Yucong Zhu

The Cool Accretion Disk in ESO 243-49 HLX-1: Further Evidence of an Intermediate-Mass Black Hole

With an inferred bolometric luminosity exceeding 10 42 erg s −1 , HLX-1 in ESO 243-49 is the most luminous of ultraluminous X-ray sources and provides one of the strongest cases for the existence of intermediate-mass black holes. We obtain good fits to disk-dominated observations of the source with BHSPEC, a fully relativistic black hole accretion disk spectral model. Due to degeneracies in the model arising from the lack of independent constraints on inclination and black hole spin, there is a factor of 100 uncertainty in the best-fit black hole mass M. Nevertheless, spectral fitting of XMM-Newton observations provides robust lower and upper limits with ∗

Global simulations of axisymmetric radiative black hole accretion discs in general relativity with a mean-field magnetic dynamo

We present a sub-grid model that emulates the magnetic dynamo operating in magnetized accretion disks. We have implemented this model in the general relativisic radiation magnetohydrodynamic (GRRMHD) code \koral, using results from local shearing sheet simulations of the magnetorotational instability to fix the parameters of the dynamo. With the inclusion of this dynamo, we are able to run 2D axisymmetric GRRMHD simulations of accretion disks for arbitrarily long times. The simulated disks exhibit sustained turbulence, with the poloidal and toroidal magnetic field components driven towards a state similar to that seen in 3D studies. Using this dynamo code, we present a set of long-duration global simulations of super-Eddington, optically-thick disks around non-spinning and spinning black holes. Super-Eddington disks around non-rotating black holes exhibit a surprisingly large efficiency,

Semi-implicit scheme for treating radiation under M1 closure in general relativistic conservative fluid dynamics codes

\eta\approx0.04

Measuring black hole spin by the continuum-fitting method: effect of deviations from the Novikov–Thorne disc model

, independent of the accretion rate, where we measure efficiency in terms of the total energy output, both radiation and mechanical, flowing out to infinity. Super-Eddington disks around spinning black holes are even more efficient, and appear to extract black hole rotational energy through a process similar to the Blandford-Znajek mechanism. All the simulated models are characterized by highly super-Eddington radiative fluxes collimated along the rotation axis. We also present a set of simulations that were designed to have Eddington or slightly sub-Eddington accretion rates (

Energy, momentum and mass outflows and feedback from thick accretion discs around rotating black holes

\dot{M} \lesssim 2\dot M_{\rm Edd}

The eye of the storm: light from the inner plunging region of black hole accretion discs

). None of these models reached a steady state. Instead, the disks collapsed as a result of runaway cooling, presumably because of a thermal instability.

An investigation of Sloan Digital Sky Survey imaging data and multiband scaling relations of spiral galaxies

A numerical scheme is described for including radiation in multi-dimensional generalrelativistic conservative fluid dynamics codes. In this met hod, a covariant form of the M1 closure scheme is used to close the radiation moments, and the radiative source terms are treated semi-implicitly in order to handle both optically t hin and optically thick regimes. The scheme has been implemented in a conservative general relativistic radiation hydrodynamics codeKORAL. The robustness of the code is demonstrated on a number of test problems, including radiative relativistic shock tubes, static radiation p ressure supported atmosphere, shadows, beams of light in curved spacetime, and radiative Bondi accretion. The advantages of M1 closure relative to other approaches such as Eddington closure and flux-limited di ffusion are discussed, and its limitations are also highlighted.

heroic: 3D general relativistic radiative post-processor with comptonization for black hole accretion discs

The X-ray spectra of accretion discs of eight stellar mass black holes have been analysed to date using the thermal continuum-fitting method, and the spectral fits have been used to estimate the spin parameters of the black holes. However, the underlying model used in this method of estimating spin is the general relativistic thin-disc model of Novikov & Thorne, which is only valid for razor-thin discs. We therefore expect errors in the measured values of spin due to inadequacies in the theoretical model. We investigate this issue by computing spectra of numerically calculated models of thin accretion discs around black holes, obtained via three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations. We apply the continuum-fitting method to these computed spectra to estimate the black hole spins and check how closely the values match the actual spin used in the GRMHD simulations. We find that the error in the dimensionless spin parameter is up to about 0.2 for a non-spinning black hole, depending on the inclination. For black holes with spins of 0.7, 0.9 and 0.98, the errors are up to about 0.1, 0.03 and 0.01, respectively. These errors are comparable to or smaller than those arising from current levels of observational uncertainty. Furthermore, we estimate that the GRMHD simulated discs from which these error estimates are obtained correspond to effective disc luminosities of about 0.4–0.7 Eddington, and that the errors will be smaller for discs with luminosities of 0.3 Eddington or less, which are used in the continuum-fitting method. We thus conclude that use of the Novikov–Thorne thin-disc model does not presently limit the accuracy of the continuum-fitting method of measuring black hole spin.

A HIGH-FREQUENCY DOPPLER FEATURE IN THE POWER SPECTRA OF SIMULATED GRMHD BLACK HOLE ACCRETION DISKS

Using long-duration general relativistic magnetohydrodynamic simulations of radiatively inefficient accretion discs, the energy, momentum and mass outflow rates from such systems are estimated. Outflows occur via two fairly distinct modes: a relativistic jet and a sub-relativistic wind. The jet power depends strongly on the black hole spin and on the magnetic flux at the horizon. Unless these are very small, the energy output in the jet dominates over that in the wind. For a rapidly spinning black hole accreting in the magnetically arrested limit, it is confirmed that jet power exceeds the total rate of accretion of rest mass energy. However, because of strong collimation, the jet probably does not have a significant feedback effect on its immediate surroundings. The power in the wind is more modest and shows a weaker dependence on black hole spin and magnetic flux. Nevertheless, because the wind subtends a large solid angle, it is expected to provide efficient feedback on a wide range of scales inside the host galaxy. Empirical formulae are obtained for the energy and momentum outflow rates in the jet and the wind.

Thermal stability in turbulent accretion discs

It is generally thought that the light coming from the inner plunging region of black hole accretion discs contributes negligibly to the disc’s overall spectrum, i.e. the plunging fluid is swallowed by the black hole before it has time to radiate. In the standard disc model used to fit X-ray observations of accretion discs, the plunging region is assumed to be perfectly dark. However, numerical simulations that include the full physics of the magnetized flow predict that a small fraction of the disc’s total luminosity emanates from this plunging region. In this work, we investigate the observational consequences of this neglected inner light. We compute radiative transfer based disc spectra that correspond to 3D general relativistic magnetohydrodynamic simulated discs (which produce light inside their plunging regions). In the context of black hole spin estimation, we find that this neglected inner light only has a modest effect (this bias is less than typical observational systematic errors). For rapidly spinning black holes, we find that the combined emission from the plunging region produces a weak power law tail at high energies. This indicates that infalling matter is the origin for some of the ‘coronal’ emission observed in the thermal dominant and steep power law states of X-ray binaries.