76 research outputs found
The BATSE Gamma-Ray Burst Spectral Catalog. I. High Time Resolution Spectroscopy of Bright Bursts using High Energy Resolution Data
This is the first in a series of gamma-ray burst spectroscopy catalogs from
the Burst And Transient Source Experiment (BATSE) on the Compton Gamma Ray
Observatory, each covering a different aspect of burst phenomenology. In this
paper, we present time-sequences of spectral fit parameters for 156 bursts
selected for either their high peak flux or fluence. All bursts have at least
eight spectra in excess of 45 sigma above background and span burst durations
from 1.66 to 278 s. Individual spectral accumulations are typically 128 ms long
at the peak of the brightest events, but can be as short as 16 ms, depending on
the type of data selected. We have used mostly high energy resolution data from
the Large Area Detectors, covering an energy range of typically 28 - 1800 keV.
The spectral model chosen is from a small empirically-determined set of
functions, such as the well-known `GRB' function, that best fits the
time-averaged burst spectra. Thus, there are generally three spectral shape
parameters available for each of the 5500 total spectra: a low-energy power-law
index, a characteristic break energy and possibly a high-energy power-law
index. We present the distributions of the observed sets of these parameters
and comment on their implications. The complete set of data that accompanies
this paper is necessarily large, and thus is archived electronically at:
http://www.journals.uchicago.edu/ApJ/journal/.Comment: Accepted for publication: ApJS, 125. 38 pages, 9 figures;
supplementary electronic archive to be published by ApJ; available from lead
author upon reques
BATSE Gamma-Ray Burst Line Search: V. Probability of Detecting a Line in a Burst
The physical importance of the apparent discrepancy between the detections by
pre-BATSE missions of absorption lines in gamma-ray burst spectra and the
absence of a BATSE line detection necessitates a statistical analysis of this
discrepancy. This analysis requires a calculation of the probability that a
line, if present, will be detected in a given burst. However, the connection
between the detectability of a line in a spectrum and in a burst requires a
model for the occurrence of a line within a burst. We have developed the
necessary weighting for the line detection probability for each spectrum
spanning the burst. The resulting calculations require a description of each
spectrum in the BATSE database. With these tools we identify the bursts in
which lines are most likely to be detected. Also, by assuming a small frequency
with which lines occur, we calculate the approximate number of BATSE bursts in
which lines of various types could be detected. Lines similar to the Ginga
detections can be detected in relatively few BATSE bursts; for example, in only
~20 bursts are lines similar to the GB 880205 pair of lines detectable. Ginga
reported lines at ~20 and ~40 keV whereas the low energy cutoff of the BATSE
spectra is typically above 20 keV; hence BATSE's sensitivity to lines is less
than that of Ginga below 40 keV, and greater above. Therefore the probability
that the GB 880205 lines would be detected in a Ginga burst rather than a BATSE
burst is ~0.2. Finally, we adopted a more appropriate test of the significance
of a line feature.Comment: 20 pages, AASTeX 4.0, 5 figures, Ap.J. in pres
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
Possible Evidence for Relativistic Shocks in Gamma-Ray Bursts
Relativistic shock models of gamma-ray bursts may be tested by comparison of
their predicted low energy asymptotic spectral indices s to observations.
Synchrotron radiation theory predicts that the instantaneous spectrum has s = -
1/3 and the spectrum integrated over the radiative decay of the electrons'
energies has s = 1/2, with other cases lying between these limits. We examine
the spectra of 11 bursts obtained by the Large Area Detectors on BATSE. One
agrees with the predicted instantaneous spectrum, as does the initial portion
of a second, and three are close to the predicted integrated spectrum. All the
observed asymptotic spectral slopes lie in the predicted range. This evidence
for relativistic shocks is independent of detailed models of bursts and of
assumptions about their distances. Radiation observed with the predicted
instantaneous spectrum has a comparatively smooth time dependence, consistent
with the necessarily long radiation time, while that with the predicted
integrated spectrum has a spiky time dependence, consistent with the
necessarily short radiation time.Comment: 12 pp., latex, 16 figure
BATSE Gamma-Ray Burst Line Search: IV. Line Candidates from the Visual Search
We evaluate the significance of the line candidates identified by a visual
search of burst spectra from BATSE's Spectroscopy Detectors. None of the
candidates satisfy our detection criteria: an F-test probability less than
10^-4 for a feature in one detector and consistency among the detectors which
viewed the burst. Most of the candidates are not very significant, and are
likely to be fluctuations. Because of the expectation of finding absorption
lines, the search was biased towards absorption features. We do not have a
quantitative measure of the completeness of the search which would enable a
comparison with previous missions. Therefore a more objective computerized
search has begun.Comment: 18 pages AASTEX 4.0; 4 POSTSCRIPT figures on request from
[email protected]
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
BATSE SD Observations of Hercules X-1
The cyclotron line in the spectrum of the accretion-powered pulsar Her X-1
offers an opportunity to assess the ability of the BATSE Spectroscopy Detectors
(SDs) to detect lines like those seen in some GRBs. Preliminary analysis of an
initial SD pulsar mode observation of Her X-1 indicated a cyclotron line at an
energy of approximately 44 keV, rather than at the expected energy of
approximately 36 keV. Our analysis of four SD pulsar mode observations of Her
X-1 made during high-states of its 35 day cycle confirms this result. We
consider a number of phenomenological models for the continuum spectrum and the
cyclotron line. This ensures that we use the simplest models that adequately
describe the data, and that our results are robust. We find modest evidence
(significance Q ~ 10^-4-10^-2) for a line at approximately 44 keV in the data
of the first observation. Joint fits to the four observations provide stronger
evidence (Q ~ 10^-7-10^-4) for the line. Such a shift in the cyclotron line
energy of an accretion-powered pulsar is unprecedented.Comment: 5 pages, LaTeX (style files aipbook.sty, aps.sty, aps10.sty,
prabib.sty, psfig.sty, and revtex.sty included with PAPER.tex), 2 embedded
PostScript figures (mongo1.ps, mongo2.ps
BATSE Gamma-Ray Burst Line Search: II. Bayesian Consistency Methodology
We describe a Bayesian methodology to evaluate the consistency between the
reported Ginga and BATSE detections of absorption features in gamma ray burst
spectra. Currently no features have been detected by BATSE, but this
methodology will still be applicable if and when such features are discovered.
The Bayesian methodology permits the comparison of hypotheses regarding the two
detectors' observations, and makes explicit the subjective aspects of our
analysis (e.g., the quantification of our confidence in detector performance).
We also present non-Bayesian consistency statistics. Based on preliminary
calculations of line detectability we find that both the Bayesian and
non-Bayesian techniques show that the BATSE and Ginga observations are
consistent given our understanding of these detectors.Comment: accepted by Ap.J., 24 pages, LaTeX with AAS macros, request 4 figures
from [email protected]
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