arXiv subjects

Julian H. Krolik

Publications and source records attributed to Julian H. Krolik.

At least 37 records · Page 2Linked to original sources

arXiv2021

High-energy emission from tidal disruption events in active galactic nuclei

Tidal disruption events (TDEs) taking place in active galactic nuclei (AGNs) are different from ordinary TDEs. In these events, the returning tidal debris stream drills through the pre-existing AGN accretion disk near the stream pericenter, destroying the inner disk in the process, and then intersects with the disk a second time at radii ranging from a few to hundreds of times the pericenter distance. The debris dynamics of such TDEs, and hence their appearance, are distinct from ordinary TDEs. Here we explore the observational signatures of this "second impact" of the stream with the disk. Strong shocks form as the dilute stream is stopped by the denser disk. Compton cooling of the shocked material produces hard X-rays, even soft gamma-rays, with most of the energy emitted between ~10 keV and 1 MeV. The luminosity follows the mass-return rate, peaking between ~$10^{42}$ and $10^{44}$ erg/s. The X-ray hardness and the smoothness of the light curve provide possible means for distinguishing the second impact from ordinary AGN flares, which exhibit softer spectra and more irregular light curves.

astro-ph.HE↗

arXiv2021

Mass-ratio and Magnetic Flux-Dependence of Modulated Accretion from Circumbinary Disks

Accreting supermassive binary black holes (SMBBHs) are potential multi-messenger sources because they emit both gravitational wave and electromagnetic (EM) radiation. Past work has shown that their EM output may be periodically modulated by an asymmetric density distribution in the circumbinary disk, often called an "overdensity" or "lump;" this modulation could possibly be used to identify a source as a binary. We explore the sensitivity of the overdensity to SMBBH mass ratio and magnetic flux through the accretion disk. We find that the relative amplitude of the overdensity and its associated EM periodic signal both degrade with diminishing mass ratio, vanishing altogether somewhere between 1:2 and 1:5. Greater magnetization also weakens the lump and any modulation of the light output. We develop a model to describe how lump formation results from internal stress degrading faster in the lump region than it can be rejuvenated through accretion inflow, and predicts a threshold value in specific internal stress below which lump formation should occur and which all our lump-forming simulations satisfy. Thus, detection of such a modulation would provide a constraint on both mass-ratio and magnetic flux piercing the accretion flow.

astro-ph.HE↗

arXiv2020

Light curves of tidal disruption events in active galactic nuclei

The black hole of an active galactic nucleus is encircled by an accretion disk. The surface density of the disk is always too low to affect the tidal disruption of a star, but it can be high enough that a vigorous interaction results when the debris stream returns to pericenter and punches through the disk. Shocks excited in the disk dissipate the kinetic energy of the disk interior to the impact point and expedite inflow toward the black hole. Radiatively efficient disks with luminosity $\gtrsim10^{-3}$ Eddington have a high enough surface density that the initial stream-disk interaction leads to energy dissipation at a super-Eddington rate. Because of the rapid inflow, only part of this dissipated energy emerges as radiation, while the rest is advected into the black hole. Dissipation, inflow, and cooling balance to keep the bolometric luminosity at an Eddington-level plateau whose duration is tens of days, with an almost linear dependence on stellar mass. After the plateau, the luminosity decreases in proportion to the disk surface density, with a power-law index between $-3$ and $-2$ at earlier times, and possibly a steeper index at later times.

astro-ph.HE↗

arXiv2020

Inverse Compton Cooling in the Coronae of Simulated Black Hole Accretion Flows

We present a formulation for a local cooling function to be employed in the diffuse, hot corona region of 3D GRMHD simulations of accreting black holes. This new cooling function calculates the cooling rate due to inverse Compton scattering by considering the relevant microphysics in each cell in the corona and approximating the radiation energy density and Compton temperature there by integrating over the thermal seed photon flux from the disk surface. The method either assumes ion and electron temperatures are equal (1T), or calculates them separately (2T) using an instantaneous equilibrium approach predicated on the actual relevant rate equations (Coulomb and Compton). The method is shown to be consistent with a more detailed ray-tracing calculation where the bulk of the cooling occurs, but is substantially less costly to perform. As an example, we apply these methods to a \textsc{harm3d} simulation of a $10 M_\odot$, non-spinning black hole, accreting at nominally 1\% the Eddington value. Both 1T and 2T approaches lead to increased radiative efficiency and a larger fraction of total cooling in the corona as compared to the original target-temperature cooling function used by \textsc{harm3d}, especially in the 1T case. Time-averaged post-processing reveals that the continuum spectral observations predicted from these simulations are qualitatively similar to actual X-ray binary data, especially so for the 1T approach which yields a harder power-law component ($Γ= 2.25$) compared to the 2T version ($Γ= 2.53$)

astro-ph.HE↗

arXiv2020

Radiative Emission Mechanisms of Tidal Disruption Events

We describe how the various outcomes of stellar tidal disruption give rise to observable radiation. We separately consider the cases where gas circularizes rapidly into an accretion disc, as well as the case when shocked debris streams provide the observable emission without having fully circularized. For the rapid circularization case, we describe how outflows, absorption by reprocessing layers, and Comptonization can cause the observed radiation to depart from that of a bare disc, possibly giving rise to the observed optical/UV emission along with soft X-rays from the disc. If, instead, most of the debris follows highly eccentric orbits for a significant time, many properties of the observed optical/UV emission can be explained by the scale of those eccentric orbits and the shocks embedded in the debris flow near orbital apocenter. In this picture, soft X-ray emission at early times results from the smaller amount of debris mass deflected into a compact accretion disc by weak shocks near the stellar pericenter. A general proposal for the near-constancy of the ultraviolet/optical color temperatures is provided, by linking it to incomplete thermalization of radiation in the atmosphere of the emitting region. We also briefly discuss the radio signals from the interaction of unbound debris and jets with the black hole environment.

astro-ph.HE↗

arXiv2020

Future Simulations of Tidal Disruption Events

Tidal disruption events involve numerous physical processes (fluid dynamics, magnetohydrodynamics, radiation transport, self-gravity, general relativistic dynamics) in highly nonlinear ways, and, because TDEs are transients by definition, frequently in non-equilibrium states. For these reasons, numerical solution of the relevant equations can be an essential tool for studying these events. In this chapter, we present a summary of the key problems of the field for which simulations offer the greatest promise and identify the capabilities required to make progress on them. We then discuss what has been---and what cannot be---done with existing numerical methods. We close with an overview of what methods now under development may do to expand our ability to understand these events.

astro-ph.HE↗

arXiv2020

PatchworkWave: A Multipatch Infrastructure for Multiphysics/Multiscale/Multiframe/Multimethod Simulations at Arbitrary Order

We present an extension of the PatchworkMHD code [1], itself an MHD-capable extension of the Patchwork code [2], for which several algorithms presented here were co-developed. Its purpose is to create a multipatch scheme compatible with numerical simulations of arbitrary equations of motion at any discretization order in space and time. In the Patchwork framework, the global simulation is comprised of an arbitrary number of moving, local meshes, or patches, which are free to employ their own resolution, coordinate system/topology, physics equations, reference frame, and in our new approach, numerical method. Each local patch exchanges boundary data with a single global patch on which all other patches reside through a client-router-server parallelization model. In generalizing Patchwork to be compatible with arbitrary order time integration, PatchworkMHD and PatchworkWave have significantly improved the interpatch interpolation accuracy by removing an interpolation of interpolated data feedback present in the original Patchwork code. Furthermore, we extend Patchwork to be multimethod by allowing multiple state vectors to be updated simultaneously, with each state vector providing its own interpatch interpolation and transformation procedures. As such, our scheme is compatible with nearly any set of hyperbolic partial differential equations. We demonstrate our changes through the implementation of a scalar wave toy-model that is evolved on arbitrary, time dependent patch configurations at 4th order accuracy.

physics.comp-ph↗

arXiv2020

Tidal disruption events in active galactic nuclei

A fraction of tidal disruption events (TDEs) occur in active galactic nuclei (AGNs) whose black holes possess accretion disks; these TDEs can be confused with common AGN flares. The disruption itself is unaffected by the disk, but the evolution of the bound debris stream is modified by its collision with the disk when it returns to pericenter. The outcome of the collision is largely determined by the ratio of the stream mass current to the azimuthal mass current of the disk rotating underneath the stream footprint, which in turn depends on the mass and luminosity of the AGN. To characterize TDEs in AGNs, we simulated a suite of stream--disk collisions with various mass current ratios. The collision excites shocks in the disk, leading to inflow and energy dissipation orders of magnitude above Eddington; however, much of the radiation is trapped in the inflow and advected into the black hole, so the actual bolometric luminosity may be closer to Eddington. The emergent spectrum may not be thermal, TDE-like, or AGN-like. The rapid inflow causes the disk interior to the impact point to be depleted within a fraction of the mass return time. If the stream is heavy enough to penetrate the disk, part of the outgoing material eventually hits the disk again, dissipating its kinetic energy in the second collision; another part becomes unbound, emitting synchrotron radiation as it shocks with surrounding gas.

astro-ph.HE↗

arXiv2019

Radio Emission from the unbound Debris of Tidal Disruption Events

When a star gets too close to a supermassive black hole, it is torn apart by the tidal forces. Roughly half of the stellar mass becomes unbound and flies away at tremendous velocities - around $10^4$ km/s. In this work we explore the idea that the shock produced by the interaction of the unbound debris with the ambient medium gives rise to the synchrotron radio emission observed in several TDEs. We use a moving mesh numerical simulation to study the evolution of the unbound debris and the bow shock around it. We find that as the periapse distance of the star decreases, the outflow becomes faster and wider. A tidal disruption event whose periapse distance is a factor of 7 smaller than the tidal radius can account for the radio emission observed in ASASSN-14li. This model also allows us to obtain a more accurate estimate for the gas density around the centre of the host galaxy of ASASSN-14li.

astro-ph.HE↗

arXiv2019

Population Estimates for Electromagnetically-Distinguishable Supermassive Binary Black Holes

Distinguishing the photon output of an accreting supermassive black hole binary system from that of a single supermassive black hole accreting at the same rate is intrinsically difficult because the majority of the light emerges from near the ISCOs of the black holes. However, there are two possible signals that can distinctively mark binaries, both arising from the gap formed in circumbinary accretion flows inside approximately twice the binary separation. One of these is a "notch" cut into the thermal spectra of these systems in the IR/optical/UV, the other a periodically-varying excess hard X-ray luminosity whose period is of order the binary orbital period. Using data from detailed galaxy evolution simulations, we estimate the distribution function in mass, mass ratio, and accretion rate for accreting supermassive black hole binaries as a function of redshift and then transform this distribution function into predicted source counts for these two potential signals. At flux levels >~10^{-13}~erg~cm^{-2}~s^{-1}, there may be ~O(10^2) such systems in the sky, mostly in the redshift range ~0.5 < z < ~1. Roughly 10% should have periods short enough (<~5~yr) to detect the X-ray modulation; this is also the period range accessible to PTA observations.

astro-ph.GA↗

arXiv2019

Quasi-Periodicity of Supermassive Binary Black Hole Accretion Approaching Merger

In this paper we continue the first ever study of magnetized mini-disks coupled to circumbinary accretion in a supermassive binary black hole (SMBBH) approaching merger reported in Bowen et al. 2018. We extend this simulation from 3 to 12 binary orbital periods. We find that relativistic SMBBH accretion acts as a resonant cavity, where quasi-periodic oscillations tied to the the frequency at which the black hole's orbital phase matches a non-linear $m=1$ density feature, or ``lump'', in the circumbinary accretion disk permeate the system. The rate of mass accretion onto each of the mini-disks around the black holes is modulated at the beat frequency between the binary frequency and the lump's mean orbital frequency, i.e., $Ω_{\rm beat} = Ω_{\rm bin} - \barΩ_{\rm lump}$, while the total mass accretion rate of this equal-mass binary is modulated at two different frequencies, $\gtrsim \barΩ_{\rm lump}$ and $\approx 2 Ω_{\rm beat}$. The instantaneous rotation rate of the lump itself is also modulated at two frequencies close to the modulation frequencies of the total accretion rate, $\barΩ_{\rm lump}$ and $2 Ω_{\rm beat}$. Because of the compact nature of the mini-disks in SMBBHs approaching merger, the inflow times within the mini-disks are comparable to the period on which their mass-supply varies, so that their masses---and the accretion rates they supply to their black holes---are strongly modulated at the same frequency. In essence, the azimuthal symmetry of the circumbinary disk is broken by the dynamics of orbits near a binary, and this $m=1$ asymmetry then drives quasi-periodic variation throughout the system, including both accretion and disk-feeding. In SMBBHs approaching merger, such time variability could introduce distinctive, increasingly rapid, fluctuations in their electromagnetic emission.

astro-ph.HE↗

arXiv2018

Predicting Stellar-Mass Black Hole X-ray Spectra from Simulations

We describe results from a new technique for the prediction of complete, self-consistent X-ray spectra from three-dimensional General Relativistic magnetohydrodynamic (GRMHD) simulations of black hole accretion flows. Density and cooling rate data from a HARM3D GRMHD simulation are processed by both an improved version of the Monte Carlo radiation transport code PANDURATA (in the corona) and the Feautrier solver PTRANSX (in the disk), with XSTAR subroutines. The codes are run in a sequential but iterative fashion to achieve globally energy-conserving and self-consistent radiation fields, temperature maps, and photoionization equilibria. The output is the X-ray spectrum as seen by a distant observer. For the example cases we consider here---a non-rotating $10 M_\odot$ black hole with solar abundances, accreting at 0.01, 0.03, 0.1, or 0.3 Eddington---we find spectra resembling actual observations of stellar-mass black holes in the soft or steep power-law state: broad thermal peaks (at 1-3 keV), steep power-laws extending to high energy ($Γ$ = 2.7-4.5), and prominent, asymmetric Fe K$α$ emission lines with equivalent widths in the range 40-400 eV (larger EW at lower accretion rates). By starting with simulation data, we obviate the need for parameterized descriptions of the accretion flow geometry---no a priori specification of the corona's shape or flux, or the disk temperature or density, etc., are needed. Instead, we apply the relevant physical principles to simulation output using appropriate numerical techniques; this procedure allows us to calculate inclination-dependent spectra after choosing only a small number of physically meaningful parameters: black hole mass and spin, accretion rate, and elemental abundances.

astro-ph.HE↗

arXiv2018

Sound Speed Dependence of Alignment in Accretion Disks Subjected to Lense-Thirring Torques

We present a series of simulations in both pure hydrodynamics (HD) and magnetohydrodynamics (MHD) exploring the degree to which alignment of disks subjected to external precessional torques (e.g., as in the `Bardeen-Petterson' effect) is dependent upon the disk sound speed c_s. Across the range of sound speeds examined, we find that the influence of the sound speed can be encapsulated in a simple "lumped-parameter" model proposed by Sorathia et al. (2013a). In this model, alignment fronts propagate outward at a speed ~0.2 rOmega_precess(r), where Omega_precess is the local test-particle precession frequency. Meanwhile, transonic radial motions transport angular momentum both inward and outward at a rate that may be described roughly in terms of an orientation diffusion model with diffusion coefficient ~2c_s^2/Omega, for local orbital frequency Omega. The competition between the two leads, in isothermal disks, to a stationary position for the alignment front at a radius proportional to c_s^(-4/5). For alignment to happen at all, the disk must either be turbulent due to the magnetorotational instability in MHD, or, in HD, it must be cool enough for the bending waves driven by disk warp to be nonlinear at their launch point. Contrary to long-standing predictions, warp propagation in MHD disks is diffusive independent of the parameter c_s/(alpha v_orb$, for orbital speed v_orb and ratio of stress to pressure of alpha. In purely HD disks, i.e., those with no internal stresses other than bulk viscosity, warmer disks align weakly or not at all; cooler disks align qualitatively similarly to MHD disks.

astro-ph.HE↗

arXiv2018

Electromagnetic Emission from Supermassive Binary Black Holes Approaching Merger

We present the first fully relativistic prediction of the electromagnetic emission from the surrounding gas of a supermassive binary black hole system approaching merger. Using a ray-tracing code to post-process data from a general relativistic 3-d MHD simulation, we generate images and spectra, and analyze the viewing angle dependence of the light emitted. When the accretion rate is relatively high, the circumbinary disk, accretion streams, and mini-disks combine to emit light in the UV/EUV bands. We posit a thermal Compton hard X-ray spectrum for coronal emission; at high accretion rates, it is almost entirely produced in the mini-disks, but at lower accretion rates it is the primary radiation mechanism in the mini-disks and accretion streams as well. Due to relativistic beaming and gravitational lensing, the angular distribution of the power radiated is strongly anisotropic, especially near the equatorial plane.

astro-ph.HE↗

arXiv2018

PATCHWORK: A Multipatch Infrastructure for Multiphysics/Multiscale/Multiframe Fluid Simulations

We present a "multipatch" infrastructure for numerical simulation of fluid problems in which sub-regions require different gridscales, different grid geometries, different physical equations, or different reference frames. Its key element is a sophisticated client-router-server framework for efficiently linking processors supporting different regions ("patches") that must exchange boundary data. This infrastructure may be used with a wide variety of fluid dynamics codes; the only requirement is that their primary dependent variables be the same in all patches, e.g., fluid mass density, internal energy density, and velocity. Its structure can accommodate either Newtonian or relativistic dynamics. The overhead imposed by this system is both problem- and computer cluster architecture-dependent. Compared to a conventional simulation using the same number of cells and processors, the increase in runtime can be anywhere from negligible to a factor of a few; however, one of the infrastructure's advantages is that it can lead to a very large reduction in the total number of zone-updates.

astro-ph.IM↗

arXiv2018

Magnetorotational instability in eccentric disks

Eccentric disks arise in such astrophysical contexts as tidal disruption events, but it is unknown whether the magnetorotational instability (MRI), which powers accretion in circular disks, operates in eccentric disks as well. We examine the linear evolution of unstratified, incompressible MRI in an eccentric disk orbiting a point mass. We consider vertical modes of wavenumber $k$ on a background flow with uniform eccentricity $e$ and vertical Alfvén speed $v_\mathrm A$ along an orbit with mean motion $n$. We find two mode families, one with dominant magnetic components, the other with dominant velocity components; the former is unstable at $(1-e)^3f^2\lesssim3$, where $f\equiv kv_\mathrm A/n$, the latter at $e\gtrsim0.8$. For $f^2\lesssim3$, MRI behaves much like in circular disks, but the growth per orbit declines slowly with increasing $e$; for $f^2\gtrsim3$, modes grow by parametric amplification, which is resonant for $0<e\ll1$. MRI growth and the attendant angular momentum and energy transport happen chiefly near pericenter, where orbital shear dominates magnetic tension.

astro-ph.HE↗

arXiv2018

Quasi-Periodic Behavior of Mini-Disks in Binary Black Holes Approaching Merger

We present the first magnetohydrodynamic simulation in which a circumbinary disk around a relativistic binary black hole feeds mass to individual accretion disks ("mini-disks") around each black hole. Mass flow through the accretion streams linking the circumbinary disk to the mini-disks is modulated quasi-periodically by the streams' interaction with a nonlinear $m=1$ density feature, or "lump", at the inner edge of the circumbinary disk: the stream supplying each mini-disk comes into phase with the lump at a frequency $0.74$ times the binary orbital frequency. Because the binary is relativistic, the tidal truncation radii of the mini-disks are not much larger than their innermost stable circular orbits; consequently, the mini-disks' inflow times are shorter than the conventional estimate and are comparable to the stream modulation period. As a result, the mini-disks are always in inflow disequilibrium, with their masses and spiral density wave structures responding to the stream's quasi-periodic modulation. The fluctuations in each mini-disk's mass are so large that as much as $75\%$ of the total mini-disk mass can be contained within a single mini-disk. Such quasi-periodic modulation of the mini-disk structure may introduce distinctive time-dependent features in the binary's electromagnetic emission.

astro-ph.HE↗

arXiv2017

Geometrically thick obscuration by radiation-driven outflow from magnetized tori of active galactic nuclei

Near-Eddington radiation from active galactic nuclei (AGNs) has significant dynamical influence on the surrounding dusty gas, plausibly furnishing AGNs with geometrically thick obscuration. We investigate this paradigm with radiative magnetohydrodynamics simulations. The simulations solve the magnetohydrodynamics equations simultaneously with the infrared (IR) and ultraviolet (UV) radiative transfer (RT) equations; no approximate closure is used for RT. We find that our torus, when given a suitable sub-Keplerian angular momentum profile, spontaneously evolves toward a state in which its opening angle, density distribution, and flow pattern change only slowly. This "steady" state lasts for as long as there is gas resupply toward the inner edge. The torus is best described as a mid-plane inflow and a high-latitude outflow. The outflow is launched from the torus inner edge by UV radiation and expands in solid angle as it ascends; IR radiation continues to drive the wide-angle outflow outside the central hole. The dusty outflow obscures the central source in soft X-rays, the IR, and the UV over three quarters of solid angle, and each decade in column density covers roughly equal solid angle around the central source; these obscuration properties are similar to what observations imply.

astro-ph.GA↗