2,081 research outputs found
Formation Rates of Black Hole Accretion Disk Gamma-Ray Bursts
While many models have been proposed for GRBs, those currently favored are
all based upon the formation of and/or rapid accretion into stellar mass black
holes. We present population synthesis calculations of these models using a
Monte Carlo approach in which the many uncertain parameters intrinsic to such
calculations are varied. We estimate the event rate for each class of model as
well as the propagation distance for those having significant delay between
formation and burst production, i.e., double neutron star (DNS) mergers and
black hole-neutron star (BH/NS) mergers. For reasonable assumptions regarding
the many uncertainties in population synthesis, we calculate a daily event rate
in the universe for i) merging neutron stars: ~100/day; ii) neutron-star black
hole mergers: ~450/day; iii) collapsars: ~10,000/day; iv) helium star black
hole mergers: ~1000/day; and v) white dwarf black hole mergers: ~20/day. The
range of uncertainty in these numbers however, is very large, typically two to
three orders of magnitude. These rates must additionally be multiplied by any
relevant beaming factor and sampling fraction (if the entire universal set of
models is not being observed). Depending upon the mass of the host galaxy, half
of the DNS and BH/NS mergers will happen within 60kpc (for a Milky-Way massed
galaxy) to 5Mpc (for a galaxy with negligible mass) from the galactic center.
Because of the delay time, neutron star and black hole mergers will happen at a
redshift 0.5 to 0.8 times that of the other classes of models. Information is
still lacking regarding the hosts of short hard bursts, but we suggest that
they are due to DNS and BH/NS mergers and thus will ultimately be determined to
lie outside of galaxies and at a closer mean distance than long complex bursts
(which we attribute to collapsars).Comment: 57 pages total, 23 figures, submitted by Ap
Iron Opacity and the Pulsar of Supernova 1987A
Neutron stars formed in Type II supernovae are likely to be initially
obscured by late-time fallback. Although much of the late-time fallback is
quickly accreted via neutrino cooling, some material remains on the neutron
star, forming an atmosphere which slowly accretes through photon emission. In
this paper, we derive structure equations of the fallback atmosphere and
present results of one-dimensional simulations of that fallback. The atmosphere
remaining after neutrino cooling becomes unimportant (less than the Compton
Eddington limit) is only a fraction of the total mass accreted (10^-8 of the
accreted mass or 10^-9 solar masses.) Recombined iron dominates the opacity in
the outer regions leading to an opacity 1000-10,000 times higher than that of
electron scattering alone. The resultant photon emission of the remnant
atmosphere is limited to 1/1000th the Compton Eddington Luminosity. The
late-time evolution of this system leads to the formation of a photon-driven
wind from the accretion of the inner portion of the atmosphere, leaving, for
most cases, a bare neutron star on timescales shorter than a year. The
degenerate remnant of 1987a may not be a black hole. Instead, the fallback
material may have already accreted or blown off in the accretion-driven wind.
If the neutron star has either a low magnetic field or a low rotational spin
frequency, we would not expect to see the neutron star remnant of 1987a.Comment: 15 pages text + 8 figures, accepted by Ap
3-Dimensional Core-Collapse
In this paper, we present the results of 3-dimensional collapse simulations
of rotating stars for a range of stellar progenitors. We find that for the
fastest spinning stars, rotation does indeed modify the convection above the
proto-neutron star, but it is not fast enough to cause core fragmentation.
Similarly, although strong magnetic fields can be produced once the
proto-neutron star cools and contracts, the proto-neutron star is not spinning
fast enough to generate strong magnetic fields quickly after collapse and, for
our simulations, magnetic fields will not dominate the supernova explosion
mechanism. Even so, the resulting pulsars for our fastest rotating models may
emit enough energy to dominate the total explosion energy of the supernova.
However, more recent stellar models predict rotation rates that are much too
slow to affect the explosion, but these models are not sophisticated enough to
determine whether the most recent, or past, stellar rotation rates are most
likely. Thus, we must rely upon observational constraints to determine the true
rotation rates of stellar cores just before collapse. We conclude with a
discussion of the possible constraints on stellar rotation which we can derive
from core-collapse supernovae.Comment: 34 pages (5 of 17 figures missing), For full paper, goto
http://qso.lanl.gov/~clf/papers/rot.ps.gz accepted by Ap
Mass Limits For Black Hole Formation
We present a series of two-dimensional core-collapse supernova simulations
for a range of progenitor masses and different input physics. These models
predict a range of supernova energies and compact remnant masses. In
particular, we study two mechanisms for black hole formation: prompt collapse
and delayed collapse due to fallback. For massive progenitors above 20 solar
masses, after a hydrodynamic time for the helium core (a few minutes to a few
hours), fallback drives the compact object beyond the maximum neutron star mass
causing it to collapse into a black hole. With the current accuracy of the
models, progenitors more massive than 40 solar masses form black holes directly
with no supernova explosion (if rotating, these black holes may be the
progenitors of gamma-ray bursts). We calculate the mass distribution of black
holes formed, and compare these predictions to the observations, which
represent a small biased subset of the black hole population. Uncertainties in
these estimates are discussed.Comment: 15 pages total, 4 figures, Modifications in Conclusion, accepted by
Ap
Constraining the Nature of X-ray Cavities in Clusters and Galaxies
We present results from an extensive survey of 64 cavities in the X-ray halos
of clusters, groups and normal elliptical galaxies. We show that the evolution
of the size of the cavities as they rise in the X-ray atmosphere is
inconsistent with the standard model of adiabatic expansion of purely
hydrodynamic models. We also note that the majority of the observed bubbles
should have already been shredded apart by Rayleigh-Taylor and
Richtmyer-Meshkov instabilities if they were of purely hydrodynamic nature.
Instead we find that the data agrees much better with a model where the
cavities are magnetically dominated and inflated by a current-dominated
magneto-hydrodynamic jet model, recently developed by Li et al. (2006) and
Nakamura et al. (2006). We conduct complex Monte-Carlo simulations of the
cavity detection process including incompleteness effects to reproduce the
cavity sample's characteristics. We find that the current-dominated model
agrees within 1sigma, whereas the other models can be excluded at >5sigma
confidence. To bring hydrodynamic models into better agreement, cavities would
have to be continuously inflated. However, these assessments are dependent on
our correct understanding of the detectability of cavities in X-ray
atmospheres, and will await confirmation when automated cavity detection tools
become available in the future. Our results have considerable impact on the
energy budget associated with active galactic nucleus feedback.Comment: 21 pages, 12 figures, emulateapj, accepted for publication in ApJ,
responded to referee's comments and added a new model, conclusions unchange
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