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NASA NTRS1992

BATSE observations of gamma-ray bursts

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.

Fishman, Gerald J.↗
NASA NTRS1992

Extragalactic scenarios for gamma-ray bursts

The cosmological distance scale to gamma-ray bursts is indicated by the observed distribution of the bursts in angle and intensity. The redshifts of the weakest and the strongest bursts detected by the BATSE experiment are estimated to be z = 1.5 and z = 0.2, respectively, while the strongest bursts detected by the PVO in its 11 years of operation were at z = 0.05. The bursts' rate is about 10 exp -6 per year in a galaxy like ours, and a typical energy release is about 10 exp 51 ergs, assuming isotropic emission. The rate of the neutron star and black hole collisions in close binaries is somewhat higher, and those collisions release 10 exp 53 - 10 exp 54 ergs in a burstlike form, making them the primary candidates for the observed gamma-ray events. Contrary to the common belief, no observations available to date are in conflict with the cosmological distance to the bursters. A detection of gravitational lensing of some bursts, the redshift of the weakest events, and the absorption of soft X-rays by the interstellar matter in the plane of our galaxy, are the specific predictions of the cosmological scenario.

Paczynski, Bohdan↗
NASA NTRS1992

Hardness/intensity correlations among BATSE bursts

Conclusions about the nature of gamma-ray bursts derived from the size-frequency distribution may be altered if a significant correlation exists between burst intensity and spectral shape. Moreover, if gamma-ray bursts have a cosmological origin, such a correlation may be expected to result from the expansion of the universe. We have performed a rudimentary search of the BATSE bursts for hardness/intensity correlations. The range of spectral shapes was determined for each burst by computing the ratio of the intensity in the range 100-300 keV to that in 55-300 keV. We find weak evidence for the existence of a correlation, the strongest effect being present when comparing the maximum hardness ratio for each burst with its maximum rate.

Paciesas, William S.↗
NASA NTRS1992

Burst detection capability of the Mars Observer gamma-ray spectrometer

This paper describes the gamma-ray burst detection capability of the gamma-ray spectrometer on board the Mars Observer spacecraft. The instrument is designed to respond to significant bursts by suspending its observation of the Martian surface in favor of optimizing the collection of pre-burst, burst, and post-burst data. The results are expected to contribute to our understanding of gamma-ray bursts through a high energy resolution search for line emission, by the examination of spectral shapes and by comprising one leg of an arrival time network to determine the direction of origin.

Metzger, Albert E.↗
NASA NTRS1993

Successive X-ray bursts from accreting neutron stars

The evolution of a neutron star undergoing a series of thermonuclear flashes in its accreted hydrogen-rich layer has been numerically followed to determine the effects of the history of the neutron star's thermal and compositional structure on the properties of the emitted X-ray bursts. Burst characteristics are studied for a range of mass accretion rates, CNO abundances in the accreted matter, and initial thermal states of the underlying neutron star core; the bursts exhibit erratic behavior for low CNO metal abundances and cool neutron star cores, with the burst recurrence time scales varying by 1-2 orders of magnitude. There is typically a continued presence of a substantial amount of unburnt hydrogen in the accreted layer throughout the series of the X-ray burst events. Convective mixing during the quiescent phase leads to the inward transport of helium to high densities and eventually to the initiation of the next outburst. The resulting bursts can be weak and, in such cases, are characterized by short recurrence time scales (1-2 hr), low peak luminosities (0.1-0.2 times the Eddington value), and low alpha-values (about 20).

Taam, Ronald E.↗
NASA NTRS1994

Integral-moment analysis of the BATSE gamma-ray burst intensity distribution

We have applied the technique of integral-moment analysis to the intensity distribution of the first 260 gamma-ray bursts observed by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory. This technique provides direct measurement of properties such as the mean, variance, and skewness of the convolved luminosity-number density distribution, as well as associated uncertainties. Using this method, one obtains insight into the nature of the source distributions unavailable through computation of traditional single parameters such as V/V(sub max)). If the luminosity function of the gamma-ray bursts is strongly peaked, giving bursts only a narrow range of luminosities, these results are then direct probes of the radial distribution of sources, regardless of whether the bursts are a local phenomenon, are distributed in a galactic halo, or are at cosmological distances. Accordingly, an integral-moment analysis of the intensity distribution of the gamma-ray bursts provides for the most complete analytic description of the source distribution available from the data, and offers the most comprehensive test of the compatibility of a given hypothesized distribution with observation.

Horack, John M.↗
NASA NTRS1994

The first BATSE gamma-ray burst catalog

The Burst and Transient Source Experiment on the Compton Gamma-Ray Observatory detected 260 cosmic gamma-ray bursts during the period 19 Apr 1991 to 5 Mar 1992. This paper presents the occurrence times, locations, peak count rates, peak fluxes, fluences, durations, and plots of time histories for these bursts. The angular distribution is consistent with isotropy. The intensity distribution shows a deficit in the number of weak bursts, which is not consistent with a homogeneous distribution of burst sources in Euclidean space. The duration distribution shows evidence for a separate class of bursts with durations less than about 2 seconds.

Fishman, Gerald J.↗
NASA NTRS1994

Do gamma-ray bursts originate from an extended Galactic Halo of high-velocity neutron stars?

The gamma-ray burst brightness distribution is inhomogeneous and the distribution on the sky is nearly isotropic. These features argue against an association of gamma-ray bursts with those Galactic objects that are known to exhibit a strong concentration toward the Galactic center or plane. The observed statistical properties indicate a cosmological origin. Circumstantial evidence suggests that neutron stars are involved in the burst phenomenon. Here we consider Population II neutron stars in an extended Galactic Halo (EGH) as an alternative to cosmological scenarios. The Burst and Transient Source Experiment (BATSE) data indicate a small deviation from isotropy near the 2 sigma level of statistical significance. If confirmed for an increasing number of bursts, these anisotropies could rule out cosmological scenarios. On the other hand, EGH models require small anisotropies like those observed by BATSE. We consider simple distribution models to determine the generic properties such halos must have to be consistent with the observations and discuss the implications of the corresponding distance scale on burst models.

Hartmann, Dieter H.↗
NASA NTRS1994

Duration versus brightness of gamma-ray bursts: Comparisons between SIGNE and BATSE

We analyze duration and brightness distributions of both the SIGNE Venera 13 and 14 and Burst and Transient Source Experiment (BATSE) gamma-ray burst databases. Choosing T(sub 50) as a measure of the burst duration and using both 64 and 1024 ms peak count rates, we search for correlations between duration, peak brightness, and the ratio V identically equals C(sub 64)/C(sub 1024), proposed as a measure of variability by Lamb, Graziani, & Smith. The duration histogram for SIGNE shows a long-duration peak that is consistent with BATSE, but does not exhibit the short population, instead appearing flat below 0.6 s; the difference is presumably caused by the failure of SIGNE to detect the short, faint bursts that were observed by BATSE. Estimating the instantaneous brightness by C(sub 64), we find that SIGNE confirms the BATSE result that the long and short bursts have similar maximum instantaneous brightnesses. Scatterplots between duration, brightness, and V are consistent for both databases; we show that SIGNE confirms the BATSE observation that there is a lack of bursts that are both bright over 1024 ms and contain a short, bright spike.

Kargatis, V. E.↗
NASA NTRS1995

Inferring the spatial and energy distribution of gamma-ray burst sources. 1: Methodology

We describe Bayesian methods for analyzing the distribution of gamma-ray burst peak photon fluxes and directions. These methods fit the differential distribution, and have the following advantages over rival methods: (1) they do not destroy information by binning or averaging the data (as do, say, chi squared, the averaged value of V/V(sub max), and angular moment analyses); (2) they straightforwardly handle uncertainties in the measured quantities; (3) they analyze the strength and direction information jointly; (4) they use information available about nondetections; and (5) they automatically identify and account for biases and selection effects given a precise description of the experiment. In these methods, the most important information needed about the instrument threshold is not its value at the times of burst triggers, as is used in the average value of V/V(sub max) analyses, but rather the value of the threshold at times when no trigger occurred. We show that this information can be summarized as an average detection efficiency that is similar to the product of the exposure and efficiency reported in the First Burst and Transient Source Experiment (BATSE) Burst (1B) Catalog, but significantly different from it at low fluxes. We also quantify an important bias that results from estimating the peak flux by scanning the burst to find the peak number of counts in a window of specified duration, as was done for the 1B Catalog. When the duration of the peak of the light curve is longer than the window duration, a simple flux estimate based on the peak counts significantly overestimates the peak flux in a nonlinear fashion that distorts the shape of the log(N)-log(P) distribution. This distortion also corrupts analyses of the V/V(sub max) distribution that use ratios of counts above background to estimate V/V(sub max). The Bayesian calculation specifies how to account for this bias. Implementation of the Bayesian approach requires some changes in the way burst data are reported that we describe in detail. Subsequent papers will report analyses of the 1B Catalog data using the methods described here.

Loredo, Thomas J.↗
NASA NTRS1994

Galactic dual population models of gamma-ray bursts

We investigate in more detail the properties of two-population models for gamma-ray bursts in the galactic disk and halo. We calculate the gamma-ray burst statistical properties, mean value of (V/V(sub max)), mean value of cos Theta, and mean value of (sin(exp 2) b), as functions of the detection flux threshold for bursts coming from both Galactic disk and massive halo populations. We consider halo models inferred from the observational constraints on the large-scale Galactic structure and we compare the expected values of mean value of (V/V(sub max)), mean value of cos Theta, and mean value of (sin(exp 2) b), with those measured by Burst and Transient Source Experiment (BATSE) and other detectors. We find that the measured values are consistent with solely Galactic populations having a range of halo distributions, mixed with local disk distributions, which can account for as much as approximately 25% of the observed BATSE bursts. M31 does not contribute to these modeled bursts. We also demonstrate, contrary to recent arguments, that the size-frequency distributions of dual population models are quite consistent with the BATSE observations.

Higdon, J. C.↗
NASA NTRS1994

An iron absorption model of gamma-ray burst spectra

Most gamma-ray bursts (GRBs) exhibit deficits of X-rays below approximately 200 keV. Here we consider a spectral model in which the burst source is shielded by an optically thick layer of circumburster material (CBM) rich in iron-group elements whose photoelectric absorption opacity exceeds the Thomson opacity below approximately 120 keV. For power-law distributions of absorption depths along the lines of sight the absorbed spectrum can indeed mimic the typial GRB spectrum. This model predicts that (a) the spectrum should evolve monotonically from hard to soft during each energy release, which is observed in most bursts, especially in fast rise exponential decay bursts; (b) Fe spectral features near 7 keV may be present in some bursts; and (c) the ratio of burst distances to the CBM and to Earth should be approximately 10(exp -11) if the spectral evolution is purely due to Fe stripping by the photons.

Liang, Edison P.↗
NASA NTRS1994

A search for the radio counterpart to the 1994 March 1 gamma-ray burst

We report on the results of a search for the radio counterpart to the bright gamma-ray burst of 1994 March 1. Using the Dominion Radio Astrophysical Observatory Synthesis Telescope sensitive, wide-field radio images at 1.4 GHz and 0.4 GHz were made of a region around GRB 940301. A total of 15 separate radio images were obtained at each frequency, sampling a near-continuous range of post-burst timescales between 3 and 15 days, as well as 26, 47, and 99 days. We place an upper limit of 3.5 mJy on a fading/flaring radio counterpart at 1.4 GHz and 55 mJy at 0.4 GHz. Unlike past efforts our counterpart search maintains high sensitivity over two decades of post-burst time durations. Time-variable radio emission after the initial gamma-ray burst is a prediction of all fireball models, currently the most popular model for gamma-ray bursts. Our observations allow us to put significant constraints on the fireball parameters for cosmological models of gamma-ray bursts.

Frail, D. A.↗
NASA NTRS1995

Constraints on the gamma-ray burst luminosity function from Pioneer Venus Orbiter and BATSE observations

We examine the width of the gamma ray burst luminosity function through the distribution of Gamma Ray Burst (GRB) peak fluxes as detected by the Pioneer Venus Orbiter (PVO) and the Burst and Transient Source Experiment (BATSE). The strength of the analysis is greatly enhanced by using a merged catalog of peak fluxes from both instruments with good cross-calibration of their sensitivities. The range of peak fluxes is increased by approximately a factor of 20 relative to the BATSE catalog. Thus, more sensitive investigations of the log N - log P distribution are possible. We place constraints on the width of the luminosity function of gamma-ray bursts brighter than the BATSE completeness limit by comparing the intensity distribution in the merged catalog with those produced by a variety of spatial density and luminosity functions. For the models examined, 90% of the detectable bursts have peak luminosities within a range of 10, indicating that the peak luminosities of gamma-ray bursts span a markedly less wide range of values than many other of their measurable properties. We also discuss for which slopes of a power-law luminosity function the observed width is at the upper end of the constrained range. This is essential in determining the power-law slopes for which luminosity-duration correlations could be important.

Ulmer, Andrew↗
NASA NTRS1995

Coincident bursts of auroral kilometric radiation and VLF emissions associted with a type 3 solar radio noise event

This paper examines an isolated magnetospheric VLF/radio noise event that is highly suggestive of the triggering of terrestrial auroral kilometric radiation (AKR) bu solar type III radio emission and of a close relation between AKR and broadband hiss. The solar type III burst was measured on polar HF riometers and was coincident with local dayside VLF/LF noise emission bursts at South Pole station. It was also coincident with AKR bursts detected onthe AMPTE/IRM satellite, at the same magnetic local time as South Pole. On the basis of the close association of AKR and VLF bursts, and from geometric considerations relating to wave propagation, it is likely that the AKR source was on the dayside and on field lines near South Pole station. The general level of geomagnetic activity was very low. However, an isolated magnetic impulse event (MIE) accompanied by a riometer absorption pulse was in progress when all of the VLF/radio noise bursts occurred. The very close association of the typew III burst at HF with the AKR is consistent with external stimulation of the AKR, is different, more immediate,triggering process than that implied by Calvert (1981) is invoked. It is suggested here that some of the HF solar radiant energy may decay into waves with frequences comparable to those of the AKR by paraetric excitation or some other process, thus providing the few background photons required for the generation of AKR by the WU and Lee (1979) cyclotron maser instability. The AKR, perhaps by modifying the magnetospheric electron velocity distribution, might have produced the observed VLF emissions. Alternatively, the VLF emissions may have arisen from the same anisotropic and unstable electron distribution function responsible for the AKR.

Rosenberg, T. J.↗
NASA NTRS1995

Notes on the diversity of the properties of radio bursts observed on the nightside of Venus

We report on further studies of radio wave bursts detected by the Orbiting Electric Field Detector (OEFD) on the Pioneer Venus Orbiter (PVO) in the nightside ionosphere of Venus. We have tested a total of 25 cases of wave burst activity for evidence of whistler-mode propagation to the spacecraft from impulsive subionospheric sources. As in a previous study of 11 of these cases (Sonwalkar et al., 1991) we find at least two distinct classes of events, one, mostly involving bursts at 100 Hz only, that passes certain tests for whistler-mode propagation, and another, mostly involving bursts in two or more of the four PVO narrowband channels (at 100 Hz, 730 Hz, 5.4 kHz, and 30 kHz), that fails to pass the tests. The subionospheric lightning hypothesis continues to be tenable as a candidate explanation for many of the 100 Hz-only events, but its number of 100 Hz-only cases that do no pass all the applicable whistler-mode tests, as well as the existence at a wide range of altitudes of multichannel cases that are clearly not propagating whistler-mode waves. The wideband bursts are often observed at altitudes above 1000 km and frequently occur in regions of locally reduced electron density. Those observed at high altitude (and possibly low altitude as well) are believed to be generated near the spacecraft, possibly by an as yet unknown mechanism responsible for similar burst observations made near Earth and other planets.

Sonwalkar, Vikas S.↗
NASA NTRS1996

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.

Meegan, Charles A.↗
NASA NTRS1993

Gravitational Waves and gamma-ray Bursts

If the gamma-ray burst sources detected by GRO are coalescing binaries at cosmological distances there should be a coincident gravitational radiation signal. Using the GRBs rate we predict the gravitational radiation detection rate as a function of the gravitational wave strain at Earth. This method of predicting the rate avoids the large statistical uncertainties in the current estimates that are based on the three neutron star binaries containing pulsars found, so far, in the Galaxy. The brightest gamma-ray bursts should be accompanied by a gravitational pulse detectable by LIGO or VIRGO, and by using the bursts as triggers for LIGO/VIRGO their sensitivity can be improved by 50% and the detection rate increases by a factor of 3. LIGO/VIRGO must reach a strain sensitivity of 10(sup-20.7)h(sub 0) to detect one burst per decade, and a failure to find coincidences at a rate of one per year with a strain sensitivity of 10(sup -20.6)h(sub 0) will rule out the binary hypothesis. If they are detected as gravitational wave sources, the time delay between the gamma-rays and the gravitational waves will help to determine the burst mechanism, and the polarization of the gravitational waves will help to determine the burst geometry.

Kochanek, Christopher S.↗