2,519 research outputs found
GRB Spectral Hardness and Afterglow Properties
A possible relationship between the presence of a radio afterglow and
gamma-ray burst spectral hardness is discussed. The correlation is marginally
significant; the spectral hardness of the bursts with radio afterglows
apparently results from a combination of the break energy Ebreak and the
high-energy spectral index beta. If valid, this relationship would indicate
that the afterglow does carry information pertaining to the GRB central engine.Comment: 5 pages, 3 figures, presented at the 5th Huntsville Gamma-Ray Burst
Symposiu
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
A Simple BATSE Measure of GRB Duty Cycle
We introduce a definition of gamma-ray burst (GRB) duty cycle that describes
the GRB's efficiency as an emitter; it is the GRB's average flux relative to
the peak flux. This GRB duty cycle is easily described in terms of measured
BATSE parameters; it is essentially fluence divided by the quantity peak flux
times duration. Since fluence and duration are two of the three defining
characteristics of the GRB classes identified by statistical clustering
techniques (the other is spectral hardness), duty cycle is a potentially
valuable probe for studying properties of these classes.Comment: 4 pages, 1 figure, presented at the 5th Huntsville Gamma-Ray Burst
Symposiu
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
On the Hardness-Intensity Correlation in Gamma-Ray Burst Pulses
We study the hardness-intensity correlation (HIC) in gamma-ray bursts (GRBs).
In particular, we analyze the decay phase of pulse structures in their light
curves. The study comprises a sample of 82 long pulses selected from 66 long
bursts observed by BATSE on the Compton Gamma-Ray Observatory. We find that at
least 57% of these pulses have HICs that can be well described by a power law.
The distribution of the power law indices, obtained by modeling the HIC of
pulses from different bursts, is broad with a mean of 1.9 and a standard
deviation of 0.7. We also compare indices among pulses from the same bursts and
find that their distribution is significantly narrower. The probability of a
random coincidence is shown to be very small. In most cases, the indices are
equal to within the uncertainties. This is particularly relevant when comparing
the external versus the internal shock models. In our analysis, we also use a
new method for studying the HIC, in which the intensity is represented by the
peak value of the E F_E spectrum. This new method gives stronger correlations
and is useful in the study of various aspects of the HIC. In particular, it
produces a better agreement between indices of different pulses within the same
burst. Also, we find that some pulses exhibit a "track jump" in their HICs, in
which the correlation jumps between two power laws with the same index. We
discuss the possibility that the "track jump" is caused by strongly overlapping
pulses. Based on our findings, the constancy of the index is proposed to be
used as a tool for pulse identification in overlapping pulses.Comment: 20 pages with 9 eps figures (emulateapj), ApJ accepte
Synchrotron Emission as the Source of GRB Spectra, Part II: Observations
We test the models of synchrotron emission presented in Part I of this series
(Lloyd & Petrosian, these proceedings) against the distributions and evolution
of GRB spectral parameters (particularly the low energy index, ). With
knowledge of the distribution and the correlation between and
presented in Part I, we show how to derive the expected distribution of
from fits to optically thin synchrotron spectra, and compare this with
the observed distribution. We show that there is no difficulty explaining
bursts below the ``line of death'', , and that these bursts
indicate that the spectrum of accelerated electrons must flatten or decline at
low energies. Bursts with low energy spectral indices that fall above this
limit are explained by the synchrotron self-absorption frequency entering the
lower end of the BATSE window. Finally, we discuss a variety of spectral
evolution behavior seen in GRBs and explain this behavior in the context of
synchrotron emission from internal shocks.Comment: To appear in the proceedings of the 5th Huntsville Symposium on Gamma
Ray Burst
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