Scale-invariant radio jets and varying black hole spin

Compact radio cores associated with relativistic jets are often observed in both active galactic nuclei and X-ray binaries. Their radiative properties follow some general scaling laws which primarily depend on their masses and accretion rates. However, it has been suggested that the black hole spin can also strongly influence the power and radio flux of these. Here, we attempt to estimate the dependency of the radio luminosity of steady jets launched by accretion disks on black hole mass, accretion rate and spin using numerical simulations. We make use of 3D GRMHD simulations of accretion disks around low-luminosity black holes in which the jet radio emission is produced by the jet sheath. We find that the radio flux increases roughly by a factor of 6 as the back hole spin increases from a~0 to a=0.98. This is comparable to the increase in accretion power with spin, meaning that the ratio between radio jet and accretion power is hardly changing. Although our jet spine power scales as expected for the Blandford-Znajek process, the dependency of jet radio luminosity on the black hole spin is somewhat weaker. Also weakly rotating black holes can produce visible radio jets. The overall scaling of the radio emission with black hole mass and accretion rate is consistent with the scale-invariant analytical models used to explain the fundamental plane of black hole activity. Spin does not introduce a significant scatter in this model. The jet-sheath model can describe well resolved accreting systems, such as SgrA* and M87, as well as the general scaling behavior of low-luminosity black holes. Hence the model should be applicable to a wide range of radio jets in sub-Eddington black holes. The black hole spin has an effect on the production of visible radio jet, but it may not be the main driver to produce visible radio jets. An extension of our findings to powerful quasars remains speculative.Comment: 10 pages, 6 figures, A&A accepte

rHARM: Accretion and Ejection in Resistive GR-MHD

Turbulent magnetic diffusivity plays an important role for accretion disks and the launching of disk winds. We have implemented magnetic diffusivity, respective resistivity in the general relativistic MHD code HARM. This paper describes the theoretical background of our implementation, its numerical realization, our numerical tests and preliminary applications. The test simulations of the new code rHARM are compared with an analytic solution of the diffusion equation and a classical shock tube problem. We have further investigated the evolution of the magneto-rotational instability (MRI) in tori around black holes for a range of magnetic diffusivities. We find indication for a critical magnetic diffusivity (for our setup) beyond which no MRI develops in the linear regime and for which accretion of torus material to the black hole is delayed. Preliminary simulations of magnetically diffusive thin accretion disks around Schwarzschild black holes that are threaded by a large-scale poloidal magnetic field show the launching of disk winds with mass fluxes of about 50% of the accretion rate. The disk magnetic diffusivity allows for efficient disk accretion that replenishes the mass reservoir of the inner disk area and thus allows for long-term simulations of wind launching for more than 5000 time units.Comment: 21 pages, 43 figures, accepted by Ap

A Numerical Study of Relativistic Fluid Collapse

We investigate the dynamics of self-gravitating, spherically-symmetric distributions of fluid through numerical means. In particular, systems involving neutron star models driven far from equilibrium in the strong-field regime of general relativity are studied. Hydrostatic solutions of Einstein's equations using a stiff, polytropic equation of state are used for the stellar models. Many of the scenarios we examine involve highly-relativistic flows that require improvements upon previously published numerical methods to simulate. Here our particular focus is on the physical behavior of the coupled fluid-gravitational system at the threshold of black hole formation--so-called black hole critical phenomena. To investigate such phenomena starting from conditions representing stable stars, we must drive the star far from its initial stable configuration. We use one of two different mechanisms to do this: setting the initial velocity profile of the star to be in-going, or collapsing a shell of massless scalar field onto the star. Both of these approaches give rise to a large range of dynamical scenarios that the star may follow. These scenarios have been extensively surveyed by using different initial star solutions, and by varying either the magnitude of the velocity profile or the amplitude of the scalar field pulse. In addition to illuminating the critical phenomena associated with the fluid collapse, the resulting phase diagram of possible outcomes provides an approximate picture of the stability of neutron stars to large, external perturbations that may occur in nature.Comment: 228 pages, 66 Postscript figures, Ph.D. Thesis, the University of Texa s at Austin, uses utdiss2.sty v

Approximate black hole binary spacetime via asymptotic matching

We construct a fully analytic, general relativistic, nonspinning black hole binary spacetime that approximately solves the vacuum Einstein equations everywhere in space and time for black holes sufficiently well separated. The metric is constructed by asymptotically matching perturbed Schwarzschild metrics near each black hole to a two-body post-Newtonian metric far from them, and a two-body post-Minkowskian metric farther still. Asymptotic matching is done without linearizing about a particular time slice, and thus it is valid dynamically and for all times, provided the binary is sufficiently well separated. This approximate global metric can be used for long dynamical evolutions of relativistic magnetohydrodynamical, circumbinary disks around inspiraling supermassive black holes to study a variety of phenomena.Comment: 17 pages, 8 figures, 1 table. Appendix added to match published versio