315 research outputs found
Black Hole - Neutron Star Mergers as Central Engines of Gamma-Ray Bursts
Hydrodynamic simulations of the merger of stellar mass black hole - neutron
star binaries (BH/NS) are compared with mergers of binary neutron stars
(NS/NS). The simulations are Newtonian, but take into account the emission and
backreaction of gravitational waves. The use of a physical nuclear equation of
state allows us to include the effects of neutrino emission. For low neutron
star to black hole mass ratios the neutron star transfers mass to the black
hole during a few cycles of orbital decay and subsequent widening before
finally being disrupted, whereas for ratios near unity the neutron star is
already distroyed during its first approach. A gas mass between about 0.3 and
about 0.7 solar masses is left in an accretion torus around the black hole and
radiates neutrinos at a luminosity of several 10^{53} erg/s during an estimated
accretion time scale of about 0.1 s. The emitted neutrinos and antineutrinos
annihilate into electron-positron pairs with efficiencies of 1-3% percent and
rates of up to 2*10^{52} erg/s, thus depositing an energy of up to 10^{51} erg
above the poles of the black hole in a region which contains less than 10^{-5}
solar masses of baryonic matter. This could allow for relativistic expansion
with Lorentz factors around 100 and is sufficient to explain apparent burst
luminosities of up to several 10^{53} erg/s for burst durations of
approximately 0.1-1 s, if the gamma emission is collimated in two moderately
focussed jets in a fraction of about 1/100-1/10 of the sky.Comment: 8 pages, LaTex, 4 postscript figures, 2 tables. ApJ Letters,
accepted; revised and shortened version, Fig. 2 change
Numerical Models of Binary Neutron Star System Mergers. I.: Numerical Methods and Equilibrium Data for Newtonian Models
The numerical modeling of binary neutron star mergers has become a subject of
much interest in recent years. While a full and accurate model of this
phenomenon would require the evolution of the equations of relativistic
hydrodynamics along with the Einstein field equations, a qualitative study of
the early stages on inspiral can be accomplished by either Newtonian or
post-Newtonian models, which are more tractable. In this paper we offer a
comparison of results from both rotating and non-rotating (inertial) frame
Newtonian calculations. We find that the rotating frame calculations offer
significantly improved accuracy as compared with the inertial frame models.
Furthermore, we show that inertial frame models exhibit significant and
erroneous angular momentum loss during the simulations that leads to an
unphysical inspiral of the two neutron stars. We also examine the dependence of
the models on initial conditions by considering initial configurations that
consist of spherical neutron stars as well as stars that are in equilibrium and
which are tidally distorted. We compare our models those of Rasio & Shapiro
(1992,1994a) and New & Tohline (1997). Finally, we investigate the use of the
isolated star approximation for the construction of initial data.Comment: 32 pages, 19 gif figures, manuscript with postscript figures
available at http://www.astro.sunysb.edu/dswesty/docs/nspap1.p
Three-dimensional simulations of non-stationary accretion by remnant black holes of compact object mergers
(abridged) Three-dimensional hydrodynamic simulations with an Eulerian PPM
code are presented for the time-dependent evolution of accretion tori around
nonrotating and rotating stellar-mass black holes (BHs), using a
pseudo-Newtonian gravitational potential to approximate the effects of general
relativity. The initial configurations are assumed to be remnants of binary
neutron star (NS) or NS+BH mergers and consist of a 4 solar mass BH with varied
spin, girded by a torus with a mass between 0.01 and 0.2 solar masses. The
evolution of tori without and with physical shear viscosity is simulated, using
a realistic equation of state and following the energy loss and lepton number
change due to neutrino emission by a neutrino-trapping scheme. The
time-dependent efficiency of converting rest-mass energy to neutrinos is found
to reach 10 percent, the efficiency of converting neutrino energy into a
pair-photon fireball by neutrino annihilation can reach several percent. The
rate of the latter process declines with time much less steeply than the total
neutrino luminosity, because the ongoing protonization of the torus ensures a
rather stable product of neutrino and antineutrino luminosities. The neutrino
emission increases steeply with higher viscosity, larger torus mass, and larger
BH spin in corotation with the torus. For rotation rates as expected for
post-merger BHs (a > 0.5) and reasonable values of the alpha viscosity (alpha ~
0.1), the considered tori release sufficient energy in neutrinos to account for
the energetics of the well-localized short gamma-ray bursts recently detected
by Hete and Swift, if collimation is invoked as predicted by hydrodynamic jet
simulations.Comment: 23 pages, 15 figures (high resolution available upon request),
accepted by Astron. Astrophys. Significantly shortened with respect to first
versio
Vacuum discharge as a possible source of gamma-ray bursts
We propose that spontaneous particle--anti-particle pair creations from the
discharged vacuum caused by the strong interactions in dense matter are major
sources of -ray bursts. Two neutron star collisions or black
hole-neutron star mergers at cosmological distance could produce a compact
object with its density exceeding the critical density for pair creations. The
emitted anti-particles annihilate with corresponding particles at the ambient
medium. This releases a large amount of energy. We discuss the spontaneous
pair creations within two neutron star collision and estimate the
exploded energy from annihilation processes. The total energy could
be around erg depending on the impact parameter of
colliding neutron stars. This value fits well into the range of the initial
energy of the most energetic -ray bursts.Comment: 12 pages, Latex, 2 figures included; replaced by the revised version,
Int. J. Mod. Phys. E in pres
Post Newtonian SPH
We introduce an adaptation of the well known Tree+SPH numerical scheme to
Post Newtonian (PN) hydrodynamics and gravity. Our code solves the (0+1+2.5)PN
equations. These equations include Newtonian hydrodynamics and gravity (0PN),
the first order relativistic corrections to those (1PN) and the lowest order
gravitational radiation terms (2.5PN). We test various aspects of our code
using analytically solvable test problems. We then proceed to study the 1PN
effects on binary neutron star coalescence by comparing calculations with and
without the 1PN terms. We find that the effect of the 1PN terms is rather
small. The largest effect arises with a stiff equation of state for which the
maximum rest mass density increases. This could induce black hole formation.
The gravitational wave luminosity is also affected.Comment: 28 pages, 13 figures, revised version published in Ap
Helium Star/Black Hole Mergers: a New Gamma-Ray Burst Model
We present a model for gamma-ray bursts (GRB's) in which a stellar mass black
hole acquires a massive accretion disk by merging with the helium core of its
red giant companion. The black hole enters the helium core after it, or its
neutron star progenitor, first experiences a common envelope phase that carries
it inwards through the hydrogen envelope. Accretion of the last several solar
masses of helium occurs on a time scale of roughly a minute and provides a
neutrino luminosity of approximately 10^51 - 10^52 erg/s. Neutrino
annihilation, 0.01% to 0.1% efficient, along the rotational axis then gives a
baryon loaded fireball of electron-positron pairs and radiation (about
10 erg total) whose beaming and relativistic interaction with
circumstellar material makes the GRB (e.g., Rees & Meszaros 1992). The useful
energy can be greatly increased if energy can be extracted from the rotational
energy of the black hole by magnetic interaction with the disk. Such events
should occur at a rate comparable to that of merging neutron stars and black
hole neutron star pairs and may be responsible for long complex GRB's, but not
short hard ones.Comment: 11 pages total, 2 Figures - altered and revised for ApJ letters,
accepte
Non-stationary hyperaccretion of stellar-mass black holes in three dimensions: Torus evolution and neutrino emission
We present three-dimensional hydrodynamic simulations of the evolution of
selfgravitating, thick accretion discs around hyperaccreting stellar-mass black
holes. The black hole-torus systems are considered to be remnants of compact
object mergers, in which case the disc is not fed by an external mass reservoir
and the accretion is non-stationary. Our models take into account viscous
dissipation, described by an alpha-law, a detailed equation of state for the
disc gas, and an approximate treatment of general relativistic effects on the
disc structure by using a pseudo-Newtonian potential for the black hole
including its possible rotation and spin-up during accretion. Magnetic fields
are ignored. The neutrino emission of the hot disc is treated by a
neutrino-trapping scheme, and the neutrino-antineutrino annihilation near the
disc is evaluated in a post-processing step. Our simulations show that the
neutrino emission and energy deposition by neutrino-antineutrino annihilation
increase sensitively with the disc mass, with the black hole spin in case of a
disc in corotation, and in particular with the alpha-viscosity. We find that
for sufficiently large alpha-viscosity neutrino-antineutrino annihilation can
be a viable energy source for gamma-ray bursts.Comment: 6 pages, 4 figures (1 colored); accepted by MNRAS, referee's
suggestions incorporated; high-resolution pictures can be obtained upon
reques
Newtonian Hydrodynamics of the Coalescence of Black Holes with Neutron Stars I: Tidally locked binaries with a stiff equation of state
We present a detailed study of the hydrodynamical interactions in a Newtonian
black hole-neutron star binary during the last stages of inspiral. We consider
close binaries which are tidally locked, use a stiff equation of state (with an
adiabatic index Gamma=3) throughout, and explore the effect of different
initial mass ratios on the evolution of the system. We calculate the
gravitational radiation signal in the quadrupole approximation. Our
calculations are carried out using a Smooth Particle Hydrodynamics (SPH) code.Comment: Replaces previous version which had figures separate from the text of
the paper. Now 47 pages long with 19 embedded figures (the figures are the
same, they were renumbered) Uses aaspp4.st
Can Naked Singularities Yield Gamma Ray Bursts?
Gamma-ray bursts are believed to be the most luminous objects in the
Universe. There has been some suggestion that these arise from quantum
processes around naked singularities. The main problem with this suggestion is
that all known examples of naked singularities are massless and hence there is
effectively no source of energy. It is argued that a globally naked singularity
coupled with quantum processes operating within a distance of the order of
Planck length of the singularity will probably yield energy burst of the order
of M_pc^2\approx2\times 10^{16} ergs, where M_p is the Planck mass.Comment: 4 pages, TeX, no figure
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