Coalescing Neutron Stars and Gamma Ray Bursters

The aim of this project is to investigate whether the model of coalescing neutron
stars in binaries or mergers of neutron stars and black holes yield sufficient
energies in gamma rays to explain the observed gamma ray bursts, assuming that
the bursters are at cosmological distances.

A short description of the numerical procedures, initial conditions and results can
be found in an extended abstract (gzip'd postscript, 37KB), which is a contribution
to the Proceedings of the 17th Texas Symposium (Munich, Dec.11-17 1994).
The full details of the procedures and initial conditions as well as results pertaining
to gravitational wave emission can be found in our first paper (gzip'd postscript, 0.8MB, A&A). A compressed summary of the results about neutrino
emission and annihilation can be found in a letter . The full paper relating to neutrinos is in preparation.

Initially two neutron stars with a mass of 1.6 solar masses each and a radius of 15km
are placed at a distance of 42km (center to center). They subsequently spiral toward
each other, emitting gravitational waves and neutrinos, and eventually merge. Apart
from the numerical resolution (mostly 64 zones per dimension, in one model 128
zones per dimension), we vary the initial spins of the neutron stars: in model A the
velocities within the neutron stars are set according to the motions of point masses
as computed from the quadrupole formula. Spins are added to the neutron stars to
take into account rotations around their axes vertical to the orbital plane. These
spins are different from model to model to describe the cases with spin vectors
parallel (model B, solid body, rigid rotator) and antiparallel (model C) to the vector
of the orbital angular momentum.

Snapshots of the evolution show two dimensional slices of the density and
temperature distribution in the orbital plane as false color plots: model A128
(0.3MB)
, model A64 (0.3MB), model B64 (0.3MB), model C64 (0.3MB).

Additionally to the traditional false color plots we also generated ray cast images
(72KB) of the neutrino emission during the neutron star coalescence simulation.
The brightness visible in the image is proportional to the luminosity at that point,
while the colors represent the mean temperature of the neutrinos, red and yellow
hues are around 4MeV, green hues around 10 MeV. The much hotter neutrinos at
the center (around 16MeV, they would be represented by a blue color) cannot
escape because the optical depth is too high.

Both the ray cast images (designated 3D) and the false color snapshots (2D) are
strung up to movies. The snapshots come in two types, one showing the density
(designated d) and the other the temperature (designated t) distribution. The number
in brackets following the designation specifies the size of the mpeg movie in MB.

    model A64     model A128     model B64     model C64
    3D (2.5)      3D (1.6)       3D (2.0)      3D (2.6) 
    2Dd(4.7)      2Dd(3.9)       2Dd(5.3)      2Dd(4.5) 
    2Dt(3.7)      2Dt(2.8)       2Dt(3.8)      2Dt(3.9) 
(Comment about movie 3D of model A128: initially the brightness of the colors are
set too brighly (0.8 ms) and then too dimly (for 0.5 ms), sorry. But the ray tracing is
done at the same time as the hydro simulation and so the frames that are produced
cannot be altered at a later time.) A comparative study was done, comparing our simulations with three previously
published models. We aim at studying the differences due to the use of different
numerical methods, different implementations of the gravitational wave
backreaction, and different equations of state.
Maximilian Ruffert / mruffert@NOSPAMmpa-garching.mpg.de / 28 April 1996