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Register a new userThe Radiative Efficiency and Spectra of Slowly Accreting Black Holes from Two-Temperature GRRMHD Simulations
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We present axisymmetric numerical simulations of radiatively inefficient accretion flows onto black holes combining general relativity, magnetohydrodynamics, self-consistent electron thermodynamics, and frequency-dependent radiation transport. We investigate a range of accretion rates up to $10^{-5} dot{M}_{mathrm{Edd}}$ onto a $10^8 M_{odot}$ black hole with spin $a_{star} = 0.5$. We report on averaged flow thermodynamics as a function of accretion rate. We present the spectra of outgoing radiation and find that it varies strongly with accretion rate, from synchrotron-dominated in the radio at low $dot{M}$ to inverse Compton-dominated at our highest $dot{M}$. In contrast to canonical analytic models, we find that by $dot{M} approx 10^{-5} dot{M}_{mathrm{Edd}}$, the flow approaches $sim 1%$ radiative efficiency, with much of the radiation due to inverse Compton scattering off Coulomb-heated electrons far from the black hole. These results have broad implications for modeling of accreting black holes across a large fraction of the accretion rates realized in observed systems.
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We present axisymmetric two-temperature general relativistic radiation magnetohydrodynamic (GRRMHD) simulations of the inner region of the accretion flow onto the supermassive black hole M87. We address uncertainties from previous modeling efforts through inclusion of models for (1) self-consistent dissipative and Coulomb electron heating (2) radiation transport (3) frequency-dependent synchrotron emission, self-absorption, and Compton scattering. We adopt a distance $D=16.7$ Mpc, an observer angle $theta = 20^{circ}$, and consider black hole masses $M/M_{odot} = (3.3times10^{9}, 6.2times10^{9})$ and spins $a_{star} = (0.5, 0.9375)$ in a four-simulation suite. For each $(M, a_{star})$, we identify the accretion rate that recovers the 230 GHz flux from VLBI measurements. We report on disk thermodynamics at these accretion rates ($dot{M}/dot{M}_{mathrm{Edd}} sim 10^{-5}$). The disk remains geometrically thick; cooling does not lead to a thin disk component. While electron heating is dominated by Coulomb rather than dissipation for $r gtrsim 10 GM/c^2$, the accretion disk remains two-temperature. Radiative cooling of electrons is not negligible, especially for $r lesssim 10 GM/c^2$. The Compton $y$ parameter is of order unity. We then compare derived and observed or inferred spectra, mm images, and jet powers. Simulations with $M/M_{odot} = 3.3times10^{9}$ are in conflict with observations. These simulations produce mm images that are too small, while the low-spin simulation also overproduces X-rays. For $M/M_{odot} = 6.2times10^{9}$, both simulations agree with constraints on radio/IR/X-ray fluxes and mm image sizes. Simulation jet power is a factor $10^2-10^3$ below inferred values, a possible consequence of the modest net magnetic flux in our models.
We report results from general relativistic radiation MHD (GRRMHD) simulations of a super-Eddington black hole (BH) accretion disk formed as a result of a tidal disruption event (TDE). We consider the fiducial case of a solar mass star on a mildly penetrating orbit disrupted by a supermassive BH of mass $10^6 , M_odot$, and consider the epoch of peak fall back rate. We post-process the simulation data to compute viewing angle dependent spectra. We perform a parameter study of the dynamics of the accretion disk as a function of BH spin and magnetic flux, and compute model spectra as a function of the viewing angle of the observer. We also consider detection limits based on the model spectra. We find that an accretion disk with a relatively weak magnetic field around the BH (so-called SANE regime of accretion) does not launch a relativistic jet, whether or not the BH is rotating. Such models reasonably reproduce several observational properties of non-jetted TDEs. The same is also true for a non-rotating BH with a strong magnetic field (MAD regime). One of our simulations has a rapidly rotating BH (spin parameter 0.9) as well as a MAD accretion disk. This model launches a powerful relativistic jet, which is powered by the BH spin energy. It reproduces the high energy emission and jet structure of the jetted TDE Swift J1644+57 surprisingly well. Jetted TDEs may thus correspond to the subset of TDE systems that have both a rapidly spinning BH and MAD accretion.
I outline the theory of accretion onto black holes, and its application to observed phenomena such as X-ray binaries, active galactic nuclei, tidal disruption events, and gamma-ray bursts. The dynamics as well as radiative signatures of black hole accretion depend on interactions between the relatively simple black-hole spacetime and complex radiation, plasma and magnetohydrodynamical processes in the surrounding gas. I will show how transient accretion processes could provide clues to these interactions. Larger global magnetohydrodynamic simulations as well as simulations incorporating plasma microphysics and full radiation hydrodynamics will be needed to unravel some of the current mysteries of black hole accretion.
A common consequence of accretion onto black holes is the formation of powerful, relativistic jets that escape the system. In the case of supermassive black holes at the centres of galaxies this has been known for decades, but for stellar-mass black holes residing within galaxies like our own, it has taken recent advances to arrive at this conclusion. Here, a review is given of the evidence that supports the existence of jets from accreting stellar-mass black holes, from observations made at optical and infrared wavelengths. In particular it is found that on occasion, jets can dominate the emission of these systems at these wavelengths. In addition, the interactions between the jets and the surrounding matter produce optical and infrared emission on large scales via thermal and non-thermal processes. The evidence, implications and applications in the context of jet physics are discussed. It is shown that many properties of the jets can be constrained from these studies, including the total kinetic power they contain. The main conclusion is that like the supermassive black holes, the jet kinetic power of accreting stellar-mass black holes is sometimes comparable to their bolometric radiative luminosity. Future studies can test ubiquities in jet properties between objects, and attempt to unify the properties of jets from all observable accreting black holes, i.e. of all masses.
A typical galaxy is thought to contain tens of millions of stellar-mass black holes, the collapsed remnants of once massive stars, and a single nuclear supermassive black hole. Both classes of black holes accrete gas from their environments. The accreting gas forms a flattened orbiting structure known as an accretion disk. During the past several years, it has become possible to obtain measurements of the spins of the two classes of black holes by modeling the X-ray emission from their accretion disks. Two methods are employed, both of which depend upon identifying the inner radius of the accretion disk with the innermost stable circular orbit (ISCO), whose radius depends only on the mass and spin of the black hole. In the Fe K method, which applies to both classes of black holes, one models the profile of the relativistically-broadened iron line with a special focus on the gravitationally redshifted red wing of the line. In the continuum-fitting method, which has so far only been applied to stellar-mass black holes, one models the thermal X-ray continuum spectrum of the accretion disk. We discuss both methods, with a strong emphasis on the continuum-fitting method and its application to stellar-mass black holes. Spin results for eight stellar-mass black holes are summarized. These data are used to argue that the high spins of at least some of these black holes are natal, and that the presence or absence of relativistic jets in accreting black holes is not entirely determined by the spin of the black hole.
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