Person: Zhu, Yucong
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Zhu
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Yucong
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Zhu, Yucong
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Publication The eye of the storm: light from the inner plunging region of black hole accretion discs(Oxford University Press (OUP), 2012) Zhu, Yucong; Davis, Shane W.; Narayan, Ramesh; Kulkarni, Akshay K.; Penna, Robert; McClintock, JeffreyIt 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 the plunging region. 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 the 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.Publication Measuring Black Hole Spin by the Continuum-Fitting Method: Effect of Deviations from the Novikov-Thorne Disc Model(Oxford University Press (OUP), 2011) Kulkarni, Akshay K.; Penna, Robert; Shcherbakov, Roman V.; Steiner, James; Narayan, Ramesh; Sadowski, Aleksander; Zhu, Yucong; McClintock, Jeffrey; Davis, Shane W.; McKinney, Jonathan C.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.Publication The Cool Accretion Disk in ESO 243-49 HLX-1: Further Evidence of an Intermediate-Mass Black Hole(IOP Publishing, 2011) Davis, Shane W.; Narayan, Ramesh; Zhu, Yucong; Barret, Didier; Farrell, Sean A.; Godet, Olivier; Servillat, Mathieu; Webb, Natalie A.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 \(3000\;M_{☉} \lesssim M \lesssim 3 × 10^{5} M_{☉}\), at 90% confidence, placing HLX-1 firmly in the intermediate-mass regime. The lower bound on M is entirely determined by matching the shape and peak energy of the thermal component in the spectrum. This bound is consistent with (but independent of) arguments based solely on the Eddington limit. Joint spectral modeling of the XMM-Newton data with more luminous Swift and Chandra observations increases the lower bound to \(6000\;M_☉\), but this tighter constraint is not independent of the Eddington limit. The upper bound on M is sensitive to the maximum allowed inclination i, and is reduced to \(M \lesssim 10^{5} M_{☉}\) if we limit \(i \lesssim 75°\).Publication Global simulations of axisymmetric radiative black hole accretion discs in general relativity with a mean-field magnetic dynamo(Oxford University Press (OUP), 2014) Sadowski, Aleksander; Narayan, Ramesh; Tchekhovskoy, Alexander; Abarca, David; Zhu, Yucong; McKinney, Jonathan C.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, η ≈ 0.04, 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 (M˙ . 2M˙ Edd). None of these models reached a steady state. Instead, the disks collapsed as a result of runaway cooling, presumably because of a thermal instability.Publication Heroic: 3D general relativistic radiative post-processor with comptonization for black hole accretion discs(Oxford University Press (OUP), 2016) Narayan, Ramesh; Zhu, Yucong; Psaltis, Dimitrios; Sadowski, AleksanderWe describe Hybrid Evaluator for Radiative Objects Including Comptonization (HEROIC), an upgraded version of the relativistic radiative post-processor code HERO described in a previous paper, but which now Includes Comptonization. HEROIC models Comptonization via the Kompaneets equation, using a quadratic approximation for the source function in a short characteristics radiation solver. It employs a simple form of accelerated lambda iteration to handle regions of high scattering opacity. In addition to solving for the radiation field, HEROIC also solves for the gas temperature by applying the condition of radiative equilibrium. We present benchmarks and tests of the Comptonization module in HEROIC with simple 1D and 3D scattering problems. We also test the ability of the code to handle various relativistic effects using model atmospheres and accretion flows in a black hole space-time. We present two applications of HEROIC to general relativistic magnetohydrodynamics simulations of accretion discs. One application is to a thin accretion disc around a black hole. We find that the gas below the photosphere in the multidimensional HEROIC solution is nearly isothermal, quite different from previous solutions based on 1D plane parallel atmospheres. The second application is to a geometrically thick radiation-dominated accretion disc accreting at 11 times the Eddington rate. Here, the multidimensional HEROIC solution shows that, for observers who are on axis and look down the polar funnel, the isotropic equivalent luminosity could be more than 10 times the Eddington limit, even though the spectrum might still look thermal and show no signs of relativistic beaming.Publication Thermal stability in turbulent accretion discs(Oxford University Press (OUP), 2013) Zhu, Yucong; Narayan, RameshThe standard thin accretion disc model predicts that discs around stellar mass black holes become radiation pressure dominated and thermally unstable once their luminosity exceeds L & 0.02LEdd. Observationally, discs in the high/soft state of X-ray binaries show little variability in the range 0.01LEdd < L < 0.5LEdd, implying that these discs in nature are in fact quite stable. In an attempt to reconcile this conflict, we investigate one-zone disc models including turbulent and convective modes of vertical energy transport. We find both mixing mechanisms to have a stabilizing effect, leading to an increase in the L threshold up to which the disc is thermally stable. In the case of stellar mass black hole systems, convection alone leads to only a minor increase in this threshold, up to ∼ 5 per cent of Eddington. However turbulent mixing has a much greater effect – the threshold rises up to 20 per cent Eddington under reasonable assumptions. In optimistic models with superefficient turbulent mixing, we even find solutions that are completely thermally stable for all accretion rates. Similar results are obtained for supermassive black holes, except that all critical accretion rates are a factor ∼10 lower in Eddington ratio.Publication The Bright Side of Black Holes: Radiation From Black Hole Accretion Disks(2015-05-15) Zhu, Yucong; Hernquist, Lars; Narayan, Ramesh; McClintock, Jeffrey E.; Davis, Shane W.An understanding of radiation is paramount for connecting observations of accretion disks with the theory of black holes. In this thesis, we explore via radiative transfer postprocessing calculations the observational signatures of black holes. We investigate disk spectra by analyzing general relativistic magnetohydrodynamic (GRMHD) simulations of accretion disks. For the most part there are no surprises -- the resulting GRMHD spectrum is very close to the analytic Novikov & Thorne (1973) prediction from decades past, except for a small modification in the case of spinning black holes, which exhibit a high-energy power-law tail that is sourced by hot Comptonized gas from within the plunging region of the accretion flow. These conclusions are borne out by both 1D and 3D radiative transfer calculations of the disk. Significant effort was spent in developing from scratch the 3D radiative code that we used for the analysis. The code is named HERO (Hybrid Evaluator for Radiative Objects) and it is a new general purpose grid-based 3D general relativistic radiative solver.