John Patrick Lestrade

The Fourth BATSE Gamma-Ray Burst Catalog (Revised)

The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) has triggered on 1637 cosmic gamma-ray bursts between 1991 April 19 and 1996 August 29. These events constitute the Fourth BATSE burst catalog. The current version (4Br) has been revised from the version first circulated on CD-ROM in 1997 September (4B) to include improved locations for a subset of bursts that have been reprocessed using additional data. A significant difference from previous BATSE catalogs is the inclusion of bursts from periods when the trigger energy range differed from the nominal 50-300 keV. We present tables of the burst occurrence times, locations, peak fluxes, fluences, and durations. In general, results from previous BATSE catalogs are confirmed here with greater statistical significance.

The Third BATSE Gamma-Ray Burst Catalog

The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) has triggered on 1122 cosmic gamma-ray bursts between 1991 April 19 and 1994 September 19. These events constitute the Third BATSE (3B) burst catalog. This catalog includes the events previously reported in the 2B catalog, which covered the time interval 1991 April 19 to 1993 March 9. We present tables of the burst occurrence times, locations, peak fluxes, fluences, and durations. In general, results from previous BATSE catalogs are confirmed here with greater statistical significance. The angular distribution is consistent with isotropy. The mean galactic dipole and quadrupole moments are within 0.6 a and 0.3 a, respectively, of the values expected for isotropy. The intensity distribution is not consistent with a homogeneous distribution of burst sources, with V/V(sub max) = 0.33 +/- 0.01. The duration distribution (T(sub 90)) exhibits bimodality, with peaks at approx. 0.5 and approx. 30 s. There is no compelling evidence for burst repetition, but only weak limits can be placed on the repetition rate.

The Hardness-Duration Diagram of Gamma-Ray Bursts

The hardness-duration diagram for gamma-ray bursts still contains undiscovered important information about the intrinsic properties of these phenomena. We analyze these diagrams for the PHEBUS and BATSE experiments. First, we show that the BATSE diagram is very similar to that for PHEBUS when we restrict the BATSE data set to events observable by PHEBUS. In this case, both diagrams present a high degree of clustering into two subclasses. This shows that the brightness of the events is a more important factor in determining the aspect of this diagram than the hardness ratio energy ranges. Second, for the subclass of long bright bursts, both experiments show evidence for a positive correlation between hardness and duration. This is a significant new result, as it represents an intrinsic property of long events. The commonly held perception of an anticorrelation (between hardness and duration) for all bursts must be replaced by a more complicated model that treats the two subclasses separately. No statistically significant correlation is found within the short-event population. The existence of such an intrinsic positive correlation for the long bursts would complicate the search for cosmological effects in the gamma-ray burst data.

High-energy spectral breaks in gamma-ray bursts

Model fits are presented for 18 gamma-ray burst spectra from 100 keV to 27 MeV made with the BATSE spectroscopy detectors on the Compton Gamma Ray Observatory. Most of the bursts are well fitted as power laws with spectral indices between -1.36 and -2.29; however, five bursts show definite departures from a simple power-law fit at high energies. Three of these bursts are well fitted with broken power-law spectra and break energies of from 400 to 690 keV, such as might arise from photon-photon interactions. If so, then the source compactness and hence distance will be sharply constrained. Two of the bursts have spectra with sharply confined slope changes and are well fitted with broken power-law spectra with break energies of 1.2 and 1.6 MeV at peak, such as might arise from photon-magnetic field interactions. If so, then these spectral breaks provide strong evidence for the existence of high magnetic fields in the burst emission region.

The Emission Time of Gamma-Ray Bursts

The concept of emission time τN is suggested as a temporal parameter which is complementary to the classical parameters of duration times T50 and T90. The emission time is defined as the time of emission of N% of the total fluence. The definition adds the time bins of high fluence in decreasing fluence rank until N% of the fluence has been reached. The emission time interval excludes low-emission intervals of bursts, and so the emission time characterizes the state of high-power emission. The distribution of this new parameter is found to be bimodal for bright bursts. The distributions of emission time τ30 and τ50, for groups based on burst intensity, are also compared.

A new variability parameter for gamma-ray burst time profiles

We present a parameter that measures the structure of gamma-ray burst time profiles. This parameter is based on the statistics of runs and is a good measure of time profile variability. It is shown to be independent of burst duration and less sensitive to burst distance than algorithms that depend directly on the intensity of a burst.

Correlations between duration, hardness and intensity in GRBs

We present here the T90 and T50 distributions for different trigger time scales from the 3B catalog and confirm the bimodality established before with the 1B data. We show that the Hardness Ratio/duration anti-correlation is present in the 3B data set and we explore Hardness and Intensity correlations by studying various burst subsets. We discuss the impact of these correlations within the framework of a cosmological GRB origin hypothesis.

The 4B BATSE gamma-ray burst catalog

The 4B catalog of gamma-ray bursts includes all BATSE triggered bursts from launch until 29 August 1996. The total number of bursts is 1637, including 515 bursts since the end of the 3B catalog. The post-3B time interval includes periods when the trigger criteria were not at the nominal energy interval or intensity. Therefore, care must be exercised in deriving global properties from this catalog.

Microsecond flares in gamma-ray bursts

It has been suggested that gamma-ray burst light curves may consist of many superposed flares with a duration shorter than 30/microsec. If true, the implications for the interpretation of burst data are enormous. With the launch of the Compton Gamma-Ray Observatory, four predictions of Mitrofanovs (1989) suggestion can be tested. Our results which contradict this suggestion are (1) the photon arrival times are not correlated between independent detectors, (2) the spectral hardness and intensity does not depend on the detector area, (3) the bursts seen by detectors which measure photon positions do not see microsecond flares, and (4) burst positions deduced from detectors with different projected areas are close to the positions deduced from time-of-flight differences between separated spacecraft. We conclude, therefore, that gamma-ray bursts are not composed of microsecond flares.

Performance Of The Large-Area Detectors For The Burst And Transient Source Experiment (BATSE) On The Gamma Ray Observatory

The Burst and Transient Source Experiment (BATSE), one of four experiments on the Gamma Ray Observatory (GRO), is expected to provide the most sensitive observations of γ-ray bursts yet obtained, as well as to provide long-term monitoring of hard x-ray and low-energy γ-ray emission from bright pulsating sources, transients, and solar flares. Eight uncollimated modules, positioned at the corners of the spacecraft to provide an unobstructed view of the sky, detect sources by various techniques based on time variability. Use of detectors with anisotropic response allows location of γ-ray bursts to be determined to an accuracy of s1° using BATSE data alone. The completed BATSE underwent intensive testing and calibration prior to its delivery in October 1988 for integration on the GRO. We describe the instrument and summarize the results of the testing and calibration as they relate to characterization of systematic uncertainties in BATSE btirst location.