537 research outputs found
Afterglows as Diagnostics of Gamma Ray Burst Beaming
If gamma ray bursts are highly collimated, radiating into only a small
fraction of the sky, the energy requirements of each event may be reduced by
several (up to 4 - 6) orders of magnitude, and the event rate increased
correspondingly. The large Lorentz factors (Gamma > 100) inferred from GRB
spectra imply relativistic beaming of the gamma rays into an angle 1/Gamma. We
are at present ignorant of whether there are ejecta outside this narrow cone.
Afterglows allow empirical tests of whether GRBs are well-collimated jets or
spherical fireballs. The bulk Lorentz factor decreases and radiation is beamed
into an ever increasing solid angle as the burst remnant expands. It follows
that if gamma ray bursts are highly collimated, many more optical and radio
transients should be observed without associated gamma rays than with them.
In addition, a burst whose ejecta are beamed into angle zeta undergoes a
qualitative change in evolution when Gamma < 1/zeta: Before this, Gamma ~
r^{-3/2}, while afterwards, Gamma decays exponentially with r. This change
results in a potentially observable break in the afterglow light curve.
Successful application of either test would eliminate the largest remaining
uncertainty in the energy requirements and space density of gamma ray bursters.Comment: 5 pages, LaTex, uses aipproc and psfig style files. To appear in the
proceedings of the Fourth Huntsville Gamma Ray Burst Symposiu
Cosmic Ray Rejection by Linear Filtering of Single Images
We present a convolution-based algorithm for finding cosmic rays in single
well-sampled astronomical images. The spatial filter used is the point spread
function (approximated by a Gaussian) minus a scaled delta function, and cosmic
rays are identified by thresholding the filtered image. This filter searches
for features with significant power at spatial frequencies too high for
legitimate objects. Noise properties of the filtered image are readily
calculated, which allows us to compute the probability of rejecting a pixel not
contaminated by a cosmic ray (the false alarm probability). We demonstrate that
the false alarm probability for a pixel containing object flux will never
exceed the corresponding probability for a blank sky pixel, provided we choose
the convolution kernel appropriately. This allows confident rejection of cosmic
rays superposed on real objects. Identification of multiple-pixel cosmic ray
hits can be enhanced by running the algorithm iteratively, replacing flagged
pixels with the background level at each iteration.Comment: Accepted for publication in PASP (May 2000 issue). An iraf script
implementing the algorithm is available from the author, or from
http://sol.stsci.edu/~rhoads/ . 16 pages including 3 figures. Uses AASTeX
aaspp4 styl
The Dynamics and Light Curves of Beamed Gamma Ray Burst Afterglows
The energy requirements of gamma ray bursts have in past been poorly
constrained because of three major uncertainties: The distances to bursts, the
degree of burst beaming, and the efficiency of gamma ray production. The first
of these has been resolved, with both indirect evidence (the distribution of
bursts in flux and position) and direct evidence (redshifted absorption
features in the afterglow spectrum of GRB 970508) pointing to cosmological
distances. We now wish to address the second uncertainty. Afterglows allow a
statistical test of beaming, described in an earlier paper. In this paper, we
modify a standard fireball afterglow model to explore the effects of beaming on
burst remnant dynamics and afterglow emission. If the burst ejecta are beamed
into angle zeta, the burst remnant's evolution changes qualitatively once its
bulk Lorentz factor Gamma < 1/zeta: Before this, Gamma declines as a power law
of radius, while afterwards, it declines exponentially. This change results in
a broken power law light curve whose late-time decay is faster than expected
for a purely spherical geometry. These predictions disagree with afterglow
observations of GRB 970508. We explored several variations on our model, but
none seems able to change this result. We therefore suggest that this burst is
unlikely to have been highly beamed, and that its energy requirements were near
those of isotropic models. More recent afterglows may offer the first practical
applications for our beamed models.Comment: 18 pages, uses emulateapj.sty, four embedded postscript figures.
Submitted to The Astrophysical Journal, 199
Optimizing Air Force Depot Programming to Maximize Operational Capability
The Air Force wants to improve the link between resources and weapons system readiness by reducing costs, improving risk-based decision making, and balancing costs with performance. With that in mind, RAND Project Air Force developed a linear programming model linking Depot Purchased Equipment Maintenance to operational capability. This thesis examined that model, provided an alternate model, and then developed a new model that determined the minimum cost necessary to maintain the force structure. The utility of using the models using Weapon System Sustainment (WSS) and additional sources of data for aircraft and engine inventories was evaluated and critiqued. While every WSS requirement has a cost, the vast majority do not have quantities associated with them. Using the sources outlined for aircraft and engine inventories does not match up with WSS data. Aircraft inventory data is more specific than the WSS data requirements. Engine inventories are managed by engine type, not by aircraft. Many engines serve multiple aircraft, and many aircraft require multiple engines. The combined result is that using WSS data to process these models and obtain meaningful results is not possible at this time
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