392 research outputs found
Non-axisymmetric relativistic Bondi-Hoyle accretion onto a Kerr black hole
In our program of studying numerically the so-called Bondi-Hoyle accretion in
the fully relativistic regime, we present here first results concerning the
evolution of matter accreting supersonically onto a rotating (Kerr) black hole.
These computations generalize previous results where the non-rotating
(Schwarzschild) case was extensively considered. We parametrize our initial
data by the asymptotic conditions for the fluid and explore the dependence of
the solution on the angular momentum of the black hole. Towards quantifying the
robustness of our numerical results, we use two different geometrical
foliations of the black hole spacetime, the standard form of the Kerr metric in
Boyer-Lindquist coordinates as well as its Kerr-Schild form, which is free of
coordinate singularities at the black hole horizon. We demonstrate some
important advantages of using such horizon adapted coordinate systems.
Our numerical study indicates that regardless of the value of the black hole
spin the final accretion pattern is always stable, leading to constant
accretion rates of mass and momentum. The flow is characterized by a strong
tail shock, which, unlike the Schwarzschild case, is increasingly wrapped
around the central black hole as the hole angular momentum increases. The
rotation induced asymmetry in the pressure field implies that besides the well
known drag, the black hole will experience also a lift normal to the flow
direction. This situation exhibits some analogies with the Magnus effect of
classical fluid dynamics.Comment: 33 pages, 20 figures, submited to MNRA
Magnetised Polish doughnuts revisited
We discuss a procedure to build new sequences of magnetised, equilibrium tori
around Kerr black holes which combines two approaches previously considered in
the literature. For simplicity we assume that the test-fluid approximation
holds, and hence we neglect the self-gravity of the fluid. The models are built
assuming a particular form of the angular momentum distribution from which the
location and morphology of equipotential surfaces can be computed. This ansatz
includes, in particular, the constant angular momentum case originally employed
in the construction of thick tori - or Polish doughnuts - and it has already
been used to build equilibrium sequences of purely hydrodynamical models. We
discuss the properties of the new models and their dependence on the initial
parameters. These new sequences can be used as initial data for
magnetohydrodynamical evolutions in general relativity.Comment: 9 pages, 6 figures. Accepted for publication in Astronomy &
Astrophysics July 12, 201
Robustness of a high-resolution central scheme for hydrodynamic simulations in full general relativity
A recent paper by Lucas-Serrano et al. indicates that a high-resolution
central (HRC) scheme is robust enough to yield accurate hydrodynamical
simulations of special relativistic flows in the presence of ultrarelativistic
speeds and strong shock waves. In this paper we apply this scheme in full
general relativity (involving {\it dynamical} spacetimes), and assess its
suitability by performing test simulations for oscillations of rapidly rotating
neutron stars and merger of binary neutron stars. It is demonstrated that this
HRC scheme can yield results as accurate as those by the so-called
high-resolution shock-capturing (HRSC) schemes based upon Riemann solvers.
Furthermore, the adopted HRC scheme has increased computational efficiency as
it avoids the costly solution of Riemann problems and has practical advantages
in the modeling of neutron star spacetimes. Namely, it allows simulations with
stiff equations of state by successfully dealing with very low-density
unphysical atmospheres. These facts not only suggest that such a HRC scheme may
be a desirable tool for hydrodynamical simulations in general relativity, but
also open the possibility to perform accurate magnetohydrodynamical simulations
in curved dynamic spacetimes.Comment: 4 pages, to be published in Phys. Rev. D (brief report
Measuring the black hole spin direction in 3D Cartesian numerical relativity simulations
We show that the so-called flat-space rotational Killing vector method for
measuring the Cartesian components of a black hole spin can be derived from the
surface integral of Weinberg's pseudotensor over the apparent horizon surface
when using Gaussian normal coordinates in the integration. Moreover, the
integration of the pseudotensor in this gauge yields the Komar angular momentum
integral in a foliation adapted to the axisymmetry of the spacetime. As a
result, the method does not explicitly depend on the evolved lapse and
shift on the respective timeslice, as they are fixed to Gaussian
normal coordinates, while leaving the coordinate labels of the spatial metric
and the extrinsic curvature unchanged. Such gauge fixing
endows the method with coordinate invariance, which is not present in integral
expressions using Weinberg's pseudotensor, as they normally rely on the
explicit use of Cartesian coordinates
Towards asteroseismology of core-collapse supernovae with gravitational-wave observations - I. Cowling approximation
Gravitational waves from core-collapse supernovae are produced by the
excitation of different oscillation modes in the proto-neutron star (PNS) and
its surroundings, including the shock. In this work we study the relationship
between the post-bounce oscillation spectrum of the PNS-shock system and the
characteristic frequencies observed in gravitational-wave signals from
core-collapse simulations. This is a fundamental first step in order to develop
a procedure to infer astrophysical parameters of the PNS formed in
core-collapse supernovae. Our method combines information from the oscillation
spectrum of the PNS, obtained through linear-perturbation analysis in general
relativity of a background physical system, with information from the
gravitational-wave spectrum of the corresponding non-linear, core-collapse
simulation. Using results from the simulation of the collapse of a 35
presupernova progenitor we show that both types of spectra are
indeed related and we are able to identify the modes of oscillation of the PNS,
namely g-modes, p-modes, hybrid modes, and standing-accretion-shock-instability
(SASI) modes, obtaining a remarkably close correspondence with the
time-frequency distribution of the gravitational-wave modes. The analysis
presented in this paper provides a proof-of-concept that asteroseismology is
indeed possible in the core-collapse scenario, and it may serve as a basis for
future work on PNS parameter inference based on gravitational-wave
observations
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