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Register a new userGeneral Relativistic Magnetohydrodynamic Simulations of Magnetically Choked Accretion Flows around Black Holes
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Jonathan C. McKinney
Publication date
2012
fields
Physics
and research's language is
English
Authors
Jonathan C. McKinney Stanford
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Black hole (BH) accretion flows and jets are qualitatively affected by the presence of ordered magnetic fields. We study fully three-dimensional global general relativistic magnetohydrodynamic (MHD) simulations of radially extended and thick (height $H$ to cylindrical radius $R$ ratio of $|H/R|sim 0.2--1$) accretion flows around BHs with various dimensionless spins ($a/M$, with BH mass $M$) and with initially toroidally-dominated ($phi$-directed) and poloidally-dominated ($R-z$ directed) magnetic fields. Firstly, for toroidal field models and BHs with high enough $|a/M|$, coherent large-scale (i.e. $gg H$) dipolar poloidal magnetic flux patches emerge, thread the BH, and generate transient relativistic jets. Secondly, for poloidal field models, poloidal magnetic flux readily accretes through the disk from large radii and builds-up to a natural saturation point near the BH. For sufficiently high $|a/M|$ or low $|H/R|$ the polar magnetic field compresses the inflow into a geometrically thin highly non-axisymmetric magnetically choked accretion flow (MCAF) within which the standard linear magneto-rotational instability is suppressed. The condition of a highly-magnetized state over most of the horizon is optimal for the Blandford-Znajek mechanism that generates persistent relativistic jets with $gtrsim 100$% efficiency for $|a/M|gtrsim 0.9$. A magnetic Rayleigh-Taylor and Kelvin-Helmholtz unstable magnetospheric interface forms between the compressed inflow and bulging jet magnetosphere, which drives a new jet-disk quasi-periodic oscillation (JD-QPO) mechanism. The high-frequency QPO has spherical harmonic $|m|=1$ mode period of $tausim 70GM/c^3$ for $a/Msim 0.9$ with coherence quality factors $Qgtrsim 10$. [abridged]
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The classical, relativistic thin-disk theory of Novikov and Thorne (NT) predicts a maximum accretion efficiency of 40% for an optically thick, radiatively efficient accretion disk around a maximally spinning black hole (BH). However, when a strong magnetic field is introduced to numerical simulations of thin disks, large deviations in efficiency are observed, in part due to mass and energy carried by jets and winds launched by the disk or BH spin. The total efficiency of accretion can be significantly enhanced beyond that predicted by NT but it has remained unclear how the radiative component is affected. In order to study the effect of a dynamically relevant large-scale magnetic field on radiatively efficient accretion, we have performed numerical 3D general relativistic - radiative - magnetohydroynamic (GRRMHD) simulations of a disk with scale height to radius ratio of $H/R~0.1$ around a moderately spinning BH (a=0.5) using the code HARMRAD. Our simulations are fully global and allow us to measure the jet, wind, and radiative properties of a magnetically arrested disk (MAD) that is kept thin via self-consistent transport of energy by radiation using the M1 closure scheme. Our fiducial disk is MAD out to a radius of ~16R_g and the majority of the total ~13% efficiency of the accretion flow is carried by a magnetically driven wind. We find that the radiative efficiency is slightly suppressed compared to NT, contrary to prior MAD GRMHD simulations with an ad hoc cooling function, but it is unclear how much of the radiation and thermal energy trapped in the outflows could ultimately escape.
The radiative and jet efficiencies of thin magnetized accretion disks around black holes (BHs) are affected by BH spin and the presence of a magnetic field that, when strong, could lead to large deviations from Novikov-Thorne (NT) thin disk theory. To seek the maximum deviations, we perform general relativistic magnetohydrodynamic (GRMHD) simulations of radiatively efficient thin (half-height $H$ to radius $R$ of $H/Rapprox 0.10$) disks around moderately rotating BHs with $a/M=0.5$. First, our simulations, each evolved for more than $70,000r_g/c$ (gravitational radius $r_g$ and speed of light $c$), show that large-scale magnetic field readily accretes inward even through our thin disk and builds-up to the magnetically-arrested disk (MAD) state. Second, our simulations of thin MADs show the disk achieves a radiative efficiency of $eta_{rm r}approx 15%$ (after estimating photon capture), which is about twice the NT value of $eta_{rm r}sim 8%$ for $a/M=0.5$ and gives the same luminosity as a NT disk with $a/Mapprox 0.9$. Compared to prior simulations with $lesssim 10%$ deviations, our result of an $approx 80%$ deviation sets a new benchmark. Building on prior work, we are now able to complete an important scaling law which suggest that observed jet quenching in the high-soft state in BH X-ray binaries is consistent with an ever-present MAD state with a weak yet sustained jet.
We investigate the dynamics of a circumbinary disc that responds to the loss of mass and to the recoil velocity of the black hole produced by the merger of a binary system of supermassive black holes. We perform the first two-dimensional general relativistic hydrodynamics simulations of textit{extended} non-Keplerian discs and employ a new technique to construct a shock detector, thus determining the precise location of the shocks produced in the accreting disc by the recoiling black hole. In this way we can study how the properties of the system, such as the spin, mass and recoil velocity of the black hole, affect the mass accretion rate and are imprinted on the electromagnetic emission from these sources. We argue that the estimates of the bremsstrahlung luminosity computed without properly taking into account the radiation transfer yield cooling times that are unrealistically short. At the same time we show, through an approximation based on the relativistic isothermal evolution, that the luminosity produced can reach a peak value above $L simeq 10^{43} {rm erg/s} $ at about $sim 30,{rm d}$ after the merger of a binary with total mass $Msimeq 10^6 M_odot$ and persist for several days at values which are a factor of a few smaller. If confirmed by more sophisticated calculations such a signal could indeed lead to an electromagnetic counterpart of the merger of binary black-hole system.
We consider a temporal response of relativistically broadened line spectrum of iron from black hole accretion irradiated by an X-ray echo under strong gravity. The physical condition of accreting gas is numerically calculated in the context of general relativistic hydrodynamics under steady-state, axisymmetry in Kerr geometry. With the onset of a point-like X-ray flare of a short finite duration just above the accretion surface, the gas is assumed to be ionized to produce a neutral iron fluorescent line. Using a fully relativistic ray-tracing approach, the response of line photons due to the X-ray illumination is traced as a function of time and energy for different source configurations around sw and Kerr black holes. Our calculations show that the X-ray echo on the accretion surface clearly imprints a characteristic time-variability in the line spectral features depending on those parameters. Simulated line profiles, aimed for the future microcalorimeter missions of large collecting area such as {it Athena}/X-IFU for typical radio-quiet Seyfert galaxies, are presented to demonstrate that state-of-the-art new observations could differentiate various source parameters by such an X-ray tomographic line reverberation.
(Abridged.) The accretion-induced collapse (AIC) of a white dwarf (WD) may lead to the formation of a protoneutron star and a collapse-driven supernova explosion. This process represents a path alternative to thermonuclear disruption of accreting white dwarfs in Type Ia supernovae. Neutrino and gravitational-wave (GW) observations may provide crucial information necessary to reveal a potential AIC. Motivated by the need for systematic predictions of the GW signature of AIC, we present results from an extensive set of general-relativistic AIC simulations using a microphysical finite-temperature equation of state and an approximate treatment of deleptonization during collapse. Investigating a set of 114 progenitor models in rotational equilibrium, with a wide range of rotational configurations, temperatures and central densities, we extend previous Newtonian studies and find that the GW signal has a generic shape akin to what is known as a Type III signal in the literature. We discuss the detectability of the emitted GWs, showing that the signal-to-noise ratio for current or next-generation interferometer detectors could be high enough to detect such events in our Galaxy. Some of our AIC models form massive quasi-Keplerian accretion disks after bounce. In rapidly differentially rotating models, the disk mass can be as large as ~0.8-Msun. Slowly and/or uniformly rotating models produce much smaller disks. Finally, we find that the postbounce cores of rapidly spinning white dwarfs can reach sufficiently rapid rotation to develop a nonaxisymmetric rotational instability.
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