2,440 research outputs found

### 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

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