635 research outputs found
Confronting Synchrotron Shock and Inverse Comptonization Models with GRB Spectral Evolution
The time-resolved spectra of gamma-ray bursts (GRBs) remain in conflict with
many proposed models for these events. After proving that most of the bursts in
our sample show evidence for spectral "shape-shifting", we discuss what
restrictions that BATSE time-resolved burst spectra place on current models. We
find that the synchrotron shock model does not allow for the steep low-energy
spectral slope observed in many bursts, including GRB 970111. We also determine
that saturated Comptonization with only Thomson thinning fails to explain the
observed rise and fall of the low-energy spectral slope seen in GRB 970111 and
other bursts. This implies that saturated Comptonization models must include
some mechanism which can cause the Thomson depth to increase intially in
pulses.Comment: (5 pages, 3 figures, to appear in Proceedings of the Fourth
Huntsville Symposium on Gamma-Ray Bursts
Testing the Invariance of Cooling Rate in Gamma-Ray Burst Pulses
Recent studies have found that the spectral evolution of pulses within
gamma-ray bursts (GRBs) is consistent with simple radiative cooling. Perhaps
more interesting was a report that some bursts may have a single cooling rate
for the multiple pulses that occur within it. We determine the probability that
the observed "cooling rate invariance" is purely coincidental by sampling
values from the observed distribution of cooling rates. We find a 0.1-26%
probability that we would randomly observe a similar degree of invariance based
on a variety of pulse selection methods and pulse comparison statistics. This
probability is sufficiently high to warrant skepticism of any intrinsic
invariance in the cooling rate.Comment: 4 pages, 1 figure, to appear in Proceedings of the Fourth Huntsville
Symposium on Gamma-Ray Burst
Local dielectric spectroscopy of near-surface glassy polymer dynamics
A non-contact scanning-probe-microscopy method was used to probe local
near-surface dielectric susceptibility and dielectric relaxation in
poly-vinyl-acetate (PVAc) near the glass transition. Dielectric spectra were
measured from 10-4 Hz to 102 Hz as a function of temperature. The measurements
probed a 20 nm thick layer below the free-surface of a bulk film. A small (4 K)
reduction in glass transition temperature and moderate narrowing of the
distribution of relaxation times was found. In contrast to results for
ultra-thin-films confined on or between metallic electrodes, no reduction in
the dielectric strength was found, inconsistent with the immobilization of
slower modes.Comment: submitte
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Waveform-level time-domain simulation comparison study of three shipboard power system architectures
Detailed waveform-level modeling and simulation of three alternative shipboard power system architectures is presented herein. The three system architectures are based on conventional 60Hz medium-voltage ac (MVAC), higherfrequency 240Hz medium-voltage ac (HFAC) and mediumvoltage dc (MVDC) technologies. To support the quantitative assessment and comparison of these three different power system architectures, each technology was modeled using a common representative, notional baseline ship. The baseline ship represents a multi-mission destroyer fitted with an 80MW next generation integrated power system (NGIPS). Modeling of each power system architecture is set forth along with simulation studies for three fault scenarios. Each of the three power system architectures was implemented within the MATLAB/ Simulink environment. Continuity of service was evaluated for each architecture along with a fault scenario using an operability metric. After a brief description of the three power system architectures and the operability metric, quantitative results are presented.Center for Electromechanic
Spectral Hardness Decay with Respect to Fluence in BATSE Gamma-Ray Bursts
We have analyzed the evolution of the spectral hardness parameter Epk as a
function of fluence in gamma-ray bursts. We fit 41 pulses within 26 bursts with
the trend reported by Liang & Kargatis (1996) which found that Epk decays
exponentially with respect to photon fluence. We also fit these pulses with a
slight modification of this trend, where Epk decays linearly with energy
fluence. In both cases, we found the set of 41 pulses to be consistent with the
trend. For the latter trend, which we believe to be more physical, the
distribution of the decay constant is roughly log-normal, with a mean of 1.75
+/- 0.07 and a FWHM of 1.0 +/- 0.1. Regarding an earlier reported invariance in
the decay constant among different pulses in a single burst, we found
probabilities of 0.49 to 0.84 (depending on the test used) that such invariance
would occur by coincidence, most likely due to the narrow distribution of decay
constant values among pulses.Comment: 17 pages, 7 figure pages, 2 table pages, submitted to The
Astrophysical Journa
Multiwavelength Observations of GX 339-4 in 1996. I. Daily Light Curves and X-ray and Gamma-Ray Spectroscopy
As part of our multiwavelength campaign of GX 339-4 observations in 1996 we
present our radio, X-ray, and gamma-ray observations made in July, when the
source was in a hard state (= soft X-ray low state). The radio observations
were made at the time when there was a possible radio jet. We show that the
radio spectrum was flat and significantly variable, and that the radio spectral
shape and amplitude at this time were not anomalous for this source. Daily
light curves from our pointed observation July 9-23 using OSSE, from BATSE, and
from the ASM on RXTE also show that there was no significant change in the X-
and gamma-ray flux or hardness during the time the possible radio jet-like
feature was seen. The higher energy portion of our pointed RXTE observation
made July 26 can be equally well fit using simple power law times exponential
(PLE) and Sunyaev-Titarchuk (ST) functions. An additional soft component is
required, as well as a broad emission feature centered on 6.4 keV. This may be
an iron line that is broadened by orbital Doppler motions and/or scattering off
a hot medium. Its equivalent width is 600 eV. Our simplistic continuum fitting
does not require an extra reflection component. Both a PLE and a ST model also
fit our OSSE spectrum on its own. Although the observations are not quite
simultaneous, combining the RXTE and CGRO spectra we find that the PLE model
easily fits the joint spectrum. However, the ST model drops off too rapidly
with increasing energies to give an acceptable joint fit.Comment: Submitted to Astrophysical Journal. 25 pages. 11 figure
Evolution of the Low-Energy Photon Spectra in Gamma-Ray Bursts
We report evidence that the asymptotic low-energy power law slope alpha
(below the spectral break) of BATSE gamma-ray burst photon spectra evolves with
time rather than remaining constant. We find a high degree of positive
correlation exists between the time-resolved spectral break energy E_pk and
alpha. In samples of 18 "hard-to-soft" and 12 "tracking" pulses, evolution of
alpha was found to correlate with that of the spectral break energy E_pk at the
99.7% and 98% confidence levels respectively. We also find that in the flux
rise phase of "hard-to-soft" pulses, the mean value of alpha is often positive
and in some bursts the maximum value of alpha is consistent with a value > +1.
BATSE burst 3B 910927, for example, has a alpha_max equal to 1.6 +/- 0.3. These
findings challenge GRB spectral models in which alpha must be negative of
remain constant.Comment: 12 pages (including 6 figures), accepted to Ap
Is the time lag-luminosity relation of GRBs a consequence of the Amati relation?
The lag-luminosity relation (LLR) provides a way of estimating GRB luminosity
by measuring the spectral lags between different energy bands. We want to
understand the origin of the LLR and test its validity. This appears especially
important if the LLR is to be used as a distance indicator. We perform a linear
analysis of the lag between two spectral bands. The lag is obtained as the time
interval between the maxima of a given pulse in the two bands. We get a simple
expression for the lag, which shows in a very simple way how it is related to
the spectral evolution of the burst via the variation of the peak energy and
spectral indices. When this expression is coupled to the Amati relation, it
leads to a LLR that agrees with the observational results only if the burst's
spectral evolution is limited to a decrease in peak energy during pulse decay.
However, when the variation of the spectral indices is also taken into account,
the predicted LLR differs from the observed one. We briefly discuss some ways
to solve this problem, such as a possible correlation between pulse spikiness
and burst luminosity.Comment: Accepted for publication in Astronomy & Astrophysics, 5 pages, 3
figure
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