15 research outputs found
Testing Approximations of Thermal Effects in Neutron Star Merger Simulations
We perform three-dimensional relativistic hydrodynamical calculations of
neutron star mergers to assess the reliability of an approximate treatment of
thermal effects in such simulations by combining an ideal-gas component with
zero-temperature, micro-physical equations of state. To this end we compare the
results of simulations that make this approximation to the outcome of models
with a consistent treatment of thermal effects in the equation of state. In
particular we focus on the implications for observable consequences of merger
events like the gravitational-wave signal. It is found that the characteristic
gravitational-wave oscillation frequencies of the post-merger remnant differ by
about 50 to 250 Hz (corresponding to frequency shifts of 2 to 8 per cent)
depending on the equation of state and the choice of the characteristic index
of the ideal-gas component. In addition, the delay time to black hole collapse
of the merger remnant as well as the amount of matter remaining outside the
black hole after its formation are sensitive to the description of thermal
effects.Comment: 10 pages, 6 figures, 9 eps files; revised with minor additions due to
referee comments; accepted by Phys.Rev.
Gravitational waves from relativistic neutron star mergers with nonzero-temperature equations of state
We analyze the gravitational wave (GW) emission from our recently published
set of relativistic neutron star (NS) merger simulations and determine
characteristic signal features that allow one to link GW measurements to the
properties of the merging binary stars. We find that the distinct peak in the
GW energy spectrum that is associated with the formation of a hypermassive
merger remnant has a frequency that depends strongly on the properties of the
nuclear equation of state (EoS) and on the total mass of the binary system,
whereas the mass ratio and the NS spins have a weak influence. If the total
mass can be determined from the inspiral chirp signal, the peak frequency of
the postmerger signal is a sensitive indicator of the EoS.Comment: 5 pages, 4 figures, revised version accepted for publication in PR
Torus Formation in Neutron Star Mergers and Well-Localized Short Gamma-Ray Bursts
Merging neutron stars (NSs) are hot candidates for the still enigmatic
sources of short gamma-ray bursts (GRBs). If the central engines of the huge
energy release are accreting relic black holes (BHs) of such mergers, it is
important to understand how the properties of the BH-torus systems, in
particular disc masses and mass and rotation rate of the compact remnant, are
linked to the characterizing parameters of the NS binaries. For this purpose we
present relativistic smoothed particle hydrodynamics simulations with
conformally flat approximation of the Einstein field equations and a physical,
non-zero temperature equation of state. Thick disc formation is highlighted as
a dynamical process caused by angular momentum transfer through tidal torques
during the merging process of asymmetric systems or in the rapidly spinning
triaxial post-merger object. Our simulations support the possibility that the
first well-localized short and hard GRBs 050509b, 050709, 050724, 050813 have
originated from NS merger events and are powered by neutrino-antineutrino
annihilation around a relic BH-torus system. Using model parameters based on
this assumption, we show that the measured GRB energies and durations lead to
estimates for the accreted masses and BH mass accretion rates which are
compatible with theoretical expectations. In particular, the low energy output
and short duration of GRB 050509b set a very strict upper limit of less than
100 ms for the time interval after the merging until the merger remnant has
collapsed to a BH, leaving an accretion torus with a small mass of only about
0.01 solar masses. This favors a (nearly) symmetric NS+NS binary with a typical
mass as progenitor system.Comment: 12 pages, 9 figures, high-resolution color figures available on
request; accepted by MNRA
Dynamical non-axisymmetric instabilities in rotating relativistic stars
We present new results on dynamical instabilities in rapidly rotating
neutron-stars. In particular, using numerical simulations in full General
Relativity, we analyse the effects that the stellar compactness has on the
threshold for the onset of the dynamical bar-mode instability, as well as on
the appearance of other dynamical instabilities. By using an extrapolation
technique developed and tested in our previous study [1], we explicitly
determine the threshold for a wide range of compactnesses using four sequences
of models of constant baryonic mass comprising a total of 59 stellar models.
Our calculation of the threshold is in good agreement with the Newtonian
prediction and improves the previous post-Newtonian estimates. In addition, we
find that for stars with sufficiently large mass and compactness, the m=3
deformation is the fastest growing one. For all of the models considered, the
non-axisymmetric instability is suppressed on a dynamical timescale with an m=1
deformation dominating the final stages of the instability. These results,
together with those presented in [1], suggest that an m=1 deformation
represents a general and late-time feature of non-axisymmetric dynamical
instabilities both in full General Relativity and in Newtonian gravity.Comment: To appear on CQG, NFNR special issue. 16 pages, 5 color figures,
movies from http://www.fis.unipr.it/numrel/BarMode/ResearchBarMode.htm
Relativistic neutron star merger simulations with non-zero temperature equations of state I. Variation of binary parameters and equation of state
An extended set of binary neutron star (NS) merger simulations is performed
with an approximative conformally flat treatment of general relativity to
systematically investigate the influence of the nuclear equation of state
(EoS), the neutron star masses, and the NS spin states prior to merging. We
employ the two non-zero temperature EoSs of Shen et al. (1998a,b) and Lattimer
& Swesty (1991). In addition, we use the cold EoS of Akmal et al. (1998) with a
simple ideal-gas-like extension according to Shibata & Taniguchi (2006), and an
ideal-gas EoS with parameters fitted to the supernuclear part of the Shen-EoS.
We estimate the mass sitting in a dilute high-angular momentum ``torus'' around
the future black hole (BH). The dynamics and outcome of the models is found to
depend strongly on the EoS and on the binary parameters. Larger torus masses
are found for asymmetric systems (up to ~0.3 M_sun for a mass ratio of 0.55),
for large initial NSs, and for a NS spin state which corresponds to a larger
total angular momentum. We find that the postmerger remnant collapses either
immediately or after a short time when employing the soft EoS of Lattimer&
Swesty, whereas no sign of post-merging collapse is found within tens of
dynamical timescales for all other EoSs used. The typical temperatures in the
torus are found to be about 3-10 MeV depending on the strength of the shear
motion at the collision interface between the NSs and thus depending on the
initial NS spins. About 10^{-3}-10^{-2} M_sun of NS matter become
gravitationally unbound during or right after the merging process. This matter
consists of a hot/high-entropy component from the collision interface and (only
in case of asymmetric systems) of a cool/low-entropy component from the spiral
arm tips. (abridged)Comment: 20 pages, 15 figures, accepted for publication in A&A, included
changes based on referee comment
Mass Ejection by Strange Star Mergers and Observational Implications
We determine the Galactic production rate of strangelets as a canonical input
to calculations of the measurable cosmic ray flux of strangelets by performing
simulations of strange star mergers and combining the results with recent
estimates of stellar binary populations. We find that the flux depends
sensitively on the bag constant of the MIT bag model of QCD and disappears for
high values of the bag constant and thus more compact strange stars. In the
latter case strange stars could coexist with ordinary neutron stars as they are
not converted by the capture of cosmic ray strangelets. An unambiguous
detection of an ordinary neutron star would then not rule out the strange
matter hypothesis.Comment: 5 pages, 2 eps figures; referee comments included, accepted by Phys.
Rev. Let
Strangeness in Astrophysics and Cosmology
Some recent developments concerning the role of strange quark matter for
astrophysical systems and the QCD phase transition in the early universe are
addressed. Causality constraints of the soft nuclear equation of state as
extracted from subthreshold kaon production in heavy-ion collisions are used to
derive an upper mass limit for compact stars. The interplay between the
viscosity of strange quark matter and the gravitational wave emission from
rotation-powered pulsars are outlined. The flux of strange quark matter nuggets
in cosmic rays is put in perspective with a detailed numerical investigation of
the merger of two strange stars. Finally, we discuss a novel scenario for the
QCD phase transition in the early universe, which allows for a small
inflationary period due to a pronounced first order phase transition at large
baryochemical potential.Comment: 8 pages, invited talk given at the International Conference on
Strangeness in Quark Matter (SQM2009), Buzios, Brasil, September 28 - October
2, 200
Merger of binary neutron stars of unequal mass in full general relativity
We present results of three dimensional numerical simulations of the merger
of unequal-mass binary neutron stars in full general relativity. A -law
equation of state is adopted, where , ,
\varep, and are the pressure, rest mass density, specific internal
energy, and the adiabatic constant, respectively. We take and the
baryon rest-mass ratio to be in the range 0.85--1. The typical grid size
is for . We improve several implementations since the
latest work. In the present code, the radiation reaction of gravitational waves
is taken into account with a good accuracy. This fact enables us to follow the
coalescence all the way from the late inspiral phase through the merger phase
for which the transition is triggered by the radiation reaction. It is found
that if the total rest-mass of the system is more than times of the
maximum allowed rest-mass of spherical neutron stars, a black hole is formed
after the merger irrespective of the mass ratios. The gravitational waveforms
and outcomes in the merger of unequal-mass binaries are compared with those in
equal-mass binaries. It is found that the disk mass around the so formed black
holes increases with decreasing rest-mass ratios and decreases with increasing
compactness of neutron stars. The merger process and the gravitational
waveforms also depend strongly on the rest-mass ratios even for the range --1.Comment: 32 pages, PRD68 to be publishe