23 research outputs found
Cosmological versus Intrinsic: The Correlation between Intensity and the Peak of the nu F_nu Spectrum of Gamma Ray Bursts
We present results of correlation studies, examining the association between
the peak of the nu F_nu spectrum of gamma ray bursts, E_p, with the burst's
energy fluence and photon peak flux. We discuss methods to account for data
truncation in E_p and fluence or flux when performing the correlation analyses.
However, because bursts near the detector threshold are not usually able to
provide reliable spectral parameters, we focus on results for the brightest
bursts in which we can better understand the selection effects relevant to E_p
and burst strength.
We find that there is a strong correlation between total fluence and E_p. We
discuss these results in terms of both cosmological and intrinsic effects.
In particular, we show that for realistic distributions of the burst
parameters, cosmological expansion alone cannot account for the correlation
between E_p and total fluence; the observed correlation is likely a result of
an intrinsic relation between the burst rest-frame peak energy and the total
radiated energy. We investigate this latter scenario in the context of
synchrotron radiation from external and internal shock models of GRBs. We find
that the internal shock model is consistent with our interpretation of the
correlation, while the external shock model cannot easily explain this
intrinsic relation between peak energy and burst radiated energy.Comment: 23 pages, including 8 postscript figures. Submitted to Ap
AI Gamma-Ray Burst Classification: Methodology/Preliminary Results
Artificial intelligence (AI) classifiers can be used to classify unknowns,
refine existing classification parameters, and identify/screen out ineffectual
parameters. We present an AI methodology for classifying new gamma-ray bursts,
along with some preliminary results.Comment: 5 pages, 2 postscript figures. To appear in the Fourth Huntsville
Gamma-Ray Burst Symposiu
Properties of Gamma-Ray Burst Classes
The three gamma-ray burst (GRB) classes identified by statistical clustering
analysis (Mukherjee et al. 1998) are examined using the pattern recognition
algorithm C4.5 (Quinlan 1986). Although the statistical existence of Class 3
(intermediate duration, intermediate fluence, soft) is supported, the
properties of this class do not need to arise from a distinct source
population. Class 3 properties can easily be produced from Class 1 (long, high
fluence, intermediate hardness) by a combination of measurement error,
hardness/intensity correlation, and a newly-identified BATSE bias (the fluence
duration bias). Class 2 (short, low fluence, hard) does not appear to be
related to Class 1.Comment: 5 pages, 4 imbedded figures, presented at the 5th Huntsville
Gamma-Ray Burst Symposiu
BATSE Observations of Gamma-Ray Burst Spectra. IV. Time-Resolved High-Energy Spectroscopy
We report on the temporal behavior of the high-energy power law continuum
component of gamma-ray burst spectra with data obtained by the Burst and
Transient Source Experiment. We have selected 126 high fluence and high flux
bursts from the beginning of the mission up until the present. Much of the data
were obtained with the Large Area Detectors, which have nearly all-sky
coverage, excellent sensitivity over two decades of energy and moderate energy
resolution, ideal for continuum spectra studies of a large sample of bursts at
high time resolution. At least 8 spectra from each burst were fitted with a
spectral form that consisted of a low-energy power law, a spectral break at
middle energies and a high-energy continuum. In most bursts (122), the
high-energy continuum was consistent with a power law. The evolution of the
fitted high-energy power-law index over the selected spectra for each burst is
inconsistent with a constant for 34% of the total sample. The sample
distribution of the average value for the index from each burst is fairly
narrow, centered on -2.12. A linear trend in time is ruled out for only 20% of
the bursts, with hard-to-soft evolution dominating the sample (100 events). The
distribution for the total change in the power-law index over the duration of a
burst peaks at the value -0.37, and is characterized by a median absolute
deviation of 0.39, arguing that a single physical process is involved. We
present analyses of the correlation of the power-law index with time, burst
intensity and low-energy time evolution. In general, we confirm the general
hard-to-soft spectral evolution observed in the low-energy component of the
continuum, while presenting evidence that this evolution is different in nature
from that of the rest of the continuum.Comment: 30 pages, with 2 tables and 9 figures To appear in The Astrophysical
Journal, April 1, 199
The Compatibility of Friedmann Cosmological Models with Observed Properties of Gamma-Ray Bursts and a Large Hubble Constant
The distance scale to cosmic gamma-ray bursts (GRB's) is still uncertain by many orders of magnitude; however, one viable scenario places GRB's at cosmological distances, thereby permitting them to be used as tracers of the cosmological expansion over a significant range of redshifts zeta. Also, several recent measurements of the Hubble constant H(sub 0) appearing in the referred literature report values of 70-80 km/s /Mpc. Although there is significant debate regarding these measurements, we proceed here under the assumption that they are evidence of a large value for H(sub 0). This is done in order to investigate the additional constraints on cosmological models that can be obtained under this hypothesis when combined with the age of the universe and the brightness distribution of cosmological gamma-ray bursts. We show that the range of cosmological models that can be consistent with the GRB brightness distribution, a Hubble constant of 70-80 km/s/Mpc, and a minimum age of the universe of 13-15 Gyr is constrained significantly, largely independent of a wide range of assumptions regarding the evolutionary nature of the burst population. Low-density, Lambda greater than 0 cosmological models with deceleration parameter in the range -1 less than q(sub 0) less than 0 and density parameter sigma(sub 0) in the range approximately equals 0.10-0.25(Omega(sub 0) approximately equals 0.2-0.5) are strongly favored
The detector response matrices of the burst and transient source experiment (BATSE) on the Compton Gamma Ray Observatory
The detector response matrices for the Burst And Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO) are described, including their creation and operation in data analysis. These response matrices are a detailed abstract representation of the gamma-ray detectors' operating characteristics that are needed for data analysis. They are constructed from an extensive set of calibration data coupled with a complex geometry electromagnetic cascade Monte Carlo simulation code. The calibration tests and simulation algorithm optimization are described. The characteristics of the BATSE detectors in the spacecraft environment are also described
FiberGLAST: a scintillating fiber approach to the GLAST mission
FiberGLAST is a scintillating fiber gamma-ray detector designed for the GLAST mission. The system described below provides superior effective area and field of view for modest cost and risk. An overview of the FiberGLAST instrument is presented, as well as a more detailed description of the principle elements of the primary detector volume. The triggering and readout electronics are described, and Monte Carlo Simulations of the instrument performance are presented
Beam test results for the FiberGLAST instrument
The FiberGLAST scintillating fiber telescope is a large-area instrument concept for NASA\u27s GLAST program. The detector is designed for high-energy gamma-ray astronomy, and uses plastic scintillating fibers to combine a photon pair tracking telescope and a calorimeter into a single instrument. A small prototype detector has been tested with high energy photons at the Thomas Jefferson National Accelerator Facility. We report on the result of this beam test, including scintillating fiber performance, photon track reconstruction, angular resolution, and detector efficiency
Estimation of GRB detection by FiberGLAST
FiberGLAST is one of several instrument concepts being developed for possible inclusion as the primary Gamma-ray Large Area Space Telescope (GLAST) instrument. The predicted FiberGLAST effective area is more than 12,000 cm2 for energies between 30 MeV and 300 GeV, with a field of view that is essentially flat from 0°–80°. The detector will achieve a sensitivity more than 10 times that of EGRET. We present results of simulations that illustrate the sensitivity of FiberGLAST for the detection of gamma-ray bursts