618 research outputs found
Radiation from hot bare strange stars
We present the results of numerical simulations of stationary, spherically
outflowing, pair winds, with total luminosities of L=10^{35}- 10^{42} ergs/s.
These results have direct relevance to the emission from hot, bare, strange
stars, which are thought to be powerful sources of electron-positron pairs
created by the Coulomb barrier at the quark surface. The spectra of emergent
photons and pairs are calculated. For L > 2x10^{35} erg/s, photons dominate the
emerging emission. As L increases from 10^{35} to 10^{42} ergs/s, the mean
photon energy decreases from ~ 400-500 keV to 40 keV, while the spectrum
changes in shape from a wide annihilation line to being nearly blackbody with a
high energy (> 100 keV) tail. Such a correlation of the photon spectrum with
the luminosity, together with the fact that super-Eddington luminosities can be
achieved, might be a good observational signature of hot, bare, strange stars.Comment: 4 pages, 4 figures, Accepted in MNRAS, includes minor correction
A Two-Dimensional Hydrostatically Equilibrium Atmosphere of a Neutron Star with Given Differential Rotation
An analytic solution has been found in the Roche approximation for the
axially symmetric structure of a hydrostatically equilibrium atmosphere of a
neutron star produced by collapse. A hydrodynamic (quasione-dimensional) model
for the collapse of a rotating iron core in a massive star gives rise to a
heterogeneous rotating protoneutron star with an extended atmosphere composed
of matter from the outer part of the iron core with differential rotation
(Imshennik and Nadyozhin, 1992). The equation of state of a completely
degenerate iron gas with an arbitrary degree of relativity is taken for the
atmospheric matter. We construct a family of toroidal model atmospheres with
total masses and total angular momenta , which are acceptable for the
outer part of the collapsed iron core, in accordance with the hydrodynamic
model, as a function of constant parameters of the
specified differential rotation law in spherical
coordinates. The assumed rotation law is also qualitatively consistent with the
hydrodynamic model for the collapse of an iron core.Comment: 9 pages, 6 figures, 1 tabl
GRBs and the thermalization process of electron-positron plasmas
We discuss the temporal evolution of the pair plasma created in Gamma-Ray
Burst sources. A particular attention is paid to the relaxation of the plasma
into thermal equilibrium. We also discuss the connection between the dynamics
of expansion and the spatial geometry of the plasma. The role of the baryonic
loading parameter is emphasized.Comment: 4 pages, 3 figures, in the Proceedings of the "Gamma Ray Bursts 2007"
meeting, November 5-9, 2007, Santa Fe, New Mexico, US
Pair plasma relaxation time scales
By numerically solving the relativistic Boltzmann equations, we compute the
time scale for relaxation to thermal equilibrium for an optically thick
electron-positron plasma with baryon loading. We focus on the time scales of
electromagnetic interactions. The collisional integrals are obtained directly
from the corresponding QED matrix elements. Thermalization time scales are
computed for a wide range of values of both the total energy density (over 10
orders of magnitude) and of the baryonic loading parameter (over 6 orders of
magnitude). This also allows us to study such interesting limiting cases as the
almost purely electron-positron plasma or electron-proton plasma as well as
intermediate cases. These results appear to be important both for laboratory
experiments aimed at generating optically thick pair plasmas as well as for
astrophysical models in which electron-positron pair plasmas play a relevant
role.Comment: Phys. Rev. E, in pres
Relativistic stars in differential rotation: bounds on the dragging rate and on the rotational energy
For general relativistic equilibrium stellar models (stationary axisymmetric
asymptotically flat and convection-free) with differential rotation, it is
shown that for a wide class of rotation laws the distribution of angular
velocity of the fluid has a sign, say "positive", and then both the dragging
rate and the angular momentum density are positive. In addition, the "mean
value" (with respect to an intrinsic density) of the dragging rate is shown to
be less than the mean value of the fluid angular velocity (in full general,
without having to restrict the rotation law, nor the uniformity in sign of the
fluid angular velocity); this inequality yields the positivity and an upper
bound of the total rotational energy.Comment: 23 pages, no figures, LaTeX. Submitted to J. Math. Phy
A hydrodynamic model for asymmetric explosions of rapidly rotating collapsing supernovae with a toroidal atmosphere
We numerically solved the two-dimensional axisymmetric hydrodynamic problem
of the explosion of a low-mass neutron star in a circular orbit. In the initial
conditions, we assumed a nonuniform density distribution in the space
surrounding the collapsed iron core in the form of a stationary toroidal
atmosphere that was previously predicted analytically and computed numerically.
The configuration of the exploded neutron star itself was modeled by a
torus with a circular cross section whose central line almost coincided with
its circular orbit. Using an equation of state for the stellar matter and the
toroidal atmosphere in which the nuclear statistical equilibrium conditions
were satisfied, we performed a series of numerical calculations that showed the
propagation of a strong divergent shock wave with a total energy of 0.2x10^51
erg at initial explosion energy release of 1.0x10^51 erg. In our calculations,
we rigorously took into account the gravitational interaction, including the
attraction from a higher-mass (1.9M_solar) neutron star located at the
coordinate origin, in accordance with the rotational explosion mechanism for
collapsing supernovae.W e compared in detail our results with previous similar
results of asymmetric supernova explosion simulations and concluded that we
found a lower limit for the total explosion energy.Comment: 13 pages, 5 figures, 2 table
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