arXiv subjects

Mitchell C. Begelman

Publications and source records attributed to Mitchell C. Begelman.

At least 55 records · Page 3Linked to original sources

arXiv2022

What really makes an accretion disc MAD

Magnetically arrested accretion discs (MADs) around black holes (BH) have the potential to stimulate the production of powerful jets and account for recent ultra-high-resolution observations of BH environments. Their main properties are usually attributed to the accumulation of dynamically significant net magnetic (vertical) flux throughout the arrested region, which is then regulated by interchange instabilities. Here we propose instead that it is mainly a dynamically important {\it toroidal} field -- the result of dynamo action triggered by the significant but still relatively weak vertical field -- that defines and regulates the properties of MADs. We suggest that rapid convection-like instabilities, involving interchange of toroidal flux tubes and operating concurrently with the magnetorotatonal instability (MRI), can regulate the structure of the disc and the escape of net flux. We generalize the convective stability criteria and disc structure equations to include the effects of a strong toroidal field and show that convective flows could be driven towards two distinct marginally stable states, one of which we associate with MADs. We confirm the plausibility of our theoretical model by comparing its quantitative predictions to simulations of both MAD and SANE (strongly magnetized but not "arrested") discs, and suggest a set of criteria that could help to distinguish MADs from other accretion states. Contrary to previous claims in the literature, we argue that MRI is not suppressed in MADs and is probably responsible for the existence of the strong toroidal field.

astro-ph.HE↗

arXiv2022

Sgr A* X-ray flares from non-thermal particle acceleration in a magnetically arrested disc

Sgr A* exhibits flares in the near-infrared and X-ray bands, with the luminosity in these bands increasing by factors of $10-100$ for $\approx 60$ minutes. One of the models proposed to explain these flares is synchrotron emission of non-thermal particles accelerated by magnetic reconnection events in the accretion flow. We use the results from PIC simulations of magnetic reconnection to post-process 3D two-temperature GRMHD simulations of a magnetically arrested disc (MAD). We identify current sheets, retrieve their properties, estimate their potential to accelerate non-thermal particles and compute the expected non-thermal synchrotron emission. We find that the flux eruptions of MADs can provide suitable conditions for accelerating non-thermal particles to energies $γ_e \lesssim 10^6$ and producing simultaneous X-ray and near-infrared flares. For a suitable choice of current-sheet parameters and a simpified synchrotron cooling prescription, the model can simultaneously reproduce the quiescent and flaring X-ray luminosities as well as the X-ray spectral shape. While the near-infrared flares are mainly due to an increase in the temperature near the black hole during the MAD flux eruptions, the X-ray emission comes from narrow current sheets bordering highly magnetized, low-density regions near the black hole and equatorial current sheets where the flux on the black hole reconnects. As a result, not all infrared flares are accompanied by X-ray ones. The non-thermal flaring emission can extend to very hard ($\lesssim 100$ keV) X-ray energies.

astro-ph.HE↗

arXiv2021

Radiation GRMHD Simulations of the Hard State of Black Hole X-ray Binaries and the Collapse of a Hot Accretion Flow

We present global radiation GRMHD simulations of strongly magnetized accretion onto a spinning, stellar mass black hole at sub-Eddington rates. Using a frequency-dependent Monte Carlo procedure for Compton scattering, we self-consistently evolve a two-temperature description of the ion-electron fluid and its radiation field. For an Eddington ratio $L/L_{\rm Edd} \gtrsim 10^{-3}$, the emergent spectrum forms an apparent power law shape from thermal Comptonization up to a cutoff at $\simeq 100$ keV, characteristic of that seen in the hard spectral states of black hole X-ray binary systems. At these luminosities, the radiative efficiency is high ($\approx 24\%$) and results in a denser midplane region where magnetic fields are dynamically important. For $L/L_{\rm Edd} \sim 10^{-2}$, our hot accretion flow appears to undergo thermal runaway and collapse. Our simulations demonstrate that hot accretion flows can be radiatively efficient and provide an estimate of their maximum luminosity.

astro-ph.HE↗

arXiv2020

Magnetic flux inversion in a peculiar changing look AGN

We argue that the changing-look event in the active galactic nucleus 1ES 1927+654, followed by a dip of 3 orders of magnitude in the X-ray luminosity, is controlled by a change in the accretion rate and an inversion of magnetic flux in a magnetically arrested disk (MAD). Before the changing-look event, strong magnetic flux on the black hole powers X-ray emission via the Blandford-Znajek process while the UV emission is produced by a radiatively inefficient magnetized disk. An advection event, bringing flux of the opposite polarity, propagates inward leading, first, to a rise in the UV/optical luminosity and, then, to a dip in the X-ray luminosity when it reaches the black hole. We estimate the timescale for magnetic flux advection and find that the observed timescale between the beginning of the changing-look event and the minimum in the X-ray luminosity, $\approx200$ days, is in agreement with the time needed to cancel the magnetic flux in a MAD extending to $\approx180\:r_g$. Although flux inversion events might be rare due to the large ratio of flux-to-mass that is needed, we argue that AGN showing an unusually high ratio of X-ray to UV luminosity are prime candidates for such events. We also suggest that similar events may lead to jet interruptions in radio-loud objects.

astro-ph.HE↗

arXiv2020

QPOs in compact binaries from small scale eruptions in an inner magnetized disk

Dwarf novæ (DNe) and low mass X-ray binaries (LMXBs) are compact binaries showing variability on time scales from years to less than seconds. Here, we focus on explaining part of the rapid fluctuations in DNe, following the framework of recent studies on the monthly eruptions of DNe that use a hybrid disk composed of an outer standard disk and an inner magnetized disk. We show that the ionization instability, that is responsible for the monthly eruptions of DNe, is also able to operate in the inner magnetized disk. Given the low density and the fast accretion time scale of the inner magnetized disk, the ionization instability generates small, rapid heating and cooling fronts propagating back and forth in the inner disk. This leads to quasi-periodic oscillations (QPOs) with a period of the order of $1000$ s. A strong prediction of our model is that these QPOs can only develop in quiescence or at the beginning/end of an outburst. We propose that these rapid fluctuations might explain a subclass of already observed QPOs in DNe as well as a, still to observe, subclass of QPOs in LMXBs. We also extrapolate to the possibility that the radiation pressure instability might be related to Type B QPOs in LMXBs.

astro-ph.HE↗

arXiv2020

Structured, relativistic jets driven by radiation

Relativistic jets, or highly collimated and fast-moving outflows, are endemic to many astrophysical phenomena. The jets produced by gamma-ray bursts and tidal disruption events are accompanied by the accretion of material onto a black hole or neutron star, with the accretion rate exceeding the Eddington limit of the compact object by orders of magnitude. In such systems, radiation dominates the energy-momentum budget of the outflow, and the dynamical evolution of the jet is governed by the equations of radiation hydrodynamics. Here we show that there are analytic solutions to the equations of radiation hydrodynamics in the viscous (i.e., diffusive) regime that describe structured, relativistic jets, which consist of a fast-moving, highly relativistic core surrounded by a slower-moving, less relativistic sheath. In these solutions, the slower-moving, outer sheath contains most of the mass, and the jet structure is mediated by local anisotropies in the radiation field. We show that, depending on the pressure and density profile of the ambient medium, the angular profile of the jet Lorentz factor is Gaussian or falls off even more steeply with angle. These solutions have implications for the nature of jet production and evolution in hyperaccreting systems, and demonstrate that such jets -- and the corresponding jet structure -- can be sustained entirely by radiative processes. We discuss the implications of these findings in the context of jetted tidal disruption events and short and long gamma-ray bursts.

astro-ph.HE↗

arXiv2020

First-principles demonstration of diffusive-advective particle acceleration in kinetic simulations of relativistic plasma turbulence

Nonthermal relativistic plasmas are ubiquitous in astrophysical systems like pulsar wind nebulae and active galactic nuclei, as inferred from their emission spectra. The underlying nonthermal particle acceleration (NTPA) processes have traditionally been modeled with a Fokker-Planck (FP) diffusion-advection equation in momentum space. In this paper, we directly test the FP framework in ab-initio kinetic simulations of driven magnetized turbulence in relativistic pair plasma. By statistically analyzing the motion of tracked particles, we demonstrate the diffusive nature of NTPA and measure the FP energy diffusion ($D$) and advection ($A$) coefficients as functions of particle energy $γm_e c^2$. We find that $D(γ)$ scales as $γ^2$ in the high-energy nonthermal tail, in line with 2nd-order Fermi acceleration theory, but has a much weaker scaling at lower energies. We also find that $A$ is not negligible and reduces NTPA by tending to pull particles towards the peak of the particle energy distribution. This study provides strong support for the FP picture of turbulent NTPA, thereby enhancing our understanding of space and astrophysical plasmas.

astro-ph.HE↗

arXiv2020

Kinetic turbulence in shining pair plasma: intermittent beaming and thermalization by radiative cooling

High-energy astrophysical systems frequently contain collisionless relativistic plasmas that are heated by turbulent cascades and cooled by emission of radiation. Understanding the nature of this radiative turbulence is a frontier of extreme plasma astrophysics. In this paper, we use particle-in-cell simulations to study the effects of external inverse Compton radiation on turbulence driven in an optically thin, relativistic pair plasma. We focus on the statistical steady state (where injected energy is balanced by radiated energy) and perform a parameter scan spanning from low magnetization to high magnetization ($0.04 \lesssim σ\lesssim 11$). We demonstrate that the global particle energy distributions are quasi-thermal in all simulations, with only a modest population of nonthermal energetic particles (extending the tail by a factor of $\sim 2$). This indicates that nonthermal particle acceleration (observed in similar non-radiative simulations) is quenched by strong radiative cooling. The quasi-thermal energy distributions are well fit by analytic models in which stochastic particle acceleration (due to, e.g., second-order Fermi mechanism or gyroresonant interactions) is balanced by the radiation reaction force. Despite the efficient thermalization of the plasma, nonthermal energetic particles do make a conspicuous appearance in the anisotropy of the global momentum distribution as highly variable, intermittent beams (for high magnetization cases). The beamed high-energy particles are spatially coincident with intermittent current sheets, suggesting that localized magnetic reconnection may be a mechanism for kinetic beaming. This beaming phenomenon may explain rapid flares observed in various astrophysical systems (such as blazar jets, the Crab nebula, and Sagittarius A*).

astro-ph.HE↗

arXiv2020

Strongly magnetized accretion disks: structure and accretion from global magnetohydrodynamic simulations

We use global magnetohydrodynamic simulations to study the influence of net vertical magnetic fields on the structure of geometrically thin ($H/r \approx 0.05$) accretion disks in the Newtonian limit. We consider initial mid-plane gas to magnetic pressure ratios $β_0 = 1000,\, 300$ and $100$, spanning the transition between weakly and strongly magnetized accretion regimes. We find that magnetic pressure is important for the disks' vertical structure in all three cases, with accretion occurring at $z/R\approx 0.2$ in the two most strongly magnetized models. The disk midplane shows outflow rather than accretion. Accretion through the surface layers is driven mainly by stress due to coherent large scale magnetic field rather than by turbulent stress. Equivalent viscosity parameters measured from our simulations show similar dependencies on initial $β_0$ to those seen in shearing box simulations, though the disk midplane is not magnetic pressure dominated even for the strongest magnetic field case. Winds are present but are not the dominant driver of disk evolution. Over the (limited) duration of our simulations, we find evidence that the net flux attains a quasi-steady state at levels that can stably maintain a strongly magnetized disk. We suggest that geometrically thin accretion disks in observed systems may commonly exist in a magnetically "elevated" state, characterized by non-zero but modest vertical magnetic fluxes, with potentially important implications for disk phenomenology in X-ray binaries (XRBs) and active galactic nuclei (AGN).

astro-ph.HE↗

arXiv2019

Electron and ion energization in relativistic plasma turbulence

Electron and ion energization (i.e., heating and nonthermal acceleration) is a fundamental, but poorly understood, outcome of plasma turbulence. In this work, we present new results on this topic from particle-in-cell simulations of driven turbulence in collisionless, relativistic electron-ion plasma. We focus on temperatures such that ions (protons) are sub-relativistic and electrons are ultra-relativistic, a regime relevant for high-energy astrophysical systems such as hot accretion flows onto black holes. We find that ions tend to be preferentially heated, gaining up to an order of magnitude more energy than electrons, and propose a simple empirical formula to describe the electron-ion energy partition as a function of the ratio of electron-to-ion gyroradii (which in turn is a function of initial temperatures and plasma beta). We also find that while efficient nonthermal particle acceleration occurs for both species in the ultra-relativistic regime, nonthermal electron populations are diminished with decreasing temperature whereas nonthermal ion populations are essentially unchanged. These results have implications for modeling and interpreting observations of hot accretion flows.

astro-ph.HE↗

arXiv2018

Internal instabilities in magnetized jets

We carry out an extensive linear stability analysis of magnetized cylindrical jets in a global framework. Foregoing the commonly invoked force-free limit, we focus on the small-scale, internal instabilities triggered in regions of the jet dominated by a toroidal magnetic field, with a weak vertical field and finite thermal pressure gradient. Such regions are likely to occur far from the jet source and boundaries, and are potential sites of magnetic energy dissipation that is essential to explain the particle acceleration and radiation observed from astrophysical jets. We validate the local stability analysis of Begelman by verifying that the eigenfunctions of the most unstable modes are radially localized. This finding allows us to propose a generic stability criterion in the presence of a weak vertical field. A stronger vertical field with a radial gradient complicates the stability criterion, due to the competition between the destabilizing thermal pressure gradient and stabilizing magnetic pressure gradients. Nevertheless, we argue that the jet interiors generically should be subject to rapidly growing, small-scale instabilities, capable of producing current sheets that lead to dissipation. We identify some new instabilities, not predicted by the local analysis, which are sensitive to the background radial profiles but have smaller growth rates than the local instabilities, and discuss the relevance of our work to the findings of recent numerical jet simulations.

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arXiv2018

Extreme AGN variability: evidence of magnetically elevated accretion?

Rapid, large amplitude variability at optical to X-ray wavelengths is now seen in an increasing number of Seyfert galaxies and luminous quasars. The variations imply a global change in accretion power, but are too rapid to be communicated by inflow through a standard thin accretion disc. Such discs are long known to have difficulty explaining the observed optical/UV emission from active galactic nuclei. Here we show that alternative models developed to explain these observations have larger scale heights and shorter inflow times. Accretion discs supported by magnetic pressure in particular are geometrically thick at all luminosities, with inflow times as short as the observed few year timescales in extreme variability events to date. Future time-resolved, multi-wavelength observations can distinguish between inflow through a geometrically thick disc as proposed here, and alternative scenarios of extreme reprocessing of a central source or instability-driven limit cycles.

astro-ph.GA↗

arXiv2018

Direct Collapse to Supermassive Black Hole Seeds with Radiation Transfer: Cosmological Halos

We have modeled direct collapse of a primordial gas within dark matter halos in the presence of radiative transfer, in high-resolution zoom-in simulations in a cosmological framework, down to the formation of the photosphere and the central object. Radiative transfer has been implemented in the flux-limited diffusion (FLD) approximation. Adiabatic models were run for comparison. We find that (a) the FLD flow forms an irregular central structure and does not exhibit fragmentation, contrary to adiabatic flow which forms a thick disk, driving a pair of spiral shocks, subject to Kelvin-Helmholtz shear instability forming fragments; (b) the growing central core in the FLD flow quickly reaches ~10 Mo and a highly variable luminosity of 10^{38}-10^{39} erg/s, comparable to the Eddington luminosity. It experiences massive recurrent outflows driven by radiation force and thermal pressure gradients, which mix with the accretion flow and transfer the angular momentum outwards; and (c) the interplay between these processes and a massive accretion, results in photosphere at ~10 AU. We conclude that in the FLD model (1) the central object exhibits dynamically insignificant rotation and slower than adiabatic temperature rise with density; (2) does not experience fragmentation leading to star formation, thus promoting the fast track formation of a supermassive black hole (SMBH) seed; (3) inclusion of radiation force leads to outflows, resulting in the mass accumulation within the central 10^{-3} pc, which is ~100 times larger than characteristic scale of star formation. The inclusion of radiative transfer reveals complex early stages of formation and growth of the central structure in the direct collapse scenario of SMBH seed formation.

astro-ph.GA↗

arXiv2018

System-size convergence of nonthermal particle acceleration in relativistic plasma turbulence

We apply collisionless particle-in-cell simulations of relativistic pair plasmas to explore whether driven turbulence is a viable high-energy astrophysical particle accelerator. We characterize nonthermal particle distributions for varying system sizes up to $L/2πρ_{e0} = 163$, where $L/2π$ is the driving scale and $ρ_{e0}$ is the initial characteristic Larmor radius. We show that turbulent particle acceleration produces power-law energy distributions that, when compared at a fixed number of large-scale dynamical times, slowly steepen with increasing system size. We demonstrate, however, that convergence is obtained by comparing the distributions at different times that increase with system size (approximately logarithmically). We suggest that the system-size dependence arises from the time required for particles to reach the highest accessible energies via Fermi acceleration. The converged power-law index of the energy distribution, $α\approx 3.0$ for magnetization $σ= 3/8$, makes turbulence a possible explanation for nonthermal spectra observed in systems such as the Crab nebula.

astro-ph.HE↗

arXiv2017

Instability in strongly magnetized accretion discs: A global perspective

We examine the properties of strongly magnetized accretion discs in a global framework, with particular focus on the evolution of magnetohydrodynamic instabilities such as the magnetorotational instability (MRI). Work by Pessah and Psaltis showed that MRI is stabilized beyond a critical toroidal field in compressible, differentially rotating flows and, also, reported the appearance of two new instabilities beyond this field. Their results stemmed from considering geometric curvature effects due to the suprathermal background toroidal field, which had been previously ignored in weak-field studies. However, their calculations were performed under the local approximation, which poses the danger of introducing spurious behavior due to the introduction of global geometric terms in an otherwise local framework. In order to avoid this, we perform a global eigenvalue analysis of the linearized MHD equations in cylindrical geometry. We confirm that MRI indeed tends to be highly suppressed when the background toroidal field attains the Pessah-Psaltis limit. We also observe the appearance of two new instabilities that emerge in the presence of highly suprathermal toroidal fields. These results were additionally verified using numerical simulations in PLUTO. There are, however, certain differences between the the local and global results, especially in the vertical wavenumber occupancies of the various instabilities, which we discuss in detail. We also study the global eigenfunctions of the most unstable modes in the suprathermal regime, which are inaccessible in the local analysis. Overall, our findings emphasize the necessity of a global treatment for accurately modeling strongly magnetized accretion discs.

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arXiv2017

Numerical investigation of kinetic turbulence in relativistic pair plasmas I: Turbulence statistics

We describe results from particle-in-cell simulations of driven turbulence in collisionless, magnetized, relativistic pair plasma. This physical regime provides a simple setting for investigating the basic properties of kinetic turbulence and is relevant for high-energy astrophysical systems such as pulsar wind nebulae and astrophysical jets. In this paper, we investigate the statistics of turbulent fluctuations in simulations on lattices of up to $1024^3$ cells and containing up to $2 \times 10^{11}$ particles. Due to the absence of a cooling mechanism in our simulations, turbulent energy dissipation reduces the magnetization parameter to order unity within a few dynamical times, causing turbulent motions to become sub-relativistic. In the developed stage, our results agree with predictions from magnetohydrodynamic turbulence phenomenology at inertial-range scales, including a power-law magnetic energy spectrum with index near $-5/3$, scale-dependent anisotropy of fluctuations described by critical balance, log-normal distributions for particle density and internal energy density (related by a $4/3$ adiabatic index, as predicted for an ultra-relativistic ideal gas), and the presence of intermittency. We also present possible signatures of a kinetic cascade by measuring power-law spectra for the magnetic, electric, and density fluctuations at sub-Larmor scales.

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arXiv2017

Kinetic turbulence in relativistic plasma: from thermal bath to non-thermal continuum

We present results from particle-in-cell simulations of driven turbulence in magnetized, collisionless, and relativistic pair plasma. We find that fluctuations are consistent with the classical $k_\perp^{-5/3}$ magnetic energy spectrum at fluid scales and a steeper $k_\perp^{-4}$ spectrum at sub-Larmor scales, where $k_\perp$ is the wavevector perpendicular to the mean field. We demonstrate the development of a non-thermal, power-law particle energy distribution, $f(E) \sim E^{-α}$, with index $α$ that decreases with increasing magnetization and increases with increasing system size (relative to the characteristic Larmor radius). Our simulations indicate that turbulence can be a viable source of energetic particles in high-energy astrophysical systems, such as pulsar wind nebulae, if scalings asymptotically become insensitive to the system size.

physics.plasm-ph↗

arXiv2016

Tidal disruption events from supermassive black hole binaries

We investigate the pre-disruption gravitational dynamics and post-disruption hydrodynamics of the tidal disruption of stars by supermassive black hole (SMBH) binaries. We focus on binaries with relatively low mass primaries ($10^6M_{\odot}$), moderate mass ratios, and separations with reasonably long gravitational wave inspiral times (tens of Myr). First, we generate a large ensemble (between 1 and 10 million) of restricted three-body integrations to quantify the statistical properties of tidal disruptions by circular SMBH binaries of initially-unbound stars. Compared to the reference case of a disruption by a single SMBH, the binary potential induces significant variance into the specific energy and angular momentum of the star at the point of disruption. Second, we use Newtonian numerical hydrodynamics to study the detailed evolution of the fallback debris from 120 disruptions randomly selected from the three-body ensemble (excluding only the most deeply penetrating encounters). We find that the overall morphology of the debris is greatly altered by the presence of the second black hole, and the accretion rate histories display a wide range of behaviors, including order of magnitude dips and excesses relative to control simulations that include only one black hole. Complex evolution persists, in some cases, for many orbital periods of the binary. We find evidence for power in the accretion curves on timescales related to the binary orbital period, though there is no exact periodicity. We discuss our results in the context of future wide-field surveys, and comment on the prospects of identifying and characterizing the subset of events occurring in nuclei with binary SMBHs.

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