Jonathan McKinney - Department of Physics & Joint Space-Science Institute, University of Maryland

Jonathan McKinney
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Name
Jonathan McKinney
Affiliation
Department of Physics & Joint Space-Science Institute, University of Maryland
Country
United States

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High Energy Astrophysical Phenomena (37)
 
Astrophysics (12)
 
General Relativity and Quantum Cosmology (8)
 
Astrophysics of Galaxies (7)
 
Cosmology and Nongalactic Astrophysics (5)
 
Instrumentation and Methods for Astrophysics (2)
 
Physics - Plasma Physics (1)
 
Solar and Stellar Astrophysics (1)

Publications Authored By Jonathan McKinney

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). Read More

2016Nov
Affiliations: 1University of Maryland, 2University of Maryland, 3University of Maryland

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. Read More

2016Aug
Affiliations: 1University of Maryland at College Park, Dept. of Physics, Joint Space-Science Institute, 2Jodrell Bank Centre for Astrophysics, University of Manchester, 3Copernicus Astronomical Center, 4MIT Kavli Institute for Astrophysics and Space Research, 5MIT Kavli Institute for Astrophysics and Space Research

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. Read More

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. Read More

2016Jan
Affiliations: 1Department of Physics & Joint Space-Science Institute, University of Maryland, 2Department of Physics & Joint Space-Science Institute, University of Maryland, 3Harvard-Smithsonian Center for Astrophysics, 4Harvard-Smithsonian Center for Astrophysics

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. Read More

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. Read More

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. Read More

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. Read More

2015Oct
Affiliations: 1Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences, 2Jet Propulsion Laboratory, 3California Institute of Technology, Cahill Center for Astronomy and Astrophysics, 4Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences, 5Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences, 6Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences, 7Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences, 8Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences, 9Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences, 10Anton Pannekoek Institute for Astronomy, 11California Institute of Technology, Cahill Center for Astronomy and Astrophysics, 12California Institute of Technology, Cahill Center for Astronomy and Astrophysics, 13Jet Propulsion Laboratory, 14Jet Propulsion Laboratory, 15Rice University, Department of Physics and Astronomy, 16Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences, 17Georgia College, Department of Chemistry, Physics, and Astronomy, 18Jet Propulsion Laboratory, 19Jet Propulsion Laboratory, 20North-West University, Centre for Space Research, 21Technical University of Denmark, DTU Space, National Space Institute, 22Yale University, Department of Astronomy, 23Washington University in Saint Louis, Physics Department and McDonnell Center for the Space Sciences, 24University of Virginia, Department of Astronomy, 25MPI for Extraterrestrial Physics Garching, 26Durham University, Centre for Extragalactic Astronomy, Department of Physics, 27Jet Propulsion Laboratory, 28North Carolina State University, Department of Physics, 29Jet Propulsion Laboratory, 30Cambridge, Institute of Astronomy, UK, 31Penn State University, Department of Astronomy and Astrophysics, 32Jet Propulsion Laboratory, 33University of California, Berkeley, Department of Physics, 34ASI Science Data Center, Italy, 35California Institute of Technology, Cahill Center for Astronomy and Astrophysics, 36Cambridge, Institute of Astronomy, UK, 37Jet Propulsion Laboratory, 38Purdue University, Department of Physics and Astronomy, 39Texas Tech University, Physics Department, 40Nagoya University, Center for Experimental Studies, Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, 41University of Maryland, Physics Department, 42RIKEN, 43Univ. of Michigan in Ann Arbor, Astronomy Dept, 44Harvard-Smithsonian Center for Astrophysics, 45Istituto di Astrofisica e Planetologia Spaziali, INAF, 46Department of Astronomy/Steward Observatory, 47Lawrence Livermore National Laboratory, 48Jet Propulsion Laboratory, 49Department of Astronomy/Steward Observatory, 50NASA Goddard Space Flight Center, 51Tohoku University, Astronomical Institute, 52NASA Goddard Space Flight Center

This paper describes the Polarization Spectroscopic Telescope Array (PolSTAR), a mission proposed to NASA's 2014 Small Explorer (SMEX) announcement of opportunity. PolSTAR measures the linear polarization of 3-50 keV (requirement; goal: 2.5-70 keV) X-rays probing the behavior of matter, radiation and the very fabric of spacetime under the extreme conditions close to the event horizons of black holes, as well as in and around magnetars and neutron stars. Read More

2015Aug
Affiliations: 1University of Maryland, College Park, 2University of Maryland, College Park, 3University of Maryland, College Park

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. Read More

2015Aug
Affiliations: 1University of Maryland, 2University of Maryland, 3University of Maryland

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. Read More

2015Jul
Affiliations: 1University of Maryland, 2University of Maryland, 3University of Maryland

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. Read More

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. Read More

Black hole (BH) accretion flows and jets are dynamic hot relativistic magnetized plasma flows whose radiative opacity can significantly affect flow structure and behavior. We describe a numerical scheme, tests, and an astrophysically relevant application using the M1 radiation closure within a new three-dimensional (3D) general relativistic (GR) radiation (R) magnetohydrodynamics (MHD) massively parallel code called HARMRAD. Our 3D GRRMHD simulation of super-Eddington accretion (about $20$ times Eddington) onto a rapidly rotating BH (dimensionless spin $j=0. Read More

Magnetically arrested accretion discs (MADs), where the magnetic pressure in the inner disc is dynamically important, provide an alternative mechanism for regulating accretion to what is commonly assumed in black hole systems. We show that a global magnetic field inversion in the MAD state can destroy the jet, significantly increase the accretion rate, and move the effective inner disc edge in to the marginally stable orbit. Reconnection of the MAD field in the inner radii launches a new type of transient outflow containing hot plasma generated by magnetic dissipation. Read More

High-frequency quasi-periodic oscillations (QPOs) appear in general-relativistic magnetohydrodynamic simulations of magnetically choked accretion flows around rapidly rotating black holes (BHs). We perform polarized radiative transfer calculations with ASTRORAY code to explore the manifestations of these QPOs for SgrA*. We construct a simulation-based model of a radiatively inefficient accretion flow and find model parameters by fitting the mean polarized source spectrum. Read More

We briefly summarize the method of simulating Sgr A* polarized sub-mm spectra from the accretion flow and fitting the observed spectrum. The dynamical flow model is based on three-dimensional general relativistic magneto hydrodynamic simulations. Fully self-consistent radiative transfer of polarized cyclo-synchrotron emission is performed. Read More

2012Nov
Affiliations: 1Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 2Center for Theoretical Science, Jadwin Hall, Princeton University, Princeton Center for Theoretical Science Fellow, 3Kavli Institute for Particle Astrophysics and Cosmology, Stanford University

Accreting black holes (BHs) produce intense radiation and powerful relativistic jets, which are affected by the BH's spin magnitude and direction. While thin disks might align with the BH spin axis via the Bardeen-Petterson effect, this does not apply to jet systems with thick disks. We used fully three-dimensional general relativistic magnetohydrodynamical simulations to study accreting BHs with various BH spin vectors and disk thicknesses with magnetic flux reaching saturation. Read More

Recent advances in general relativistic magnetohydrodynamic modeling of jets offer unprecedented insights into the inner workings of accreting black holes that power the jets in active galactic nuclei (AGN) and other accretion systems. I will present the results of recent studies that determine spin-dependence of jet power and discuss the implications for the AGN radio loud/quiet dichotomy and recent observations of high jet power in a number of AGN. Read More

Ongoing millimeter VLBI observations with the Event Horizon Telescope allow unprecedented study of the innermost portion of black hole accretion flows. Interpreting the observations requires relativistic, time-dependent physical modeling. We discuss the comparison of radiative transfer calculations from general relativistic MHD simulations of Sagittarius A* and M87 with current and future mm-VLBI observations. Read More

The outflow efficiency (eta) 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 eta than for prograde BHs. We self-consistently determine the amount of large-scale magnetic flux trapped by rapidly spinning (a = -0. Read More

Black hole (BH) accretion flows and jets are qualitatively affected by the presence of ordered magnetic fields. We study fully three-dimensional global general relativistic magnetohydrodynamic (MHD) simulations of radially extended and thick (height $H$ to cylindrical radius $R$ ratio of $|H/R|\sim 0.2--1$) accretion flows around BHs with various dimensionless spins ($a/M$, with BH mass $M$) and with initially toroidally-dominated ($\phi$-directed) and poloidally-dominated ($R-z$ directed) magnetic fields. Read More

We present an analytical solution for thin disk accretion onto a Kerr black hole that extends the standard Novikov-Thorne alpha-disk in three ways: (i) it incorporates nonzero stresses at the inner edge of the disk, (ii) it extends into the plunging region, and (iii) it uses a corrected vertical gravity formula. The free parameters of the model are unchanged. Nonzero boundary stresses are included by replacing the Novikov-Thorne no torque boundary condition with the less strict requirement that the fluid velocity at the innermost stable circular orbit is the sound speed, which numerical models show to be the correct behavior for luminosities below ~30% Eddington. Read More

The supermassive black hole candidate at the center of M87 drives an ultra-relativistic jet visible on kiloparsec scales, and its large mass and relative proximity allow for event horizon scale imaging with very long baseline interferometry at millimeter wavelengths (mm-VLBI). Recently, relativistic magneto-hydrodynamic (MHD) simulations of black hole accretion flows have proven capable of launching magnetically-dominated jets. We construct time-dependent disc/jet models of the innermost portion of the M87 nucleus by performing relativistic radiative transfer calculations from one such simulation. Read More

2011Sep
Affiliations: 1Purdue University, 2Stanford University

The "no hair" theorem, a key result in General Relativity, states that an isolated black hole is defined by only three parameters: mass, angular momentum, and electric charge; this asymptotic state is reached on a light-crossing time scale. We find that the "no hair" theorem is not formally applicable for black holes formed from collapse of a rotating neutron star. Rotating neutron stars can self-produce particles via vacuum breakdown forming a highly conducting plasma magnetosphere such that magnetic field lines are effectively "frozen-in" the star both before and during collapse. Read More

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 center, 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. Read More

The X-ray spectra of accretion discs of eight stellar-mass black holes have been analyzed to date using the thermal continuum fitting method, and the spectral fits have been used to estimate the spin parameters of the black holes. However, the underlying model used in this method of estimating spin is the general relativistic thin-disc model of Novikov & Thorne, which is only valid for razor-thin discs. We therefore expect errors in the measured values of spin due to inadequacies in the theoretical model. Read More

2010Nov
Affiliations: 1Department of Physics and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 2Center for Integrated Plasma Studies, Department of Physics, University of Colorado, Boulder

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. Read More

We obtain estimates of Sgr A* accretion flow and black hole parameters by fitting polarized sub-mm observations with spectra computed using three-dimensional (3D) general relativistic (GR) magnetohydrodynamical (MHD) (GRMHD) simulations. Observations are compiled from averages over many epochs from reports in 29 papers for estimating the mean fluxes Fnu, linear polarization (LP) fractions, circular polarization (CP) fractions, and electric vector position angles (EVPAs). GRMHD simulations are computed with dimensionless spins a_*=0,0. Read More

Magnetic reconnection, a fundamental plasma process associated with a rapid dissipation of magnetic energy, is believed to power many disruptive phenomena in laboratory plasma devices, the Earth magnetosphere, and the solar corona. Traditional reconnection research, geared towards these rather tenuous environments, has justifiably ignored the effects of radiation on the reconnection process. However, in many reconnecting systems in high-energy astrophysics (e. Read More

For the first time it has become possible to compare global 3D general relativistic magnetohydrodynamic (GRMHD) jet formation simulations directly to very-long baseline interferometric multi-frequency polarization observations of the pc-scale structure of active galactic nucleus (AGN) jets. Unlike the jet emission, which requires post hoc modeling of the non-thermal electrons, the Faraday rotation measures (RMs) depend primarily upon simulated quantities and thus provide a robust way in which to confront simulations with observations. We compute RM distributions of 3D global GRMHD jet formation simulations, with which we explore the dependence upon model and observational parameters, emphasizing the signatures of structures generic to the theory of MHD jets. Read More

Recent high resolution observations of the Galactic center black hole allow for direct comparison with accretion disk simulations. We compare two-temperature synchrotron emission models from three dimensional, general relativistic magnetohydrodynamic simulations to millimeter observations of Sgr A*. Fits to very long baseline interferometry and spectral index measurements disfavor the monochromatic face-on black hole shadow models from our previous work. Read More

The standard general relativistic model of a razor-thin accretion disk around a black hole, developed by Novikov & Thorne (NT) in 1973, assumes the shear stress vanishes at the radius of the innermost stable circular orbit (ISCO) and that, outside the ISCO, the shear stress is produced by an effective turbulent viscosity. However, astrophysical accretion disks are not razor-thin, it is uncertain whether the shear stress necessarily vanishes at the ISCO, and the magnetic field, which is thought to drive turbulence in disks, may contain large-scale structures that do not behave like a simple local scalar viscosity. We describe three-dimensional general relativistic magnetohydrodynamic simulations of accretion disks around black holes with a range of spin parameters, and we use the simulations to assess the validity of the NT model. Read More

The connection between collimation and acceleration of magnetized relativistic jets is discussed. The focus is on recent numerical simulations which shed light on some longstanding problems. Read More

2009Nov
Affiliations: 1Harvard-CfA, 2Harvard-CfA, 3Kavli Institute for Particle Astrophysics and Cosmology, Stanford University

Radio loud active galactic nuclei (AGN) are on average 1000 times brighter in the radio band compared to radio quiet AGN. We investigate whether this radio loud/quiet dichotomy can be due to differences in the spin of the central black holes that power the radio-emitting jets. Using general relativistic magnetohydrodynamic simulations, we construct steady state axisymmetric numerical models for a wide range of black hole spins (dimensionless spin parameter 0. Read More

2009Aug
Affiliations: 1Harvard-CfA, 2Harvard-CfA, 3Department of Physics and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University

Achromatic breaks in afterglow light curves of gamma-ray bursts (GRBs) arise naturally if the product of the jet's Lorentz factor \gamma and opening angle \Theta_j satisfies (\gamma \Theta_j) >> 1 at the onset of the afterglow phase, i.e., soon after the conclusion of the prompt emission. Read More

We have developed time-dependent models of FU Ori accretion outbursts to explore the physical properties of protostellar disks. Our two-dimensional, axisymmetric models incorporate full vertical structure with a new treatment of the radiative boundary condition for the disk photosphere. We find that FU Ori-type outbursts can be explained by a slow accumulation of matter due to gravitational instability. Read More

2009Jan
Affiliations: 1Harvard-CfA, 2Department of Physics and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 3Harvard-CfA

Unconfined relativistic outflows from rotating, magnetized compact objects are often well-modeled by assuming the field geometry is approximately a split-monopole at large radii. Earlier work has indicated that such an unconfined flow has an inefficient conversion of magnetic energy to kinetic energy. This has led to the conclusion that ideal magnetohydrodynamical (MHD) processes fail to explain observations of, e. Read More

2008Dec
Affiliations: 1Department of Physics and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 2Department of Physics and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University
Category: Astrophysics

Rotating magnetized compact objects and their accretion discs can generate strong toroidal magnetic fields driving highly magnetized plasmas into relativistic jets. Of significant concern, however, has been that a strong toroidal field in the jet should be highly unstable to the non-axisymmetric helical kink (screw) $m=1$ mode leading to rapid disruption. In addition, a recent concern has been that the jet formation process itself may be unstable due to the accretion of non-dipolar magnetic fields. Read More

2008Aug
Affiliations: 1Harvard University, Department of Physics, 2Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 3Harvard-Smithsonian Center for Astrophysics, 4Harvard-Smithsonian Center for Astrophysics, 5Center for Theoretical Astrophysics, University of Illinois at Urbana-Champaign, 6Harvard-Smithsonian Center for Astrophysics
Category: Astrophysics

We describe three-dimensional general relativistic magnetohydrodynamic simulations of a geometrically thin accretion disk around a non-spinning black hole. The disk has a thickness $h/r\sim0.05-0. Read More

2008Mar
Affiliations: 1Harvard CfA/ITC, 2Stanford University/KIPAC, 3Harvard CfA/ITC
Category: Astrophysics

Long-duration gamma-ray bursts (GRBs) require an engine capable of driving a jet of plasma to ultrarelativistic bulk Lorentz factors of up to several hundred and into narrow opening angles of a few degrees. We use global axisymmetric stationary solutions of magnetically-dominated (force-free) ultrarelativistic jets to test whether the popular magnetic-driving paradigm can generate the required Lorentz factors and opening angles. Our global solutions are obtained via time-dependent relativistic ideal magnetodynamical numerical simulations which follow the jet from the central engine to beyond six orders of magnitude in radius. Read More

Active galactic nuclei, x-ray binaries, pulsars, and gamma-ray bursts are all believed to be powered by compact objects surrounded by relativistic plasma flows driving phenomena such as accretion, winds, and jets. These flows are often accurately modelled by the relativistic magnetohydrodynamics (MHD) approximation. Time-dependent numerical MHD simulations have proven to be especially insightful, but one regime that remains difficult to simulate is when the energy scales (kinetic, thermal, magnetic) within the plasma become disparate. Read More

The role of the equation of state for a perfectly conducting, relativistic magnetized fluid is the main subject of this work. The ideal constant $\Gamma$-law equation of state, commonly adopted in a wide range of astrophysical applications, is compared with a more realistic equation of state that better approximates the single-specie relativistic gas. The paper focus on three different topics. Read More

2006Oct
Affiliations: 1Harvard University, 2Institute for Theory and Computation, 3Harvard University
Category: Astrophysics

We consider a self-similar force-free wind flowing out of an infinitely thin disk located in the equatorial plane. On the disk plane, we assume that the magnetic stream function $P$ scales as $P\propto R^\nu$, where $R$ is the cylindrical radius. We also assume that the azimuthal velocity in the disk is constant: $v_\phi = Mc$, where $M<1$ is a constant. Read More

2006Jul
Affiliations: 1Institute for Theory and Computation, 2Institute for Theory and Computation
Category: Astrophysics

In paper I, we showed that time-dependent general relativistic magnetohydrodynamic (GRMHD) numerical models of accretion disks, although being highly turbulent, have surprisingly simple electromagnetic properties. In particular, the toroidal current density in the disk takes the form $dI_\phi/dr \propto r^{-5/4}$. Guided by this simplicity, we use a time-dependent general relativistic force-free electrodynamics (GRFFE) code to study an idealized problem in which the accretion disk is replaced by an infinitely thin rotating equatorial current sheet. Read More

2006Jul
Affiliations: 1Institute for Theory and Computation, 2Institute for Theory and Computation
Category: Astrophysics

General relativistic numerical simulations of magnetized accretion flows around black holes show a disordered electromagnetic structure in the disk and corona and a highly relativistic, Poynting-dominated funnel jet in the polar regions. The polar jet is nearly consistent with the stationary paraboloidal Blandford-Znajek model of an organized field threading the polar regions of a rotating black hole. How can a disordered accretion disk and corona lead to an ordered jet? We show that the polar jet is associated with a strikingly simple angular-integrated toroidal current distribution $dI_\phi/dr \propto r^{-5/4}$, where $I_\phi(r)$ is the toroidal current enclosed inside radius $r$. Read More

2006Mar
Affiliations: 1Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics
Category: Astrophysics

The formation and large-scale propagation of Poynting-dominated jets produced by accreting, rapidly rotating black hole systems are studied by numerically integrating the general relativistic magnetohydrodynamic equations of motion to follow the self-consistent interaction between accretion disks and black holes. This study extends previous similar work by studying jets till $t\approx 10^4GM/c^3$ out to $r\approx 10^4GM/c^2$, by which the jet is super- fast magnetosonic and moves at a lab-frame bulk Lorentz factor of $\Gamma\sim 10$ with a maximum terminal Lorentz factor of $\Gamma_\infty\lesssim 10^3$. The radial structure of the Poynting-dominated jet is piece-wise self-similar, and fits to flow quantities along the field line are provided. Read More

2006Jan
Affiliations: 1Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics
Category: Astrophysics

The luminosity and structure of neutron star magnetospheres are crucial to our understanding of pulsar and plerion emission. A solution found using the force-free approximation would be an interesting standard with which any model with more physics could be compared. Prior quasi-analytic force-free solutions may not be stable, while prior time-dependent magnetohydrodynamic models used unphysical model parameters. Read More

2006Jan
Affiliations: 1Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics
Category: Astrophysics

The force-free limit of magnetohydrodynamics (MHD) is often a reasonable approximation to model black hole and neutron star magnetospheres. We describe a general relativistic force-free (GRFFE) formulation that allows general relativistic magnetohydrodynamic (GRMHD) codes to directly evolve the GRFFE equations of motion. Established, accurate, and well-tested conservative GRMHD codes can simply add a new inversion piece of code to their existing code, while continuing to use all the already-developed facilities present in their GRMHD code. Read More

Conservative numerical schemes for general relativistic magnetohydrodynamics (GRMHD) require a method for transforming between ``conserved'' variables such as momentum and energy density and ``primitive'' variables such as rest-mass density, internal energy, and components of the four-velocity. The forward transformation (primitive to conserved) has a closed-form solution, but the inverse transformation (conserved to primitive) requires the solution of a set of five nonlinear equations. Here we discuss the mathematical properties of the inverse transformation and present six numerical methods for performing the inversion. Read More