492 research outputs found
General Relativistic Simulations of Magnetized Plasmas around Merging Supermassive Black Holes
Coalescing supermassive black hole binaries are produced by the mergers of
galaxies and are the most powerful sources of gravitational waves accessible to
space-based gravitational observatories. Some such mergers may occur in the
presence of matter and magnetic fields and hence generate an electromagnetic
counterpart. In this Letter, we present the first general relativistic
simulations of magnetized plasma around merging supermassive black holes using
the general relativistic magnetohydrodynamic code Whisky. By considering
different magnetic field strengths, going from non-magnetically dominated to
magnetically dominated regimes, we explore how magnetic fields affect the
dynamics of the plasma and the possible emission of electromagnetic signals. In
particular we observe a total amplification of the magnetic field of ~2 orders
of magnitude which is driven by the accretion onto the binary and that leads to
much stronger electromagnetic signals, more than a factor of 10^4 larger than
comparable calculations done in the force-free regime where such amplifications
are not possible.Comment: 7 pages, 5 figures. Minor changes to match version accepted for
publication on The Astrophysical Journal Letter
Accurate evolutions of inspiralling neutron-star binaries: assessment of the truncation error
We have recently presented an investigation in full general relativity of the
dynamics and gravitational-wave emission from binary neutron stars which
inspiral and merge, producing a black hole surrounded by a torus (see
arXiv:0804.0594). We here discuss in more detail the convergence properties of
the results presented in arXiv:0804.0594 and, in particular, the deterioration
of the convergence rate at the merger and during the survival of the merged
object, when strong shocks are formed and turbulence develops. We also show
that physically reasonable and numerically convergent results obtained at
low-resolution suffer however from large truncation errors and hence are of
little physical use. We summarize our findings in an "error budget", which
includes the different sources of possible inaccuracies we have investigated
and provides a first quantitative assessment of the precision in the modelling
of compact fluid binaries.Comment: 13 pages, 5 figures. Minor changes to match published version. Added
figure 5 right pane
WhiskyMHD: a new numerical code for general relativistic magnetohydrodynamics
The accurate modelling of astrophysical scenarios involving compact objects
and magnetic fields, such as the collapse of rotating magnetized stars to black
holes or the phenomenology of gamma-ray bursts, requires the solution of the
Einstein equations together with those of general-relativistic
magnetohydrodynamics. We present a new numerical code developed to solve the
full set of general-relativistic magnetohydrodynamics equations in a dynamical
and arbitrary spacetime with high-resolution shock-capturing techniques on
domains with adaptive mesh refinements. After a discussion of the equations
solved and of the techniques employed, we present a series of testbeds carried
out to validate the code and assess its accuracy. Such tests range from the
solution of relativistic Riemann problems in flat spacetime, over to the
stationary accretion onto a Schwarzschild black hole and up to the evolution of
oscillating magnetized stars in equilibrium and constructed as consistent
solutions of the coupled Einstein-Maxwell equations.Comment: minor changes to match the published versio
Local simulations of the magnetized Kelvin-Helmholtz instability in neutron-star mergers
Context. Global MHD simulations show Kelvin-Helmholtz (KH) instabilities at
the contact surface of two merging neutron stars. That region has been
identified as the site of efficient amplification of magnetic fields. However,
these global simulations, due to numerical limitations, were unable to
determine the saturation level of the field strength, and thus the possible
back-reaction of the magnetic field onto the flow. Aims. We investigate the
amplification of initially weak fields in KH unstable shear flows, and the
back-reaction of the field onto the flow. Methods. We use a high-resolution
ideal MHD code to perform 2D and 3D local simulations of shear flows. Results.
In 2D, the magnetic field is amplified in less than 0.01ms until it reaches
locally equipartition with the kinetic energy. Subsequently, it saturates due
to resistive instabilities that disrupt the KH vortex and decelerate the shear
flow on a secular time scale. We determine scaling laws of the field
amplification with the initial field strength and the grid resolution. In 3D,
this hydromagnetic mechanism may be dominated by purely hydrodynamic
instabilities limiting the amplification. We find maximum magnetic fields of
10^16 G locally, and r.m.s. maxima within the box of 10^15 G. However, such
strong fields exist only for a short period. In the saturated state, the
magnetic field is mainly oriented parallel to the shear flow for strong initial
fields, while weaker initial fields tend to lead to a more balanced
distribution of the field energy. In all models the flow shows small-scale
features. The magnetic field is at most in equipartition with the decaying
shear flow. (abridged)Comment: 26 pages, 22 figures (figure quality reduced); accepted for
publication in Astronomy & Astrophysic
An improved formulation of the relativistic hydrodynamics equations in 2D Cartesian coordinates
A number of astrophysical scenarios possess and preserve an overall
cylindrical symmetry also when undergoing a catastrophic and nonlinear
evolution. Exploiting such a symmetry, these processes can be studied through
numerical-relativity simulations at smaller computational costs and at
considerably larger spatial resolutions. We here present a new
flux-conservative formulation of the relativistic hydrodynamics equations in
cylindrical coordinates. By rearranging those terms in the equations which are
the sources of the largest numerical errors, the new formulation yields a
global truncation error which is one or more orders of magnitude smaller than
those of alternative and commonly used formulations. We illustrate this through
a series of numerical tests involving the evolution of oscillating spherical
and rotating stars, as well as shock-tube tests.Comment: 19 pages, 9 figure
Accurate evolutions of inspiralling neutron-star binaries: prompt and delayed collapse to black hole
Binary neutron-star (BNS) systems represent primary sources for the
gravitational-wave (GW) detectors. We present a systematic investigation in
full GR of the dynamics and GW emission from BNS which inspiral and merge,
producing a black hole (BH) surrounded by a torus. Our results represent the
state of the art from several points of view: (i) We use HRSC methods for the
hydrodynamics equations and high-order finite-differencing techniques for the
Einstein equations; (ii) We employ AMR techniques with "moving boxes"; (iii) We
use as initial data BNSs in irrotational quasi-circular orbits; (iv) We exploit
the isolated-horizon formalism to measure the properties of the BHs produced in
the merger; (v) Finally, we use two approaches, based either on gauge-invariant
perturbations or on Weyl scalars, to calculate the GWs. These techniques allow
us to perform accurate evolutions on timescales never reported before (ie ~30
ms) and to provide the first complete description of the inspiral and merger of
a BNS leading to the prompt or delayed formation of a BH and to its ringdown.
We consider either a polytropic or an ideal fluid EOS and show that already
with this idealized EOSs a very interesting phenomenology emerges. In
particular, we show that while high-mass binaries lead to the prompt formation
of a rapidly rotating BH surrounded by a dense torus, lower-mass binaries give
rise to a differentially rotating NS, which undergoes large oscillations and
emits large amounts of GWs. Eventually, also the NS collapses to a rotating BH
surrounded by a torus. Finally, we also show that the use of a non-isentropic
EOS leads to significantly different evolutions, giving rise to a delayed
collapse also with high-mass binaries, as well as to a more intense emission of
GWs and to a geometrically thicker torus.Comment: 35 pages, 29 figures, corrected few typos to match the published
version. High-resolution figures and animations can be found at
http://numrel.aei.mpg.de/Visualisations/Archive/BinaryNeutronStars/Relativistic_Meudon/index.htm
On the Shear Instability in Relativistic Neutron Stars
We present new results on instabilities in rapidly and differentially
rotating neutron stars. We model the stars in full general relativity and
describe the stellar matter adopting a cold realistic equation of state based
on the unified SLy prescription. We provide evidence that rapidly and
differentially rotating stars that are below the expected threshold for the
dynamical bar-mode instability, beta_c = T/|W| ~ 0.25, do nevertheless develop
a shear instability on a dynamical timescale and for a wide range of values of
beta. This class of instability, which has so far been found only for small
values of beta and with very small growth rates, is therefore more generic than
previously found and potentially more effective in producing strong sources of
gravitational waves. Overall, our findings support the phenomenological
predictions made by Watts, Andersson and Jones on the nature of the low-T/|W|.Comment: 20 pages; accepted to the Classical and Quantum Gravity special issue
for MICRA200
Critical Phenomena in Neutron Stars I: Linearly Unstable Nonrotating Models
We consider the evolution in full general relativity of a family of linearly
unstable isolated spherical neutron stars under the effects of very small,
perturbations as induced by the truncation error. Using a simple ideal-fluid
equation of state we find that this system exhibits a type-I critical
behaviour, thus confirming the conclusions reached by Liebling et al. [1] for
rotating magnetized stars. Exploiting the relative simplicity of our system, we
are able carry out a more in-depth study providing solid evidences of the
criticality of this phenomenon and also to give a simple interpretation of the
putative critical solution as a spherical solution with the unstable mode being
the fundamental F-mode. Hence for any choice of the polytropic constant, the
critical solution will distinguish the set of subcritical models migrating to
the stable branch of the models of equilibrium from the set of subcritical
models collapsing to a black hole. Finally, we study how the dynamics changes
when the numerically perturbation is replaced by a finite-size, resolution
independent velocity perturbation and show that in such cases a nearly-critical
solution can be changed into either a sub or supercritical. The work reported
here also lays the basis for the analysis carried in a companion paper, where
the critical behaviour in the the head-on collision of two neutron stars is
instead considered [2].Comment: 15 pages, 9 figure
Radio precursors to neutron star binary mergings
We discuss a possible generation of radio bursts preceding final stages of
binary neutron star mergings which can be accompanied by short gamma-ray
bursts. Detection of such bursts appear to be advantageous in the low-frequency
radio band due to a time delay of ten to several hundred seconds required for
radio signal to propagate in the ionized intergalactic medium. This delay makes
it possible to use short gamma-ray burst alerts to promptly monitor specific
regions on the sky by low-frequency radio facilities, especially by LOFAR. To
estimate the strength of the radio signal, we assume a power-law dependence of
the radio luminosity on the total energy release in a magnetically dominated
outflow, as found in millisecond pulsars. Based on the planned LOFAR
sensitivity at 120 MHz, we estimate that the LOFAR detection rate of such radio
transients could be about several events per month from redshifts up to
in the most optimistic scenario. The LOFAR ability to detect such
events would crucially depend on exact efficiency of low-frequency radio
emission mechanism.Comment: 6 pages, 2 figures, Accepted for publication in Astrophysics & Space
Science. Largely extended version of ArXiv:0912.521
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