arXiv subjects

Jonathan C. McKinney

Publications and source records attributed to Jonathan C. McKinney.

At least 19 recordsLinked to original sources

arXiv2022

PATOKA: Simulating Electromagnetic Observables of Black Hole Accretion

The Event Horizon Telescope (EHT) has released analyses of reconstructed images of horizon-scale millimeter emission near the supermassive black hole at the center of the M87 galaxy. Parts of the analyses made use of a large library of synthetic black hole images and spectra, which were produced using numerical general relativistic magnetohydrodynamics fluid simulations and polarized ray tracing. In this article, we describe the PATOKA pipeline, which was used to generate the Illinois contribution to the EHT simulation library. We begin by describing the relevant accretion systems and radiative processes. We then describe the details of the three numerical codes we use, iharm, ipole, and igrmonty, paying particular attention to differences between the current generation of the codes and the originally published versions. Finally, we provide a brief overview of simulated data as produced by PATOKA and conclude with a discussion of limitations and future directions.

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arXiv2019

The Event Horizon General Relativistic Magnetohydrodynamic Code Comparison Project

Recent developments in compact object astrophysics, especially the discovery of merging neutron stars by LIGO, the imaging of the black hole in M87 by the Event Horizon Telescope (EHT) and high precision astrometry of the Galactic Center at close to the event horizon scale by the GRAVITY experiment motivate the development of numerical source models that solve the equations of general relativistic magnetohydrodynamics (GRMHD). Here we compare GRMHD solutions for the evolution of a magnetized accretion flow where turbulence is promoted by the magnetorotational instability from a set of nine GRMHD codes: Athena++, BHAC, Cosmos++, ECHO, H-AMR, iharm3D, HARM-Noble, IllinoisGRMHD and KORAL. Agreement between the codes improves as resolution increases, as measured by a consistently applied, specially developed set of code performance metrics. We conclude that the community of GRMHD codes is mature, capable, and consistent on these test problems.

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arXiv2018

General Relativistic Radiation Magnetohydrodynamic Simulations of Thin Magnetically Arrested Disks

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.

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arXiv2018

Angular Momentum Transport in Thin Magnetically Arrested Disks

In accretion disks with large-scale ordered magnetic fields, the magnetorotational instability (MRI) is marginally suppressed, so other processes may drive angular momentum transport leading to accretion. Accretion could then be driven by large-scale magnetic fields via magnetic braking, but large-scale magnetic flux can build-up onto the black hole and within the disk leading to a magnetically-arrested disk (MAD). Such a MAD state is unstable to the magnetic Rayleigh-Taylor (RT) instability, which itself leads to vigorous turbulence and the emergence of low-density highly-magnetized bubbles. This instability was studied in a thin (ratio of half-height H to radius R, $H/R \approx 0.1$) MAD simulation, where it has a more dramatic effect on the dynamics of the disk than for thicker disks. We find that the low-density bubbles created by the magnetic RT instability decrease the stress (leading to angular momentum transport) in the disk rather than increasing magnetic torques. Indeed, we find that the dominant component of the stress is due to turbulent magnetic fields, despite the suppression of the axisymmetric MRI and the dominant presence of large-scale magnetic fields. This suggests that the magnetic RT instability plays a significant role in driving angular momentum transport in MADs.

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arXiv2018

A unified model for tidal disruption events

In the past few years wide-field optical and UV transient surveys as well as 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 expected to be 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.

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arXiv2017

Observational Signatures of Mass-Loading in Jets Launched by Rotating Black Holes

It is widely believed that relativistic jets in X-ray binaries and active-galactic nuclei are powered by the rotational energy of black holes. This idea is supported by general-relativistic magnetohydrodynamic (GRMHD) simulations of accreting black holes, which demonstrate efficient energy extraction via the Blandford-Znajek mechanism. However, due to uncertainties in the physics of mass-loading, and the failure of GRMHD numerical schemes in the highly-magnetized funnel region, the matter content of the jet remains poorly constrained. We investigate the observational signatures of mass-loading in the funnel by performing general-relativistic radiative transfer calculations on a range of 3D GRMHD simulations of accreting black holes. We find significant observational differences between cases in which the funnel is empty and cases where the funnel is filled with plasma, particularly in the optical and X-ray bands. In the context of Sgr A*, current spectral data constrains the jet filling only if the black hole is rapidly rotating with $a\gtrsim0.9$. In this case, the limits on the infrared flux disfavour a strong contribution from material in the funnel. We comment on the implications of our models for interpreting future Event Horizon Telescope observations. We also scale our models to stellar-mass black holes, and discuss their applicability to the low-luminosity state in X-ray binaries.

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arXiv2017

Blazar Variability From Turbulence in Jets Launched by Magnetically Arrested Accretion Flows

Blazars show variability on timescales ranging from minutes to years, the former being comparable to and in some cases even shorter than the light-crossing time of the central black hole. The observed gamma-ray lightcurves can be described by a power-law power density spectrum (PDS), with a similar index for both BL Lacs and flat-spectrum radio quasars. We show that this variability can be produced by turbulence in relativistic jets launched by magnetically arrested accretion flows (MADs). We perform radiative transport calculations on the turbulent, highly-magnetized jet launching region of a MAD with a rapidly rotating supermassive black hole. The resulting synchrotron and synchrotron self-Compton emission, originating from close to the black hole horizon, is highly variable. This variability is characterized by PDS which is remarkably similar to the observed power-law spectrum at frequencies less than a few per day. Furthermore, turbulence in the jet launching region naturally produces fluctuations in the plasma on scales much smaller than the horizon radius. We speculate that similar turbulent processes, operating in the jet at large radii (and therefore high bulk Lorentz factor), are responsible for blazar variability over many decades in frequency, including on minute timescales.

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arXiv2016

Energetic constraints on electromagnetic signals from double black hole mergers

The possible Fermi detection of an electromagnetic counterpart to the double black hole merger GW150914 has inspired many theoretical models, some of which propose that the holes spiraled together inside a massive star. However, we show that the heat produced by the dynamical friction on such black hole orbits can exceed the stellar binding energy by a large factor, which means that this heat could destroy the star and thus make it difficult for enough gas to be near the holes at merger to produce detectable photons. These considerations must be taken into account when models are proposed for electromagnetic counterparts to the coalescence of two stellar-mass black holes. We find that only when the two black holes form very close to the center can the star avoid destruction. In that case, dynamical friction can make the black holes coalesce faster than they would in vacuum, which leads to a modification of the gravitational waveform that is potentially observable by advanced LIGO.

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arXiv2016

Double Compton and Cyclo-Synchrotron in Super-Eddington Disks, Magnetized Coronae, and Jets

We present an extension to the general relativistic radiation magnetohydrodynamic code HARMRAD to account for emission and absorption by thermal cyclo-synchrotron, double Compton, bremsstrahlung, low-temperature OPAL opacities as well as Thomson and Compton scattering. We approximate the radiation field as a Bose-Einstein distribution and evolve it using the radiation number-energy-momentum conservation equations in order to track photon hardening. We perform various simulations to study how these extensions affect the radiative properties of magnetically-arrested disks accreting at Eddington to super-Eddington rates. We find that double Compton dominates bremsstrahlung in the disk within a radius of $r\sim 15r_g$ (gravitational radii) at a hundred times the Eddington accretion rate, and within smaller radii at lower accretion rates. Double Compton and cyclo-synchrotron regulate radiation and gas temperatures in the corona, while cyclo-synchrotron regulates temperatures in the jet. Interestingly, as the accretion rate drops to Eddington, an optically thin corona develops whose gas temperature of $T\sim 10^9$K is $\sim 100$ times higher than the disk's black body temperature. Our results show the importance of double Compton and synchrotron in super-Eddington disks, magnetized coronae, and jets.

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arXiv2016

Effects of Spin on High-Energy Radiation from Accreting Black Holes

Observations of jets in X-ray binaries show a correlation between radio power and black hole spin. This correlation, if confirmed, points towards the idea that relativistic jets may be powered by the rotational energy of black holes. In order to examine this further, we perform general-relativistic radiative transport calculations on magnetically arrested accretion flows, which are known to produce powerful jets via the Blandford-Znajek (BZ) mechanism. We find that the X-ray and gamma-ray emission strongly depend on spin and inclination angle. Surprisingly, the high-energy power does not show the same dependence on spin as the BZ jet power, but instead can be understood as a redshift effect. In particular, photons observed perpendicular to the spin axis suffer little net redshift until originating from close to the horizon. Such observers see deeper into the hot, dense, highly-magnetized inner disk region. This effect is largest for rapidly rotating black holes due to a combination of frame dragging and decreasing horizon radius. While the X-ray emission is dominated by the near horizon region, the near-infrared radiation originates at larger radii. Therefore, the ratio of X-ray to near-infrared power is an observational signature of black hole spin.

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arXiv2016

Efficiency of Thin Magnetically-Arrested Disks Around Black Holes

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 $H/R\approx 0.10$) 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 $η_{\rm r}\approx 15\%$ (after estimating photon capture), which is about twice the NT value of $η_{\rm r}\sim 8\%$ for $a/M=0.5$ and gives the same luminosity as a NT disk with $a/M\approx 0.9$. 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.

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arXiv2016

Jet Signatures in the Spectra of Accreting Black Holes

Jets are observed as radio emission in active galactic nuclei and during the low/hard state in X-ray binaries (XRBs), but their contribution at higher frequencies has been uncertain. We study the dynamics of jets in XRBs using the general-relativistic magnetohydrodynamic code HARM. We calculate the high-energy spectra and variability properties using a general-relativistic radiative transport code based on grmonty. We find the following signatures of jet emission (i) a significant gamma-ray peak above ~10^22 Hz, (ii) a break in the optical/UV spectrum, with a change in luminosity from L ~ nu^0 to L ~ nu, followed by another break at higher frequencies where the spectrum roughly returns to L ~ nu^0, and (iii) a pronounced synchrotron peak near or below ~10^14 Hz indicates that a significant fraction of any observed X-ray emission originates in the jet. We investigate the variability during a large-scale magnetic field inversion in which the Blandford-Znajek (BZ) jet is quenched and a new transient hot reconnecting plasmoid is launched by the reconnecting field. The ratio of the gamma-rays to X-rays changes from L_gamma / L_X > 1 in the BZ jet to L_gamma / L_X < 1 during the launching of the transient plasmoid.

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arXiv2016

Probing the magnetic field structure in Sgr A* on Black Hole Horizon Scales with Polarized Radiative Transfer Simulations

Magnetic fields are believed to drive accretion and relativistic jets in black hole accretion systems, but the magnetic-field structure that controls these phenomena remains uncertain. We perform general relativistic (GR) polarized radiative transfer of time-dependent three-dimensional GR magnetohydrodynamical (MHD) simulations to model thermal synchrotron emission from the Galactic Center source Sagittarius A$^\ast$ (Sgr A*). We compare our results to new polarimetry measurements by the Event Horizon Telescope (EHT) and show how polarization in the visibility (Fourier) domain distinguishes and constrains accretion flow models with different magnetic field structures. These include models with small-scale fields in disks driven by the magnetorotational instability (MRI) as well as models with large-scale ordered fields in magnetically-arrested disks (MAD). We also consider different electron temperature and jet mass-loading prescriptions that control the brightness of the disk, funnel-wall jet, and Blandford-Znajek-driven funnel jet. Our comparisons between the simulations and observations favor models with ordered magnetic fields near the black hole event horizon in Sgr A*, although both disk- and jet-dominated emission can satisfactorily explain most of the current EHT data. We show that stronger model constraints should be possible with upcoming circular polarization and higher frequency ($349~{\rm GHz}$) measurements.

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arXiv2015

Electromagnetic vs. Lense-Thirring alignment of black hole accretion discs

Accretion discs and black holes (BHs) have angular momenta that are generally misaligned with respect to each other, which can lead to warps in the discs and bends in any jets produced. We consider a disc that is misaligned at large radii and torqued by Lense-Thirring (LT) precession and a Blandford-Znajek (BZ) jet torque. We consider a variety of disc states that include radiatively inefficient thick discs, radiatively efficient thin discs, and super-Eddington accretion discs. The magnetic field strength of the BZ jet is chosen as either from standard equipartition arguments or from magnetically arrested disc (MAD) simulations. We show that standard thin accretion discs can reach spin-disc alignment out to large radii long before LT would play a role, as caused by the slow infall time that gives even a weak BZ jet time to align the disc. We show that geometrically thick radiatively inefficient discs and super-Eddington discs in the MAD state reach disc-spin alignment near the black hole when density profiles are shallow as in magnetohydrodynamical simulations, while the BZ jet aligns discs with steep density profiles (as in advection-dominated accretion flows) with the BH spin out to larger radii. Our results imply that the BZ jet torque should affect the cosmological evolution of BH spin magnitude and direction, BH spin measurements in active galactic nuclei and X-ray binaries, and the interpretations for Event Horizon Telescope observations of discs or jets in strong-field gravity regimes.

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arXiv2015

Resolved Magnetic-Field Structure and Variability Near the Event Horizon of Sagittarius A*

Near a black hole, differential rotation of a magnetized accretion disk is thought to produce an instability that amplifies weak magnetic fields, driving accretion and outflow. These magnetic fields would naturally give rise to the observed synchrotron emission in galaxy cores and to the formation of relativistic jets, but no observations to date have been able to resolve the expected horizon-scale magnetic-field structure. We report interferometric observations at 1.3-millimeter wavelength that spatially resolve the linearly polarized emission from the Galactic Center supermassive black hole, Sagittarius A*. We have found evidence for partially ordered fields near the event horizon, on scales of ~6 Schwarzschild radii, and we have detected and localized the intra-hour variability associated with these fields.

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arXiv2015

Soft X-ray Temperature Tidal Disruption Events from Stars on Deep Plunging Orbits

One of the puzzles associated with tidal disruption event candidates (TDEs) is that there is a dichotomy between the color temperatures of ${\rm few}\times 10^4$~K for TDEs discovered with optical and UV telescopes, and the color temperatures of ${\rm few}\times 10^5 - 10^6$~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 which 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 $\lesssim 5\times 10^6 M_\odot$ 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.

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arXiv2015

Efficiency of Super-Eddington Magnetically-Arrested Accretion

The radiative efficiency of super-Eddington accreting black holes (BHs) is explored for magnetically-arrested disks (MADs), where magnetic flux builds-up to saturation near the BH. Our three-dimensional general relativistic radiation magnetohydrodynamic (GRRMHD) simulation of a spinning BH (spin $a/M=0.8$) accreting at $\sim 50$ times Eddington shows a total efficiency $\sim 50\%$ when time-averaged and total efficiency $\gtrsim 100\%$ in moments. Magnetic compression by the magnetic flux near the rotating BH leads to a thin disk, whose radiation escapes via advection by a magnetized wind and via transport through a low-density channel created by a Blandford-Znajek (BZ) jet. The BZ efficiency is sub-optimal due to inertial loading of field lines by optically thick radiation, leading to BZ efficiency $\sim 40\%$ on the horizon and BZ efficiency $\sim 5\%$ by $r\sim 400r_g$ (gravitational radii) via absorption by the wind. Importantly, radiation escapes at $r\sim 400r_g$ with efficiency $η\approx 15\%$ (luminosity $L\sim 50L_{\rm Edd}$), similar to $η\approx 12\%$ for a Novikov-Thorne thin disk and beyond $η\lesssim 1\%$ seen in prior GRRMHD simulations or slim disk theory. Our simulations show how BH spin, magnetic field, and jet mass-loading affect the radiative and jet efficiencies of super-Eddington accretion.

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arXiv2014

Global simulations of axisymmetric radiative black hole accretion disks in general relativity with a sub-grid magnetic dynamo

We present a sub-grid model that emulates the magnetic dynamo operating in magnetized accretion disks. We have implemented this model in the general relativisic radiation magnetohydrodynamic (GRRMHD) code \koral, using results from local shearing sheet simulations of the magnetorotational instability to fix the parameters of the dynamo. With the inclusion of this dynamo, we are able to run 2D axisymmetric GRRMHD simulations of accretion disks for arbitrarily long times. The simulated disks exhibit sustained turbulence, with the poloidal and toroidal magnetic field components driven towards a state similar to that seen in 3D studies. Using this dynamo code, we present a set of long-duration global simulations of super-Eddington, optically-thick disks around non-spinning and spinning black holes. Super-Eddington disks around non-rotating black holes exhibit a surprisingly large efficiency, $η\approx0.04$, independent of the accretion rate, where we measure efficiency in terms of the total energy output, both radiation and mechanical, flowing out to infinity. Super-Eddington disks around spinning black holes are even more efficient, and appear to extract black hole rotational energy through a process similar to the Blandford-Znajek mechanism. All the simulated models are characterized by highly super-Eddington radiative fluxes collimated along the rotation axis. We also present a set of simulations that were designed to have Eddington or slightly sub-Eddington accretion rates ($\dot{M} \lesssim 2\dot M_{\rm Edd}$). None of these models reached a steady state. Instead, the disks collapsed as a result of runaway cooling, presumably because of a thermal instability.

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