the properties of wind and jet. It is found that a larger spin and stronger magnetic field usually enhance the wind and jet. The formulae describing the mass flux, poloidal velocity, and fluxes of momentum, kinetic energy, and total energy of wind and jet are presented. One interesting finding, among others, is that even in the case of very rapidly spinning black hole where the jet is supposed to be the strongest, the momentum flux of jet is smaller than that of wind, while the total energy flux of jet is larger than that of wind by at most a factor of 10. This result suggests that wind potentially plays a more important role than jet at least for some problems in active galactic nuclei feedback.
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The magnitude of the viscosity and magnetic field parameters in hot accretion flows is investigated in low luminosity active galactic nuclei (LLAGNs). Theoretical studies show that a geometrically thin, optically thick disk is truncated at mass accretion rates less than a critical value by mass evaporated vertically from the disk to the corona, with the truncated region replaced by an advection dominated accretion flow (ADAF). The critical accretion rate for such a truncation is a function of the viscosity and magnetic field. Observations of X-ray photon indices and spectral fits of a number of LLAGNs published in the literature provide an estimate of the critical rate of mass accretion and the truncation radius respectively. By comparing the observational results with theoretical predictions, the viscosity and magnetic field parameters in the hot accretion flow region are estimated. Specifically, the mass accretion rates inferred in different sources constrain the viscosity parameter, whereas the truncation radii of the disk, as inferred from spectral fits, further constrain the magnetic field parameter. It is found that the value of the viscosity parameter in the corona/ADAF ranges from 0.17 to 0.5, with values clustered about 0.2-0.3. Magnetic pressure is required by the relatively small truncation radii for some LLAGNs and is found to be as high as its equipartition value with the gas pressure. The inferred values of the viscosity parameter are in agreement with those obtained from the observations of non-stationary accretion in stellar mass black hole X-ray transients. This consistency provides support for the paradigm that a geometrically thin disk is truncated by means of a mass evaporation process from the disk to the corona at low mass accretion rates.
We solved the set of two-dimensional magnetohydrodynamic (MHD) equations for optically thin black hole accretion flows incorporating toroidal component of magnetic field. Following global and local MHD simulations of black hole accretion disks, the magnetic field inside the disk is decomposed into a large scale field and a fluctuating field. The effects of the fluctuating magnetic field in transferring the angular momentum and dissipating the energy are described through the usual $ alpha $ description. We solved the MHD equations by assuming steady state and radially self-similar approximation in $ r-theta $ plane of spherical coordinate system. We found that as the amount of magnetic field at the equatorial plane increases, the heating by the viscosity decreases. In addition, the maximum amount of the heating by the viscous dissipation is produced at the mid-plane of the disk, while that of the heating by the magnetic field dissipation is produced at the surface of the disk. Our main conclusion is that in terms of the no-outflow solution, thermal equilibrium still exists for the strong magnetic filed at the equatorial plane of the disk.
We analyze two 3D general-relativistic magnetohydrodynamic accretion simulations in the context of how they would manifest in Event Horizon Telescope (EHT) observations of supermassive black holes. The two simulations differ only in whether the initial angular momentum of the plasma is aligned with the rapid (a = 0.9) spin of the black hole. Both have low net magnetic flux. Ray tracing is employed to generate resolved images of the synchrotron emission. When using parameters appropriate for Sgr A* and assuming a viewing angle aligned with the black hole spin, we find the most prominent difference is that the central shadow in the image is noticeably eccentric in tilted models, with the ring of emission relatively unchanged. Applying this procedure to M87 with a viewing angle based on the large-scale jet, we find that adding tilt increases the angular size of the ring for fixed black hole mass and distance, while at the same time increasing the number of bright spots in the image. Our findings illustrate observable features that can distinguish tilted from aligned flows. They also show that tilted models can be viable for M87, and that not accounting for tilt can bias inferences of physical parameters. Future modeling of horizon-scale observations should account for potential angular momentum misalignment, which is likely generic at the low accretion rates appropriate for EHT targets.
We report on a 120 ks Chandra/HETG spectrum of the black hole GRS 1915+105. The observation was made during an extended and bright soft state in June, 2015. An extremely rich disk wind absorption spectrum is detected, similar to that observed at lower sensitivity in 2007. The very high resolution of the third-order spectrum reveals four components to the disk wind in the Fe K band alone; the fastest has a blue-shift of v = 0.03c. Broadened re-emission from the wind is also detected in the first-order spectrum, giving rise to clear accretion disk P Cygni profiles. Dynamical modeling of the re-emission spectrum gives wind launching radii of r ~ 10^(2-4) GM/c^2. Wind density values of n ~ 10^(13-16) cm^-3 are then required by the ionization parameter formalism. The small launching radii, high density values, and inferred high mass outflow rates signal a role for magnetic driving. With simple, reasonable assumptions, the wind properties constrain the magnitude of the emergent magnetic field to B ~ 10^(3-4) Gauss if the wind is driven via magnetohydrodynamic (MHD) pressure from within the disk, and B ~ 10^(4-5) Gauss if the wind is driven by magnetocentrifugal acceleration. The MHD estimates are below upper limits predicted by the canonical alpha-disk model (Shakura & Sunyaev 1973). We discuss these results in terms of fundamental disk physics and black hole accretion modes.
We have studied the relationship between the solar-wind speed $[V]$ and the coronal magnetic-field properties (a flux expansion factor [$f$] and photospheric magnetic-field strength [$B_{mathrm{S}}$]) at all latitudes using data of interplanetary scintillation and solar magnetic field obtained for 24 years from 1986 to 2009. Using a cross-correlation analyses, we verified that $V$ is inversely proportional to $f$ and found that $V$ tends to increase with $B_{mathrm{S}}$ if $f$ is the same. As a consequence, we find that $V$ has extremely good linear correlation with $B_{mathrm{S}}/f$. However, this linear relation of $V$ and $B_{mathrm{S}}/f$ cannot be used for predicting the solar-wind velocity without information on the solar-wind mass flux. We discuss why the inverse relation between $V$ and $f$ has been successfully used for solar-wind velocity prediction, even though it does not explicitly include the mass flux and magnetic-field strength, which are important physical parameters for solar-wind acceleration.
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