202 research outputs found
Unveiling the Secrets of Gamma Ray Bursts
Gamma Ray Bursts (GRBs) are unpredictable and brief flashes of gamma rays
that occur about once a day in random locations in the sky. Since gamma rays do
not penetrate the Earth's atmosphere, they are detected by satellites, which
automatically trigger ground-based telescopes for follow-up observations at
longer wavelengths. In this introduction to Gamma Ray Bursts we review how
building a multi-wavelength picture of these events has revealed that they are
the most energetic explosions since the Big Bang and are connected with stellar
deaths in other galaxies. However, in spite of exceptional observational and
theoretical progress in the last 15 years, recent observations raise many
questions which challenge our understanding of these elusive phenomena. Gamma
Ray Bursts therefore remain one of the hottest topics in modern astrophysics.Comment: 20 pages, 11 figures, review article, draft version, final version
will appear in Contemporary Physic
Making Black Holes in Supernovae
The possibility of making stellar mass black holes in supernovae that
otherwise produce viable Type II and Ib supernova explosions is discussed and
estimates given of their number in the Milky Way Galaxy. Observational
diagnostics of stellar mass black hole formation are reviewed. While the
equation of state sets the critical mass, fall back during the explosion is an
equally important (and uncertain) element in determining if a black hole is
formed. SN 1987A may or may not harbor a black hole, but if the critical mass
for neutron stars is 1.5 - 1.6 M\sun, as Brown and Bethe suggest, it probably
does. Observations alone do not yet resolve the issue. Reasons for this state
of ambiguity are discussed and suggestions given as to how gamma-ray and x-ray
observations in the future might help.Comment: 14 pages, uuencoded gzipped postscript, Accepted Nuclear Physics A,
Gerry Brown Festschrift contributio
Massive Star Evolution: Nucleosynthesis and Nuclear Reaction Rate Uncertainties
We present a nucleosynthesis calculation of a 25 solar mass star of solar
composition that includes all relevant isotopes up to polonium. In particular,
all stable isotopes and necessary nuclear reaction rates are covered. We follow
the stellar evolution from hydrogen burning till iron core collapse and
simulate the explosion using a ``piston'' approach. We discuss the influence of
two key nuclear reaction rates, C12(a,g) and Ne22(a,n), on stellar evolution
and nucleosynthesis. The former significantly influences the resulting core
sizes (iron, silicon, oxygen) and the overall presupernova structure of the
star. It thus has significant consequences for the supernova explosion itself
and the compact remnant formed. The later rate considerably affects the
s-process in massive stars and we demonstrate the changes that different
currently suggested values for this rate cause.Comment: 6 pages, including 4 PostScript figures, to appear in Proc.
"Astronomy with Radioactivities III", New Astronomy Review
Observational constraints on the neutron star mass distribution
Radio observations of neutron star binary pulsar systems have constrained
strongly the masses of eight neutron stars. Assuming neutron star masses are
uniformly distributed between lower and upper bounds and , the
observations determine with 95\% confidence that and . These limits give observational
support to neutron star formation scenarios that suggest that masses should
fall predominantly in the range , and will also be
important in the interpretation of binary inspiral observations by the Laser
Interferometer Gravitational-wave Observatory.Comment: Postscript, 4 pages, NU-GR-
Optical Bumps in Cosmological GRBs as Supernovae
From both photometric and broadband spectral monitoring of gamma-ray burst
(GRB) lightcurve ``bumps,'' particularly in GRB 011121, a strong case grew for
a supernova (SN) origin. The GRB-SN connection was finally solidified beyond a
reasonable doubt with the discovery that the bump in GRB 030329 was
spectroscopically similar to a bright Type Ic SN. In light of this result, I
redress the previous SN bump claims and conclude that 1) the distribution of
GRB-SN bump peak magnitudes is consistent with the local Type Ibc SNe peak
distribution and suggest that 2) the late-time bumps in all long-duration GRBs
are likely supernovae.Comment: 5 pages, 2 figures. To be published in Proc. IAU Colloquium #192
``Supernovae (10 years of SN1993J),'' held 22-26 April 2003, Valencia, Spain.
Editors: J.M. Marcaide and K.W. Weiler. Uses svmult.cl
Exact Solutions for Matter-Enhanced Neutrino Oscillations
The analogy between supersymmetric quantum mechanics and matter-enhanced
neutrino oscillations is exploited to obtain exact solutions for a class of
electron density profiles. This integrability condition is analogous to the
shape-invariance in supersymmetric quantum mechanics. This method seems to be
the most direct way to obtain the exact survival probabilities for a number of
density profiles of interest, such as linear and exponential density profiles.
The resulting neutrino amplitudes can also be utilized as comparison amplitudes
for the uniform semiclassical treatment of neutrino propagation in arbitrary
electron density profiles.Comment: Submitted to Physical Review D. Latex file, 8 pages. This paper is
also available at http://nucth.physics.wisc.edu/preprints
The Supernova Relic Neutrino Background
An upper bound to the supernova relic neutrino background from all past Type
II supernovae is obtained using observations of the Universal metal enrichment
history. We show that an unambiguous detection of these relic neutrinos by the
Super-Kamiokande detector is unlikely. We also analyze the event rate in the
Sudbury Neutrino Observatory (where coincident neutrons from anti-nu_e + D -->
n + n + e+ might enhance background rejection), and arrive at the same
conclusion. If the relic neutrino flux should be observed to exceed our upper
bound and if the observations of the metal enrichment history (for z<1) are not
in considerable error, then either the Type II supernova rate does not track
the metal enrichment history or some mechanism may be responsible for
transforming anti-nu_{mu,tau} --> anti-nu_e.Comment: Matches version accepted for publication in Phys. Rev.
Recommended from our members
Models for Gamma-Ray Bursts and Diverse Transients
The observational diversity of ''gamma-ray bursts'' (GRBs) has been increasing, and the natural inclination is a proliferation of models. We explore the possibility that at least part of this diversity is a consequence of a single basic model for the central engine operating in a massive star of variable mass, differential rotation rate, and mass loss rate. Whatever that central engine may be--and here the collapsar is used as a reference point--it must be capable of generating both a narrowly collimated, highly relativistic jet to make the GRB, and a wide angle, sub-relativistic outflow responsible for exploding the star and making the supernova bright. To some extent, the two components may vary independently, so it is possible to produce a variety of jet energies and supernova luminosities. We explore, in particular, the production of low energy bursts and find a lower limit, {approx} 10{sup 48} erg s{sup -1} to the power required for a jet to escape a massive star before that star either explodes or is accreted. Lower energy bursts and ''suffocated'' bursts may be particularly prevalent when the metallicity is high, i.e., in the modern universe at low redshift
Neutrino transport in accretion disks
We test approximate approaches to solving a neutrino transport problem that
presents itself in the analysis of some accretion-disk models. Approximation #1
consists of replacing the full, angular- dependent, distribution function by a
two-stream simulation, where the streams are respectively outwardly and
inwardly directed, with angles to the vertical. In
this approximation the full energy dependence of the distribution function is
retained, as are the energy and temperature dependences of the scattering
rates. Approximation #2, used in recent works on the subject, replaces the
distribution function by an intensity function and the scattering rates by
temperature-energy-averaged quantities. We compare the approximations to the
results of solving the full Boltzmann equation. Under some interesting
conditions, approximation #1 passes the test; approximation #2 does not. We
utilize the results of our analysis to construct a toy model of a disc at a
temperature and density such that relativistic particles are more abundant than
nucleons, and dominate both the opacity and pressure. The nucleons will still
provide most of the energy density. In the toy model we take the rate of heat
generation (which drives the radiative transfer problem) to be proportional to
the nucleon density. The model allows the simultaneous solution of the neutrino
transport and hydrostatic equilibrium problems in a disk in which the nucleon
density decreases approximately linearly as one moves from the median plane of
the disk upwards, reaching zero on the upper boundary.Comment: 8 pages, 5 figures Parentheses added in eqs. 10-1
Stochastic background of gravitational waves
A continuous stochastic background of gravitational waves (GWs) for burst
sources is produced if the mean time interval between the occurrence of bursts
is smaller than the average time duration of a single burst at the emission,
i.e., the so called duty cycle must be greater than one. To evaluate the
background of GWs produced by an ensemble of sources, during their formation,
for example, one needs to know the average energy flux emitted during the
formation of a single object and the formation rate of such objects as well. In
many cases the energy flux emitted during an event of production of GWs is not
known in detail, only characteristic values for the dimensionless amplitude and
frequencies are known. Here we present a shortcut to calculate stochastic
backgrounds of GWs produced from cosmological sources. For this approach it is
not necessary to know in detail the energy flux emitted at each frequency.
Knowing the characteristic values for the ``lumped'' dimensionless amplitude
and frequency we show that it is possible to calculate the stochastic
background of GWs produced by an ensemble of sources.Comment: 6 pages, 4 eps figures, (Revtex) Latex. Physical Review D (in press
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