457 research outputs found

### Multimessenger Parameter Estimation of GW170817

We combine gravitational wave (GW) and electromagnetic (EM) data to perform a
Bayesian parameter estimation of the binary neutron star (NS) merger GW170817.
The EM likelihood is constructed from a fit to a large number of numerical
relativity simulations which we combine with a lower bound on the mass of the
remnant's accretion disk inferred from the modeling of the EM light curve. In
comparison with previous works, our analysis yields a more precise
determination of the tidal deformability of the binary, for which the EM data
provide a lower bound, and of the mass ratio of the binary, with the EM data
favoring a smaller mass asymmetry. The 90\% credible interval for the areal
radius of a $1.4\ M_\odot$ NS is found to be $12.2^{+1.0}_{-0.8} \pm 0.2\ {\rm
km}$ (statistical and systematic uncertainties).Comment: 7 pages, 3 figures, accepted to the EPJA Topical Issue: The first
Neutron Star Merger Observation - Implications for Nuclear Physic

### Discontinuous Galerkin methods for general-relativistic hydrodynamics: formulation and application to spherically symmetric spacetimes

We have developed the formalism necessary to employ the
discontinuous-Galerkin approach in general-relativistic hydrodynamics. The
formalism is firstly presented in a general 4-dimensional setting and then
specialized to the case of spherical symmetry within a 3+1 splitting of
spacetime. As a direct application, we have constructed a one-dimensional code,
EDGES, which has been used to asses the viability of these methods via a series
of tests involving highly relativistic flows in strong gravity. Our results
show that discontinuous Galerkin methods are able not only to handle strong
relativistic shock waves but, at the same time, to attain very high orders of
accuracy and exponential convergence rates in smooth regions of the flow. Given
these promising prospects and their affinity with a pseudospectral solution of
the Einstein equations, discontinuous Galerkin methods could represent a new
paradigm for the accurate numerical modelling in relativistic astrophysics.Comment: 24 pages, 19 figures. Small changes; matches version to appear in PR

### Universality and intermittency in relativistic turbulent flows of a hot plasma

With the aim of determining the statistical properties of relativistic
turbulence and unveiling novel and non-classical features, we resent the
results of direct numerical simulations of driven turbulence in an
ultrarelativistic hot plasma using high-order numerical schemes. We study the
statistical properties of flows with average Mach number ranging from $\sim
0.4$ to $\sim 1.7$ and with average Lorentz factors up to $\sim 1.7$. We find
that flow quantities, such as the energy density or the local Lorentz factor,
show large spatial variance even in the subsonic case as compressibility is
enhanced by relativistic effects. The velocity field is highly intermittent,
but its power-spectrum is found to be in good agreement with the predictions of
the classical theory of Kolmogorov. Overall, our results indicate that
relativistic effects are able to significantly enhance the intermittency of the
flow and affect the high-order statistics of the velocity field, while leaving
unchanged the low-order statistics, which instead appear to be universal and in
good agreement with the classical Kolmogorov theory. To the best of our
knowledge, these are the most accurate simulations of driven relativistic
turbulence to date.Comment: 5 pages, 4 figures. Minor changes to match the version accepted on
ApJ

### Universality and intermittency in relativistic turbulent flows of a hot gas

With the aim of determining the statistical properties of relativistic
turbulence and unveiling novel and non-classical features, we present the
results of direct numerical simulations of driven turbulence in an
ultrarelativistic hot plasma using high-order numerical schemes. We study the
statistical properties of flows with average Mach number ranging from $\sim
0.4$ to $\sim 1.7$ and with average Lorentz factors up to $\sim 1.7$. We find
that flow quantities, such as the energy density or the local Lorentz factor,
show large spatial variance even in the subsonic case as compressibility is
enhanced by relativistic effects. The velocity field is highly intermittent,
but its power-spectrum is found to be in good agreement with the predictions of
the classical theory of Kolmogorov.Comment: Talk given at the ASTRONUM2012 conference on the 25th of June 201

### Critical Phenomena in Neutron Stars II: Head-on Collisions

We consider the head-on collision of equal-mass neutron stars boosted towards
each other and we study the behavior of such systems near the threshold of
black-hole formation. In particular, we confirm the previous findings by [1]
that a type-I critical phenomenon can be observed by fine-tuning the initial
mass of the two neutron stars. At the same time, we argue against the
interpretation that the critical solution is not a perturbed spherical star and
show instead that the metastable star corresponds to a (perturbed) equilibrium
solution on the unstable branch of the equilibrium configurations. As a result,
the head-on collision of two neutron stars near the critical threshold can be
seen as a transition in the space of configurations from an initial stable
solution over to a critical metastable one which can either migrate to a stable
solution or collapse to a black hole. The critical exponent for this process
shows a fine structure which was already observed in the case of the critical
collapse of scalar fields but never before for perfect fluids.Comment: 15 pages, 19 figure

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