Search

Three short very soft gamma-ray (SGR) transient events from a location consistent with that of the SGR 1900 + 14, first described by Mazets et al. (1979), were detected by the Burst and Transient Source Experiment. The results of observations of the temporal and spectral properties of the SGR 1900 + 14 suggest that the SGR phase lasts at least 13 years, lending support to the suggestion by Kouveliotou et al. (1987) and Fishman et al. (1989) that SGRs are related to galactic (possibly population I) objects, perhaps neutron stars.

An analysis of 153 gamma-ray bursts detected by the Burst and Transient Source Experiment on the Compton Gamma Ray Observatory is reported. The number versus intensity distribution does not follow the -3/2 power law expected for a spatially extended homogeneous distribution of sources, but at the same time the angular distribution is isotropic within statistical limits. Taken together these results are consistent with the spatial distribution of any known population of galactic objects, but may be consistent with the bursts being at cosmological distances.

We use dipole and quadrupole statistics to test the large-scale isotropy of the first 1005 gamma-ray bursts observed by the Burst and Transient Source Experiment (BATSE). In addition to the entire sample of 1005 gamma-ray bursts, many subsets are examined. We use a variety of dipole and quadrupole statistics to search for Galactic and other predicted anisotropies and for anisotropies in a coordinate-system independent manner. We find the gamma-ray burst locations to be consistent with isotropy, e.g., for the total sample the observed Galactic dipole moment differs from the value predicted for isotropy by 0.9 sigma and the observed Galactic quadrupole moment by 0.3 sigma. We estimate for various models the anisotropies that could have been detected. If one-half of the locations were within 86 degrees of the Galactic center, or within 28 degrees of the Galactic plane, the ensuing dipole or quadrupole moment would have typically been detected at the 99% confidence level. We compare the observations with the dipole and quadrupole moments of various Galactic models. Several Galactic gamma-ray bursts models have moments within 2 sigma of the observations; most of the Galactic models proposed to date are no longer in acceptable agreement with the data. Although a spherical Dark Matter Halo distribution could be consistent with the data, the required core radius is larger than the core radius of the Dark Matter Halo used to explain the Galaxy's rotation curve. Gamma-ray bursts are much more isotropic than any observed Galactic population, strongly favoring but not requiring an origin at cosmological distances., Comment: Accepted for publication in ApJ, 1996 March 1. 40 pages of text and tables. A plain Postscript version, including additional tables and the figures (2.6 Mb) is available under "Research Articles" at http://xanth.msfc.nasa.gov/astro/batse/batse_home.shtml

A time dependent two-point correlation-function analysis of the BATSE 2B catalog finds no evidence of burst repetition. As part of this analysis, we discuss the effects of sky exposure on the observability of burst repetition and present the equation describing the signature of burst repetition in the data. For a model of all burst repetition from a source occurring in less than five days we derive upper limits on the number of bursts in the catalog from repeaters and model-dependent upper limits on the fraction of burst sources that produce multiple outbursts., To appear in the Astrophysical Journal Letters, uuencoded compressed PostScript, 11 pages with 4 embedded figures

If classical GRBs were shown to repeat, cosmological models involving merging neutron stars inside galaxies would in effect be ruled out. We investigate the question of burst repetition by searching for the corresponding signal in the correlation function and nearest neighbor distribution. We analyze data for 742 γ‐ray bursts observed with BATSE. Preliminary results derived from this study do not support the recent claim by Lamb & Quashnock that a significant fraction of bursts repeat on time scales less than ∼1 year.

The current performance of BATSE is described, emphasizing the aspects related to gamma‐ray burst observations. Flight software modifications since launch are briefly described which affect burst data, along with the methods of distinguishing true cosmic gamma‐ray bursts from other sources of triggers. Finally, we describe a previously unreported, rare type of trigger that may have significance for atmospheric, ionospheric, and lightning research: terrestrial gamma‐ray flashes.

Galactic corona/extended halo gamma‐ray burst source distributions are analyzed and compared to BATSE observations. Models have radial densities of the form n(r)=n c [1+(r/r c )α]−1, where α=2 for coronal dark matter, and the density is constant inside the core radius r c . M31 is assumed to have a burst population similar to that of the Milky Way, while other Local Group galaxies are ignored. The parameter space of all such single‐population models is bounded, with 1.5≤α

We summarize the global properties of 743 gamma‐ray bursts observed by BATSE since launch in April of 1991. We emphasize those observations relevant to the central problem of the spatial distribution of the burst sources. The high degree of isotropy, combined with an intensity distribution inconsistent with homogeneity, severely constrains geometrical models for source distributions. Various representations of the intensity distribution are discussed. Evidence for repeating sources in the first BATSE catalog do not appear to be confirmed in later bursts. The distribution of burst durations shows a bimodality with peaks at about 0.3 seconds and 30 seconds. Spectral characteristics are summarized.

We search for dipole or quadrupole anisotropies in the locations of the first 743 gamma‐ray bursts observed by BATSE. No convincing anisotropies have been found.

In the BATSE data there exist cosmic gamma‐ray bursts which do not meet the on‐board trigger criteria. We report here the results of an off‐line, expost‐facto; search for bursts which would trigger on 1.024 s or 4.096 s timescale integrations. These bursts did not set the on‐board trigger.

Angular correlation analyses are standard tools used to understand the distributions of numerous celestial objects, including gamma‐ray bursts. Three useful methods of testing for clustering are binning analyses, nearest neighbor tests, and the correlation function. We present the results of a preliminary investigation into the clustering properties of the bursts detected by BATSE. We find no statistically significant evidence of bursts clustering around the directions of M31, the LMC, or the Virgo Cluster, and there is no evidence for bursts tending to cluster among themselves.

The paradigm that gamma‐ray bursts originate from Galactic populations is studied in detail using the BATSE angular and intensity distributions. Monte Carlo models of gamma‐ray burst spatial distributions and luminosity functions are created, then folded through mathematical models of BATSE selection effects. The BATSE Burst Catalog and the Monte Carlo Burst Catalogs are analyzed using modified statistical homogeneity and isotropy tests. Numerous catalog results are studied so that errors can be established.Analysis of BATSE angular and intensity distributions greatly constrains origins and luminosities of bursts. No single population of Galactic Disk, Halo, or Local Spiral Arm sources satisfactorily explains BATSE observations (and burst luminosity functions are of secondary importance when considering such models). One family of models still satisfying BATSE data is comprised of sources in a spherical Galactic Corona. Coronal models are limited to small ranges of burst luminosity and core radius. Multip...

Two statistics and the corresponding tests of isotropy are discussed. These tests are coordinate‐system independent and thus model‐independent, and they yield an analytic significance for any detected anisotropy. The Rayleigh‐Watson statistic W measures the size of the dipole moment of the locations, while the Bingham statistic B measures the deviation of the quadrupole moments from the values expected for isotropy. These tests are applied to the locations of the first 447 BATSE gamma‐ray bursts.

The large area detectors on the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) are being used to monitor about 40 sources in the energy range of 20–1800 keV and to search for transients on a daily basis. We present an overview of the sources which have been detected with the BATSE instrument, and discuss some of the properties of these sources which can be studied. Time histories of the flux and energy spectra of recently detected transient hard x‐ray sources are presented.

The Large Area Detectors of the BATSE instrument combine the benefits of large collecting area for good statistics and good energy resolution, thus they should be a very useful tool for the detection of spectral features in gamma ray bursts. The best candidate line features would have to be moderately broad (≥20 keV at around 50 keV) to be resolved. The BATSE Spectroscopy Detectors have been at the focus of efforts to confirm previous observations of line‐like spectral features, particularly those observed by the GINGA instrument. Here, we have made a study to characterize the spectroscopic capabilities of the Large Area Detectors in their response to GINGA‐like features. With two essentially distinct all‐sky monitoring spectroscopy instruments, the Large Area Detectors and the Spectroscopy Detectors, BATSE has the potential to confirm its own observations of spectral features.

To understand the source of gamma‐ray bursts, we must distinguish between theories placing the sources within and around our galaxy and theories placing them well beyond our galaxy. To make that distinction, we must accurately determine the direction to bursts. Previous directions determined by the Burst and Transient Source Experiment on the Compton Gamma ray Observatory included systematic errors of around 7 degrees in addition to statistical errors between 1 and 20 degrees resulting from the Poisson uncertainty in photon flux measurements. Although these errors did not significantly affect our reported measurement of moments of the γ‐ray burst distribution, we made a concerted effort to understand and reduce the systematic errors in BATSE’s burst directions. We report here on the result of our efforts.

Gamma‐ray bursts are being detected with unprecedented sensitivity by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray observatory since its launch in April, 1991. The experiment is detecting about one gamma‐ray burst per day. A brief description is presented of the on‐orbit performance of BATSE, the methods of identification of bursts, and examples of the diverse time profiles of the gamma‐ray bursts observed. The most significant finding thus far is the apparent isotropy of the bursts together with the observed inhomogeneity of the sources.

Dipole and quadrupole tests of isotropy have been and will continue to be applied to the locations of the gamma‐ray bursts observed by BATSE. To date, these tests show the locations to be statistically consistent with isotropy. This paper examines three effects that limit BATSE’s ability to detect a small dipole or quadrupole anisotropy: 1) the nonzero errors on the locations, 2) the finite number of bursts observed, and 3) the nonuniform sky exposure. The dominant limitation is, and will continue to be, the finite number of bursts observed.

The gamma‐ray background on the BATSE experiment is a complex, time‐varying combination of several factors. These include diffuse galactic and cosmic background; activation induced by particle fluxes in the SAA; atmospheric gamma‐rays; discrete source contributions; and other factors. We have developed techniques for fitting the Continuous and Discriminator BATSE data with multi‐component models. Some of these components are intended to directly represent factors comprising the background, while others are more empirical in nature. The model is used to ‘‘detrend’’ the data and can be used for on‐ground folding in pulsar analysis and searches for long period pulsations, searches for weak, long duration gamma‐ray bursts and transients, background subtraction from known gamma‐ray bursts and other transients, and construction of the burst sky exposure map. Components of these models can be used to measure the diffuse background and obtain refined measurements of discrete sources.

The Burst and Transient Source Experiment (BATSE) consists of eight anisotropic γ‐ray spectrometers at the corners of the Compton Gamma Ray Observatory. BATSE monitors the full sky from a fixed orientation and determines the direction of γ‐ray bursts with an accuracy appropriate for studying the bursts’ celestial distribution. We describe the calculation of γ‐ray burst directions from measurements made by BATSE. We present a sample of calculated directions from BATSE’s measurement of solar flares and compare the calculated directions with the solar direction. We describe the systematic errors apparent in these data and discuss ongoing efforts to correct them.

The Burst and Transient Source Experiment (BATSE) detects any γ‐ray burst not obscured by the earth and intense enough to produce a sufficiently strong signal in at least two of its eight large area NaI detectors (LADs). For a given intensity, a LAD records a stronger signal from a source which directly faces it than from a source to its side. Thus, BATSE’s detection of a γ‐ray burst may depend upon the burst’s direction. We discuss the effect of this uneven exposure to the sky on the instrument’s measurement of the distribution of γ‐ray burst directions and intensities, and we describe our method of accounting for the effect.

The Burst and Transient Source Experiment (BATSE) on the Gamma Ray Observatory, having an all‐sky observing capability and high sensitivity in the low energy gamma‐ray region, has a reasonable change of detecting emission from a nearby nova within the first few hours of its outburst.

The spatial distribution of gamma‐ray burst source is investigated using the intensity distribution and the angular distribution and the angular distribution of bursts observed by BATSE. The intensity distribution exhibits fewer weak burst than would be expected for a homogeneous distribution of sources. However, the angular distribution is consistent with an isotropic distribution. These results are compared to galactic disk and halo models of gamma‐ray burst sources.

Preliminary results pertaining to burst location using BATSE large Area Detector measurements of solar flares are presented. These solar flare measurements are currently being used to fine tune the calibration of our data analysis software. The current status of techniques for identifying and eliminating systematic errors from the data analysis tools is discussed. Data revealing the effects of the atmospheric scattering algorithm that will be implemented in the data analysis software are presented.

GAMMA-RAY bursts (GRBs) vary in duration from hundreds of seconds down to several milliseconds. Early studies1 suggested that bursts with durations of

The demonstration of repeated gamma-ray bursts from an individual source would severely constrain burst source models. Recent reports (Quashnock and Lamb 1993; Wang and Lingenfelter 1993) of evidence for repetition in the first BATSE burst catalog have generated renewed interest in this issue. Here, we analyze the angular distribution of 585 bursts of the second BATSE catalog (Meegan et al. 1994). We search for evidence of burst recurrence using the nearest and farthest neighbor statistic and the two-point angular correlation function. We find the data to be consistent with the hypothesis that burst sources do not repeat; however, a repeater fraction of up to about 20% of the observed bursts cannot be excluded., ApJ Letters, in press, 13 pages, including three embedded figures. uuencoded Unix-compressed PostScript

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.

Two hard state (X-ray low state) outbursts in the GX 339-4 system in 1991 June-October and 1992 September-1993 January were detected and monitored by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO). The outbursts were remarkably similar in both temporal and spectral aspects. The spectrum in the 20-300 keV band evolves from a hard, Comptonized shape to a steeper (alpha approximately equals -3) power law during each outburst.

Three scenarios of burst distributions seem geometricaly compatible with the data obtained by BATSE: association of bursts with a heliocentric distribution, bursts in an extended galactic halo, and bursts at cosmological distances. Detection of a slight deviation from isotropy in the angular distribution would provide an important clue to determining the distances to the bursts, by discrimination among the three scenarios. The ability to tightly constrain the amount of anisotropy present in the distribution is limited, in part, by uncertainties in the individual burst locations. Using bursts whose locations have been determined accurately and independently by the Third Interplanetary Network, we derive an estimate of the BATSE location uncertainties. We then incorporate these into 3—dimensional Monte Carlo simulations of burst distributions to show that despite individual location uncertainties of ∼5°, the ability of BATSE to place stringent limits on the anisotropy present in the angular distribution of the gamma‐ray bursts is not significantly degraded. Once the amount of allowable anisotropy is known, constaints can be placed on the possible parent distributions of the burst sources. We conclude with a brief discussion of these constraints for galactic halo models.