4,055 research outputs found
A Plasma Instability Theory of Gamma-Ray Burst Emission
A new theory for gamma-ray burst radiation is presented. In this theory,
magnetic fields and relativistic electrons are created through plasma processes
arising as a relativistic shell passes through the interstellar medium. The
gamma-rays are produced through synchrotron self-Compton emission. It is found
that shocks do not arise in this theory, and that efficient gamma-ray emission
only occurs for a high Lorentz factor and a high-density interstellar medium.
The former explains the absence of gamma-ray bursts with thermal spectra. The
latter provides the Compton attenuation theory with an explanation of why the
interstellar medium density is always high. The theory predicts the existence
of a class of extragalactic optical transient that emit no gamma-rays.Comment: Presented at the 20 Texas Symposium on Relativistic Astrophysics,
December 1998, Paris, France. To appear on the proceedings compact dis
Analysis of thermal environment in the thrustor cavity of space vehicles
Wind tunnel tests to determine the mechanisms involved in producing the high local heating rates in and around the thrustor nozzle in a non-firing condition are reported. The geometry of a typical nozzle installation is described. Procedures for reducing the effects of hypersonic flow interactions in order to prevent excess local heating are explained
Synchrotron Self Absorption in GRB Afterglow
GRB afterglow is reasonably described by synchrotron emission from
relativistic blast waves at cosmological distances. We perform detailed
calculations taking into account the effect of synchrotron self absorption. We
consider emission from the whole region behind the shock front, and use the
Blandford McKee self similar solution to describe the fluid behind the shock.
We calculate the spectra and the observed image of a GRB afterglow near the
self absorption frequency and derive an accurate expression for
. We show that the image is rather homogeneous for , as
opposed to the bright ring at the outer edge and dim center, which appear at
higher frequencies. We compare the spectra we obtain to radio observations of
GRB970508. We combine the calculations of the spectra near the self absorption
frequency with other parts of the spectra and obtain revised estimates for the
physical parameters of the burst: , ,
, . These estimates are different by up to two
orders of magnitude than the estimates based on an approximate spectrum.Comment: 19 page latex file including 6 figures and 1 tabl
Galaxy-Galaxy Lensing by Non-Spherical Haloes I:Theoretical Considerations
We use Monte Carlo simulations to investigate the theory of galaxy-galaxy
lensing by non-spherical dark matter haloes. The simulations include a careful
accounting of the effects of multiple deflections. In a typical data set where
the mean tangential shear of sources with redshifts zs ~ 0.6 is measured with
respect to the observed symmetry axes of foreground galaxies with redshifts zl
~ 0.3, the signature of anisotropic galaxy-galaxy lensing differs substantially
from the expectation that one would have in the absence of multiple
deflections. The observed ratio of the mean tangential shears, g+/g-, is
strongly suppressed compared to the function that one would measure if the
intrinsic symmetry axes of the foreground galaxies were known. Depending upon
the characteristic masses of the lenses, the observed ratio of the mean
tangential shears may be consistent with an isotropic signal (despite the fact
that the lenses are non-spherical), or it may even be reversed from the
expected signal (i.e., the mean tangential shear for sources close to the
observed minor axes of the lenses may exceed the mean tangential shear for
sources close to the observed major axes of the lenses). These effects are
caused primarily by the fact that the lens galaxies have, themselves, been
lensed and therefore the observed symmetry axes of the lenses differ from their
intrinsic symmetry axes. The effects of lensing of the foreground galaxies on
the observed function g+/g- cannot be eliminated by the rejection of foreground
galaxies with small image ellipticities, nor by focusing the analysis on
sources that are located very close to the observed symmetry axes of the
foreground galaxies. We conclude that any attempt to use a measurement of g+/g-
to constrain the shapes of dark matter galaxy haloes must include Monte Carlo
simulations that take multiple deflections properly into account.Comment: 15 pages, 17 figures, submitted to MNRAS, full manuscript with
high-resolution version of Fig. 4 can be found at
http://firedrake.bu.edu/preprints/preprints.htm
Cosmological gamma-ray burst model
A cosmological gamma-ray burst model that reproduces the observed gamma-ray spectra was developed. This model, which is an outgrowth of work on synchrotron emission from cosmological sources, creates the observed spectra from a power law spectrum through Compton attenuation in dense molecular clouds. It restricts the burst source to the centers of galaxies, and it is easily tested through comparisons with time dependent burst spectra. Dr. Brainerd continued development of a Monte Carlo code that tracks the random walk of a gamma-ray through a plasma in a strong magnetic field and used this code to show that a two temperature plasma in a strong magnetic field can suppress soft x-ray emission
Time Dependent Clustering Analysis of the Second BATSE Gamma-Ray Burst Catalog
A time dependent two-point correlation-function analysis of the BATSE 2B
catalog finds no evidence of burst repetition. As part of this analysis, we
discuss the effects of sky exposure on the observability of burst repetition
and present the equation describing the signature of burst repetition in the
data. For a model of all burst repetition from a source occurring in less than
five days we derive upper limits on the number of bursts in the catalog from
repeaters and model-dependent upper limits on the fraction of burst sources
that produce multiple outbursts.Comment: To appear in the Astrophysical Journal Letters, uuencoded compressed
PostScript, 11 pages with 4 embedded figure
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