ABSTRACT
We describe global, 3D, time‐dependent, non‐radiative, general‐relativistic, magnetohydrodynamic simulations of accreting black holes (BHs). The simulations are designed to transport a large ...amount of magnetic flux to the centre, more than the accreting gas can force into the BH. The excess magnetic flux remains outside the BH, impedes accretion, and leads to a magnetically arrested disc. We find powerful outflows. For a BH with spin parameter a = 0.5, the efficiency with which the accretion system generates outflowing energy in jets and winds is η≈ 30 per cent. For a = 0.99, we find η≈ 140 per cent, which means that more energy flows out of the BH than flows in. The only way this can happen is by extracting spin energy from the BH. Thus the a = 0.99 simulation represents an unambiguous demonstration, within an astrophysically plausible scenario, of the extraction of net energy from a spinning BH via the Penrose–Blandford–Znajek mechanism. We suggest that magnetically arrested accretion might explain observations of active galactic nuclei with apparent η≈ few × 100 per cent.
Black hole (BH) accretion flows and jets are qualitatively affected by the presence of ordered magnetic fields. We study fully 3D global general relativistic magnetohydrodynamic (MHD) simulations of ...radially extended and thick (height H-to-cylindrical radius R ratio of |H/R| ∼ 0.2-1) accretion flows around BHs with various dimensionless spins (a/M, with BH mass M) and with initially toroidally dominated (φ-directed) and poloidally dominated (R−z directed) magnetic fields. First, for toroidal field models and BHs with high enough |a/M|, coherent large-scale (i.e. ≫H) dipolar poloidal magnetic flux patches emerge, thread the BH, and generate transient relativistic jets. Second, for poloidal field models, poloidal magnetic flux readily accretes through the disc from large radii and builds up to a natural saturation point near the BH. While models with |H/R| ∼ 1 and |a/M| ≤ 0.5 do not launch jets due to quenching by mass infall, 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 magnetorotational instability is suppressed. The condition of a highly magnetized state over most of the horizon is optimal for the Blandford-Znajek mechanism which generates persistent relativistic jets with ≳100 per cent efficiency for |a/M| ≳ 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-disc quasi-periodic oscillation (JD-QPO) mechanism. The high-frequency QPO has spherical harmonic |m| = 1 mode period of τ∼ 70GM/c
3 for a/M∼ 0.9 with coherence quality factors Q≳ 10. Overall, our models are qualitatively distinct from most prior MHD simulations (typically, |H/R| ≪ 1 and poloidal flux is limited by initial conditions), so they should prove useful for testing accretion-jet theories and measuring a/M in systems such as SgrA* and M87.
ABSTRACT
The outflow efficiency (η) from black hole (BH) accretion disc systems is known to depend upon both the BH spin (a) and the amount of large-scale magnetic flux threading the BH and disc. ...Semi-analytical flux-trapping models suggest retrograde BHs should trap much more large-scale magnetic flux near the BH leading to much higher η than for prograde BHs. We self-consistently determine the amount of large-scale magnetic flux trapped by rapidly spinning (a=−0.9 and 0.9) BHs using global 3D time-dependent non-radiative general relativistic magnetohydrodynamic simulations of thick (h/r≈ 0.3-0.6) discs. We find that BH-trapped flux builds up until it is strong enough to disrupt the inner accretion disc. Contrary to prior flux-trapping models, which do not include the back-reaction of magnetic flux on the disc, our simulations show prograde BHs trap more magnetic flux, leading to about three times higher η than retrograde BHs for |a|= 0.9. Both spin orientations can produce highly efficient jets, η∼ 100 per cent, with increasing η for increasing disc thickness. The similarity of η for prograde and retrograde BHs makes it challenging to infer the sign of a based on jet energetics alone.
A Unified Model for Tidal Disruption Events Dai, Lixin; McKinney, Jonathan C.; Roth, Nathaniel ...
Astrophysical journal. Letters,
06/2018, Volume:
859, Issue:
2
Journal Article
Peer reviewed
Open access
In the past few years wide-field optical and UV transient surveys and X-ray telescopes have allowed us to identify a few dozen candidate tidal disruption events (TDEs). While in theory the physical ...processes in TDEs are ubiquitous, a few distinct classes of TDEs have been observed. Some TDEs radiate mainly in NUV/optical, while others produce prominent X-rays. Moreover, relativistic jets have been observed in only a handful of TDEs. This diversity might be related to the details of the super-Eddington accretion and emission physics relevant to TDE disks. In this Letter, we utilize novel three-dimensional general relativistic radiation magnetohydrodynamics simulations to study the super-Eddington compact disk phase expected in TDEs. Consistent with previous studies, geometrically thick disks, wide-angle optically thick fast outflows, and relativistic jets are produced. The outflow density and velocity depend sensitively on the inclination angle, and hence so does the reprocessing of emission produced from the inner disk. We then use Monte Carlo radiative transfer to calculate the reprocessed spectra and find that that the observed ratio of optical to X-ray fluxes increases with increasing inclination angle. This naturally leads to a unified model for different classes of TDEs in which the spectral properties of the TDE depend mainly on the viewing angle of the observer with respect to the orientation of the disk.
A new general relativistic radiation magnetohydrodynamical code koral is described, which employs the M1 scheme to close the radiation moment equations. The code has been successfully verified ...against a number of tests. Axisymmetric simulations of super-critical magnetized accretion on non-rotating (a
* = 0.0) and spinning (a
* = 0.9) black holes are presented. The accretion rates in the two models are
. These first general relativistic simulations of super-critical black hole accretion are potentially relevant to tidal disruption events and hyper-accreting supermassive black holes in the early Universe. Both simulated models are optically and geometrically thick, and have funnels through which energy escapes in the form of relativistic gas, Poynting flux and radiative flux. The jet is significantly more powerful in the a
* = 0.9 run. The net energy outflow rate in the two runs correspond to efficiencies of 5 per cent (a
* = 0) and 33 per cent (a
* = 0.9), as measured with respect to the mass accretion rate at the black hole. These efficiencies agree well with those measured in previous simulations of non-radiative geometrically thick discs. Furthermore, in the a
* = 0.9 run, the outflow power appears to originate in the spinning black hole, suggesting that the associated physics is again similar in non-radiative and super-critical accretion flows. While the two simulations are efficient in terms of total energy outflow, both runs are radiatively inefficient. Their luminosities are only ∼1-10L
Edd, which corresponds to a radiative efficiency ∼0.1 per cent. Interestingly, most of the radiative luminosity emerges through the funnels where the local radiative flux is highly super-Eddington.
Radio loud active galactic nuclei (AGNs) are on average 1000 times brighter in the radio band compared to radio quiet AGNs. We investigate whether this radio loud/quiet dichotomy can be due to ...differences in the spin of the central black holes (BHs) that power the radio-emitting jets. Using general relativistic magnetohydrodynamic simulations, we construct steady state axisymmetric numerical models for a wide range of BH spins (dimensionless spin parameter 0.1 <= a <= 0.9999) and a variety of jet geometries. We assume that the total magnetic flux through the BH horizon at radius r{sub H}(a) is held constant. If the BH is surrounded by a thin accretion disk, we find that the total BH power output depends approximately quadratically on the angular frequency of the hole, P {proportional_to} OMEGA{sup 2}{sub H} {proportional_to} (a/r{sub H}){sup 2}. We conclude that, in this scenario, differences in the BH spin can produce power variations of only a few tens at most. However, if the disk is thick such that the jet subtends a narrow solid angle around the polar axis, then the power dependence becomes much steeper, P {proportional_to} OMEGA{sup 4}{sub H} or even {proportional_to}OMEGA{sup 6}{sub H}. Power variations of 1000 are then possible for realistic BH spin distributions. We derive an analytic solution that accurately reproduces the steeper scaling of jet power with OMEGA{sub H} and we provide a numerical fitting formula that reproduces all our simulation results. We discuss other physical effects that might contribute to the observed radio loud/quiet dichotomy of AGNs.
Prompt gamma-ray burst (GRB) emission requires some mechanism to dissipate an ultrarelativistic jet. Internal shocks or some form of electromagnetic dissipation are candidate mechanisms. Any ...mechanism needs to answer basic questions, such as what is the origin of variability, what radius does dissipation occur at, and how does efficient prompt emission occur. These mechanisms also need to be consistent with how ultrarelativistic jets form and stay baryon pure despite turbulence and electromagnetic reconnection near the compact object and despite stellar entrainment within the collapsar model. We use the latest magnetohydrodynamical models of ultrarelativistic jets to explore some of these questions in the context of electromagnetic dissipation due to the slow collisional and fast collisionless reconnection mechanisms, as often associated with Sweet-Parker and Petschek reconnection, respectively. For a highly magnetized ultrarelativistic jet and typical collapsar parameters, we find that significant electromagnetic dissipation may be avoided until it proceeds catastrophically near the jet photosphere at large radii (r ∼ 1013-1014 cm), by which the jet obtains a high Lorentz factor (γ∼ 100-1000), has a luminosity of L
j∼ 1050-1051 erg s−1, has observer variability time-scales of the order of 1 s (ranging from 0.001 to 10 s), achieves γθj∼ 10-20 (for opening half-angle θj) and so is able to produce jet-breaks, and has comparable energy available for both prompt and afterglow emission. A range of model parameters are investigated and simplified scaling laws are derived. This reconnection switch mechanism allows for highly efficient conversion of electromagnetic energy into prompt emission and associates the observed prompt GRB pulse temporal structure with dissipation time-scales of some number of reconnecting current sheets embedded in the jet. We hope this work helps to motivate the development of self-consistent radiative compressible relativistic reconnection models.
Transcription of ribosomal RNA (rRNA) by RNA Polymerase (Pol) I in the nucleolus is necessary for ribosome biogenesis, which is intimately tied to cell growth and proliferation. Perturbation of ...ribosome biogenesis results in tissue specific disorders termed ribosomopathies in association with alterations in nucleolar structure. However, how rRNA transcription and ribosome biogenesis regulate nucleolar structure during normal development and in the pathogenesis of disease remains poorly understood. Here we show that homozygous null mutations in Pol I subunits required for rRNA transcription and ribosome biogenesis lead to preimplantation lethality. Moreover, we discovered that Polr1a.sup.-/-, Polr1b.sup.-/-, Polr1c.sup.-/- and Polr1d.sup.-/- mutants exhibit defects in the structure of their nucleoli, as evidenced by a decrease in number of nucleolar precursor bodies and a concomitant increase in nucleolar volume, which results in a single condensed nucleolus. Pharmacological inhibition of Pol I in preimplantation and midgestation embryos, as well as in hiPSCs, similarly results in a single condensed nucleolus or fragmented nucleoli. We find that when Pol I function and rRNA transcription is inhibited, the viscosity of the granular compartment of the nucleolus increases, which disrupts its phase separation properties, leading to a single condensed nucleolus. However, if a cell progresses through mitosis, the absence of rRNA transcription prevents reassembly of the nucleolus and manifests as fragmented nucleoli. Taken together, our data suggests that Pol I function and rRNA transcription are required for maintaining nucleolar structure and integrity during development and in the pathogenesis of disease.
ABSTRACT One of the puzzles associated with tidal disruption event candidates (TDEs) is that there is a dichotomy between the color temperatures of a few × 104 K for TDEs discovered with optical and ...UV telescopes and the color temperatures of a few × 105-106 K for TDEs discovered with X-ray satellites. Here, we propose that high-temperature TDEs are produced when the tidal debris of a disrupted star self-intersects relatively close to the supermassive black hole, in contrast to the more distant self-intersection that leads to lower color temperatures. In particular, we note from simple ballistic considerations that greater apsidal precession in an orbit is the key to closer self-intersection. Thus, larger values of β, the ratio of the tidal radius to the pericenter distance of the initial orbit, are more likely to lead to higher temperatures of more compact disks that are super-Eddington and geometrically and optically thick. For a given star and β, apsidal precession also increases for larger black hole masses, but larger black hole masses imply a lower temperature at the Eddington luminosity. Thus, the expected dependence of the temperature on the mass of the black hole is non-monotonic. We find that in order to produce a soft X-ray temperature TDE, a deep plunging stellar orbit with β > 3 is needed and a black hole mass of 5 × 106M is favored. Although observations of TDEs are comparatively scarce and are likely dominated by selection effects, it is encouraging that both expectations are consistent with current data.
Abstract
We present a numerical method that evolves a two-temperature, magnetized, radiative, accretion flow around a black hole, within the framework of general relativistic radiation ...magnetohydrodynamics. As implemented in the code koral, the gas consists of two sub-components – ions and electrons – which share the same dynamics but experience independent, relativistically consistent, thermodynamical evolution. The electrons and ions are heated independently according to a prescription from the literature for magnetohydrodynamical turbulent dissipation. Energy exchange between the particle species via Coulomb collisions is included. In addition, electrons gain and lose energy and momentum by absorbing and emitting synchrotron and bremsstrahlung radiation and through Compton scattering. All evolution equations are handled within a fully covariant framework in the relativistic fixed-metric space–time of the black hole. Numerical results are presented for five models of low-luminosity black hole accretion. In the case of a model with a mass accretion rate
$\dot{M}\sim 4\times 10^{-8} \dot{M}_{\rm Edd}$
, we find that radiation has a negligible effect on either the dynamics or the thermodynamics of the accreting gas. In contrast, a model with a larger
$\dot{M}\sim 4\times 10^{-4} \dot{M}_{\rm Edd}$
behaves very differently. The accreting gas is much cooler and the flow is geometrically less thick, though it is not quite a thin accretion disc.
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