823 research outputs found
Testing the Gamma-Ray Burst Pulse Start Conjecture
We test the hypothesis that prompt gamma-ray burst pulse emission starts
simultaneously at all energies (the Pulse Start Conjecture). Our analysis,
using a sample of BATSE bursts observed with four channel, 64-ms data and
performed using a pulse fit model, generally supports this hypothesis for the
Long GRB class, although a few discrepant pulses belong to bursts observed
during times characterized by low signal-to-noise, hidden pulses, and/or
significant pulse overlap. The typical uncertainty in making this statement is
< 0.4 s for pulses in Long GRBs (and < 0.2 s for 40% of the pulses) and perhaps
< 0.1 s for pulses in Short GRBs. When considered along with the Epk decline
found in GRB pulse evolution, this result implies that energy is injected at
the beginning of each and every GRB pulse, and the subsequent spectral
evolution, including the pulse peak intensity, represents radiated energy
losses from this initial injection.Comment: 34 pages, 17 figures, 3 tables, accepted for publication in The
Astrophysical Journa
Extension of an Exponential Light Curve GRB Pulse Model Across Energy Bands
A simple mathematical model of GRB pulses in time, suggested in Norris et al.
(2005), is extended across energy. For a class of isolated pulses, two of those
parameters appear effectively independent of energy. Specifically, statistical
fits indicate that pulse amplitude and pulse width are energy
dependent, while pulse start time and pulse shape are effectively energy
independent. These results bolster the Pulse Start and Pulse Scale conjectures
of Nemiroff (2000) and add a new Pulse Shape conjecture which states that a
class of pulses all have the same shape. The simple resulting pulse counts
model is , where is the
time since the start of the pulse. This pulse model is found to be an
acceptable statistical fit to many of the fluent separable BATSE pulses listed
in Norris et al. (2005). Even without theoretical interpretation, this
cross-energy extension may be immediately useful for fitting prompt emission
from GRB pulses across energy channels with a minimal number of free
parameters.Comment: 11 pages, 5 figures. Accepted by MNRA
Unification of Pulses in Long and Short Gamma-Ray Bursts: Evidence from Pulse Properties and their Correlations
We demonstrate that distinguishable gamma-ray burst pulses exhibit similar
behaviors as evidenced by correlations among the observable pulse properties of
duration, peak luminosity, fluence, spectral hardness, energy-dependent lag,
and asymmetry. Long and Short burst pulses exhibit these behaviors, suggesting
that a similar process is responsible for producing all GRB pulses. That these
properties correlate in the observer's frame indicates that intrinsic
correlations are strong enough to not be diluted into insignificance by the
dispersion in distances and redshift. We show how all correlated pulse
characteristics can be explained by hard-to-soft pulse evolution, and we
demonstrate that "intensity tracking" pulses not having these properties are
not single pulses; they instead appear to be composed of two or more
overlapping hard-to-soft pulses. In order to better understand pulse
characteristics, we recognize that hard-to-soft evolution provides a more
accurate definition of a pulse than its intensity variation. This realization,
coupled with the observation that pulses begin near-simultaneously across a
wide range of energies, leads us to conclude that the observed pulse emission
represents the energy decay resulting from an initial injection, and that one
simple and as yet unspecified physical mechanism is likely to be responsible
for all gamma-ray burst pulses regardless of the environment in which they form
and, if GRBs originate from different progenitors, then of the progenitors that
supply them with energy.Comment: 35 pages including 11 figures and 4 tables, accepted for publication
in The Astrophysical Journa
BATSE software for the analysis of the gamma ray burst spatial distribution
The Burst and Transient Source Experiment (BATSE) on the Gamma Ray Observatory (GRO) is designed to study astronomical gamma ray sources and to provide better positional, spectral, and time resolution about these objects than has previously been possible from one experiment. The procedure to be used in the analysis of the gamma ray burst spatial distribution is presented. Data is input from BATSE via the Gamma Ray Burst Catalog (listing individual burst positions, flux values, and associated errors) and the Sky Sensitivity Map (which summarizes observational selection effects in table format). A FORTRAN program generates Monte Carlo burst catalogs, which are models to be compared to the actual distribution. The Monte Carlo models are then filtered through the Sky Sensitivity Map so that they suffer from the same selection effects as the actual catalog data. Additionally, each burst position is converted into a probability distribution to mimic BATSE positional sensitivity. The Burst Catalog, Monte Carlo burst catalog, and Sky Sensitivity Map are then passed onto an IDL program that compares the catalogs for statistical significance. The Sky Sensitivity Map is used to estimate how often each sky area is observed above the minimum flux level in question. Each burst found in this sky area is then weighted according to the frequency with which this sky area is observed. The catalogs are then compared via tests of homogeneity (based on their radial distributions) and isotropy (based upon their angular distributions). The results of the statistical comparisons along with graphs and charts of the summaries, are output from the IDL program for study
Monte Carlo models and analysis of galactic disk gamma-ray burst distributions
Gamma-ray bursts are transient astronomical phenomena which have no quiescent counterparts in any region of the electromagnetic spectrum. Although temporal and spectral properties indicate that these events are likely energetic, their unknown spatial distribution complicates astrophysical interpretation. Monte Carlo samples of gamma-ray burst sources are created which belong to Galactic disk populations. Spatial analysis techniques are used to compare these samples to the observed distribution. From this, both quantitative and qualitative conclusions are drawn concerning allowed luminosity and spatial distributions of the actual sample. Although the Burst and Transient Source Experiment (BATSE) experiment on Gamma Ray Observatory (GRO) will significantly improve knowledge of the gamma-ray burst source spatial characteristics within only a few months of launch, the analysis techniques described herein will not be superceded. Rather, they may be used with BATSE results to obtain detailed information about both the luminosity and spatial distributions of the sources
The Two-Point Angular Correlation Function and BATSE Sky Exposure
The Two-Point Angular Correlation Function is a standard analysis tool used
to study angular anisotropies. Since BATSE's sky exposure (the angular sampling
of gamma-ray bursts) is anisotropic, the TPACF should at some point identify
anisotropies in BATSE burst catalogs due to sky exposure. The effects of BATSE
sky exposure are thus explored here for BATSE 3B and 4B catalogs. Sky-exposure
effects are found to be small.Comment: 5 pages, 1 postscript figure. To appear in the Fourth Huntsville
Gamma-Ray Burst Symposiu
Possible structure in the GRB sky distribution at redshift two
Context. Research over the past three decades has revolutionized cosmology
while supporting the standard cosmological model. However, the cosmological
principle of Universal homogeneity and isotropy has always been in question,
since structures as large as the survey size have always been found each time
the survey size has increased. Until 2013, the largest known structure in our
Universe was the Sloan Great Wall, which is more than 400 Mpc long located
approximately one billion light years away.
Aims. Gamma-ray bursts are the most energetic explosions in the Universe. As
they are associated with the stellar endpoints of massive stars and are found
in and near distant galaxies, they are viable indicators of the dense part of
the Universe containing normal matter. The spatial distribution of gamma-ray
bursts can thus help expose the large scale structure of the Universe.
Methods. As of July 2012, 283 GRB redshifts have been measured. Subdividing
this sample into nine radial parts, each containing 31 GRBs, indicates that the
GRB sample having 1.6 < z < 2.1 differs significantly from the others in that
14 of the 31 GRBs are concentrated in roughly 1/8 of the sky. A two-dimensional
Kolmogorov-Smirnov test, a nearest-neighbour test, and a Bootstrap Point-Radius
Method explore the significance of this clustering.
Results. All tests used indicate that there is a statistically significant
clustering of the GRB sample at 1.6 < z < 2.1. Furthermore, this angular excess
cannot be entirely attributed to known selection biases, making its existence
due to chance unlikely.
Conclusions. This huge structure lies ten times farther away than the Sloan
Great Wall, at a distance of approximately ten billion light years. The size of
the structure defined by these GRBs is about 2000-3000 Mpc, or more than six
times the size of the Sloan Great Wall.Comment: accepted for publication in Astronomy & Astrophysic
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
Correlation between the isotropic energy and the peak energy at zero fluence for the individual pulses of GRBs: towards an universal physical correlation for the prompt emission
We find a strong correlation between the peak energy at zero fluence () and the isotropic energy () of the 22 pulses
of 9 Gamma Ray Bursts (GRB) detected by the Fermi satellite. The correlation
holds for the individual pulses of each GRB, which shows the reality of the
correlation. The derived correlation (Spearman correlation coefficient, , is
0.96) is much stronger compared to the correlations using (in
place of ) determined from the time-integrated spectrum ( =
0.8), or the time-resolved spectrum not accounting for broad pulse structures
( = 0.37), or the pulse-wise spectrum ( = 0.89). Though the improvement
in the - relation (the Amati relation) for
a pulse-wise analysis is known earlier, this is the first time a parameter
derived from a joint spectral and timing fit to the data is shown to improve
the correlation. We suggest that , rather than ,
is intrinsic to a GRB pulse and a natural choice as the parameter in the
pulse-wise correlation studies.Comment: 6 pages, 2 tables, 2 figures, accepted for publication in Ap
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