751 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
GRB990123: The Case for Saturated Comptonization
The recent simultaneous detection of optical, X-ray and gamma-ray photons
from GRB990123 during the burst provides the first broadband multi-wavelength
characterization of the burst spectrum and evolution. Here we show that a
direct correlation exists between the time-varying gamma-ray spectral shape and
the prompt optical emission. This combined with the unique signatures of the
time-resolved spectra of GRB990123 convincingly supports earlier predictions of
the saturated Comptonization model. Contrary to other suggestions, we find that
the entire continuum from optical to gamma-rays can be generated from a single
source of leptons (electrons and pairs). The optical flux only appears to lag
the gamma-ray flux due to the high initial Thomson depth of the plasma. Once
the plasma has completely thinned out, the late time afterglow behavior of our
model is the same as in standard models based on the Blandford-McKee (1976)
solution.Comment: 10 pages, including 3 figures and 1 table, submitted to The
Astrophysical Journal Letter
A Thermal-Nonthermal Inverse Compton Model for Cyg X-1
Using Monte Carlo methods to simulate the inverse Compton scattering of soft
photons, we model the spectrum of the Galactic black hole candidate Cyg X-1,
which shows evidence of a nonthermal tail extending beyond a few hundred keV.
We assume an ad hoc sphere of leptons, whose energy distribution consists of a
Maxwellian plus a high energy power-law tail, and inject 0.5 keV blackbody
photons. The spectral data is used to constrain the nonthermal plasma fraction
and the power-law index assuming a reasonable Maxwellian temperature and
Thomson depth. A small but non-negligible fraction of nonthermal leptons is
needed to explain the power-law tail.Comment: 5 pages, 2 PostScript figure, uses aipproc.sty, to appear in
Proceedings of Fourth Compton Symposiu
Interpreting the Ionization Sequence in Star-Forming Galaxy Emission-Line Spectra
High ionization star forming (SF) galaxies are easily identified with strong
emission line techniques such as the BPT diagram, and form an obvious
ionization sequence on such diagrams. We use a locally optimally emitting cloud
model to fit emission line ratios that constrain the excitation mechanism,
spectral energy distribution, abundances and physical conditions along the
star-formation ionization sequence. Our analysis takes advantage of the
identification of a sample of pure star-forming galaxies, to define the
ionization sequence, via mean field independent component analysis. Previous
work has suggested that the major parameter controlling the ionization level in
SF galaxies is the metallicity. Here we show that the observed SF- sequence
could alternatively be interpreted primarily as a sequence in the distribution
of the ionizing flux incident on gas spread throughout a galaxy. Metallicity
variations remain necessary to model the SF-sequence, however, our best models
indicate that galaxies with the highest and lowest observed ionization levels
(outside the range -0.37 < log [O III]/H\b{eta} < -0.09) require the variation
of an additional physical parameter other than metallicity, which we determine
to be the distribution of ionizing flux in the galaxy.Comment: 41 pages, 17 figures, 9 tables, accepted to MNRA
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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
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
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
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