arXiv subjects

Kris Beckwith

Publications and source records attributed to Kris Beckwith.

26 records · Page 2Linked to original sources

arXiv2008

Simulations of Magnetorotational Turbulence with a Higher-Order Godunov Scheme

(abridged) We apply a second-order Godunov code, Athena, to studies of the magnetorotational instability using unstratified shearing box simulations with a uniform net vertical field and a sinusoidally varying zero net vertical field. The Athena results agree well with similar studies that used different numerical algorithms. We conduct analyses to study the flow of energy from differential rotation to turbulent fluctuations to thermalization. A study of the temporal correlation between the time derivatives of volume-averaged energy components shows that energy injected into turbulent fluctuations dissipates on a timescale of $Ω^{-1}$, where $Ω$ is the orbital frequency of the local domain. Magnetic dissipation dominates over kinetic dissipation, although not by as great a factor as the ratio of magnetic to kinetic energy. We Fourier-transform the magnetic and kinetic energy evolution equations and, using the assumption that the time-averaged energies are constant, determine the level of numerical dissipation as a function of length scale and resolution. By modeling numerical dissipation as if it were physical in origin, we characterize numerical resistivity and viscosity in terms of effective Reynolds and Prandtl numbers. The resulting effective magnetic Prandtl number is $\sim 2$, independent of resolution or initial field geometry. MRI simulations with effective Reynolds and Prandtl numbers determined by numerical dissipation are not equivalent to those where these numbers are set by physical resistivity and viscosity. These results serve, then, as a baseline for future shearing box studies where dissipation is controlled by the inclusion of explicit viscosity and resistivity.

astro-ph↗

arXiv2008

Where is the Radiation Edge in Magnetized Black Hole Accretion discs?

General Relativistic (GR) Magnetohydrodynamic (MHD) simulations of black hole accretion find significant magnetic stresses near and inside the innermost stable circular orbit (ISCO), suggesting that such flows could radiate in a manner noticeably different from the prediction of the standard model, which assumes that there are no stresses in that region. We provide estimates of how phenomenologically interesting parameters like the ``radiation edge", the innermost ring of the disc from which substantial thermal radiation escapes to infinity, may be altered by stresses near the ISCO. These estimates are based on data from a large number of three-dimensional GRMHD simulations combined with GR ray-tracing. For slowly spinning black holes ($a/M<0.9$), the radiation edge lies well inside where the standard model predicts, particularly when the system is viewed at high inclination. For more rapidly spinning black holes, the contrast is smaller. At fixed total luminosity, the characteristic temperature of the accretion flow increases between a factor of $1.2-2.4$ over that predicted by the standard model, whilst at fixed mass accretion rate, there is a corresponding enhancement of the accretion luminosity which may be anywhere from tens of percent to order unity. When all these considerations are combined, we find that, for fixed black hole mass, luminosity, and inclination angle, our uncertainty in the characteristic temperature of the radiation reaching distant observers due to uncertainty in dissipation profile (around a factor of 3) is {\it greater} than the uncertainty due to a complete lack of knowledge of the black hole's spin (around a factor of 2) and furthermore that spin estimates based on the stress-free inner boundary condition provide an upper limit to $a/M$.

astro-ph↗

arXiv2008

The Influence of Magnetic Field Geometry on the Evolution of Black Hole Accretion Flows: Similar Disks, Drastically Different Jets

Because the magneto-rotational instability is capable of exponentially amplifying weak preexisting magnetic fields, it might be hoped that the character of the magnetic field in accretion disks is independent of the nature of the seed field. However, the divergence-free nature of magnetic fields in highly conducting fluids ensures that their large-scale topology is preserved, no matter how greatly the field intensity is changed. By performing global two-dimensional and three-dimensional general relativistic magnetohydrodynamic disk simulations with several different topologies for the initial magnetic field, we explore the degree to which the character of the flows around black holes depends on the initial topology. We find that while the qualitative properties of the accretion flow are nearly independent of field topology, jet-launching is very sensitive to it: a sense of vertical field consistent for at least an inner disk inflow time is essential to the support of strong jets.

astro-ph↗

arXiv2006

Estimating the Radiative Efficiency of Magnetized Accretion Disks Around Black Holes

Simulations of black hole accretion have shown that magnetic stresses are present near and inside the innermost stable circular orbit (ISCO). This finding suggests that such flows may be more luminous than predicted by the standard accretion disk model. Here we apply a prescription for heat dissipation within the simulated accretion flows to estimate their implied radiative efficiency. We assume that dissipation is proportional to the current density squared, and find that the resulting azimuthally-averaged and shell-integrated radial profile is well-matched to the radial heat dissipation profile of the standard disk model for the region outside the ISCO, particularly when it is adjusted to account for additional stress at the ISCO. In contrast to the standard model, however, the dissipation profile derived from the current density continues past the ISCO and through the plunging region. The total predicted dissipation rate is between $\simeq 30%$ and $\simeq 100%$ greater than that predicted by the standard model, depending on the black hole spin. Most of the additional dissipation takes place just outside the ISCO. To predict luminosities, we assume instantaneous radiation and zero optical depth, but allow for photon capture. The net radiative efficiency seen by a distant observer is increased relative to the standard model by $\simeq 25%$--80%, with the largest fractional increase for intermediate black hole spins because the increase in dissipation from enhanced stress that occurs for rapid spin is partially offset by the increased likelihood that the additional photons will be captured by the hole.

astro-ph↗

arXiv2004

Extreme Gravitational Lensing near Rotating Black Holes

We describe a new approach to calculating photon trajectories and gravitational lensing effects in the strong gravitational field of the Kerr black hole. These techniques are applied to explore both the imaging and spectral properties of photons that perform multiple orbits of the central mass before escaping to infinity. Viewed at large inclinations, these higher order photons contribute $\sim 20 %$ of the total luminosity of the system for a Schwarzschild hole, whilst for an extreme Kerr black hole this fraction rises to $\sim 60 %$. In more realistic models these photons will be re-absorbed by the disc at large distances from the hole, but this returning radiation could provide a physical mechanism to resolve the discrepancy between the predicted and observed optical/UV colours in AGN. Conversely, at low inclinations, higher order images re-intercept the disc plane close to the black hole, so need not be absorbed by the disc if this is within the plunging region. These photons form a bright ring carrying approximately 10% of the total disc luminosity for a Schwarzchild black hole. The spatial separation between the inner edge of the disc and the ring is similar to the size of the event horizon. This is resolvable for supermassive black holes with proposed X-ray interferometery missions such as MAXIM, so has the potential to provide an observational test of strong field gravity.

astro-ph↗

arXiv2004

Observing braneworld black holes

Spacetime in the vicinity of an event horizon can be probed using observations which explore the dynamics of the accretion disc. Many high energy theories of gravity lead to modifications of the near horizon regime, potentially providing a testing ground for these theories. In this paper, we explore the impact of braneworld gravity on this region by formulating a method of deriving the general behaviour of the as yet unknown braneworld black hole solution. We use simple bounds to constrain the solution close to the horizon.

hep-th↗

arXiv2004

Iron Line Profiles in Strong Gravity

We describe a new code which can accurately calculate the relativistic effects which distort the emission from an accretion disc around a black hole. We compare our results for a disk which extends from the innermost stable orbit to $20r_{g}$ in both Schwarzschild and maximal ($a=0.998$) Kerr spacetimes with the two line profile codes which are on general release in the XSPEC spectral fitting package. These models generally give a very good description of the relativistic smearing of the line for this range of radii. However, these models have some limitations. In particular we show that the assumed form of the {\em angular} emissivity law (limb darkening or brightening) can make significant changes to the derived line profile where lightbending is important. This is {\em always} the case for extreme Kerr spacetimes or high inclination systems, where the observed line is produced from a very large range of different emitted angles. In these situations the assumed angular emissivity can affect the derived {\em radial} emissivity. The line profile is not simply determined by the well defined (but numerically difficult) physical effects of strong gravity, but is also dependent on the poorly known astrophysics of the disc emission.

astro-ph↗

arXiv2004

Accurate Modelling of Relativistic Iron Lines from Accretion Discs

Observations of fluorescent iron lines from accreting black holes provide one of the best tests of strong field gravity available to date, and the only current observational tool to probe black hole spacetime. However, the two most widely used models for spectral fitting (diskline, laor) are over a decade old and have significant limitations. We present a new code for calculating these effects which will be incorporated within the XSPEC package

astro-ph↗