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The first observations in gamma-ray astronomy were made in the late 1960's, primarily by balloon-borne observations. In the early 1970's, gamma-ray bursts were discovered, completely by accident, by satellites looking for man-made nuclear explosions in space. The celestial nature of these events were soon confirmed by other satellites. The first large detector system designed for cosmic gamma-ray bursts observations was the BATSE instrument on the Compton Gamma-Ray Observatory. Some of the details of the instrumentation onboard ballons and satellites and the gamma-ray bursts observational properties they determined are presented.

Aims: In this paper we study the main spectral and temporal properties of gamma-ray bursts (GRBs) observed by Fermi/GBM. We investigate these key properties of GRBs in the rest-frame of the progenitor and test for possible intra-parameter correlations to better understand the intrinsic nature of these events. Methods: Our sample comprises 32 GRBs with measured redshift that were observed by GBM until August 2010. 28 of them belong to the long-duration population and 4 events were classified as short/hard bursts. For all of these events we derive, where possible, the intrinsic peak energy in the $\nu F_{\nu}$ spectrum (\eprest), the duration in the rest-frame, defined as the time in which 90% of the burst fluence was observed (\tninetyrest) and the isotropic equivalent bolometric energy (\eiso). Results: The distribution of \eprest has mean and median values of 1.1 MeV and 750 keV, respectively. A log-normal fit to the sample of long bursts peaks at ~800 keV. No high-\ep population is found but the distribution is biased against low \ep values. We find the lowest possible \ep that GBM can recover to be ~ 15 keV. The \tninetyrest distribution of long GRBs peaks at ~10 s. The distribution of \eiso has mean and median values of $8.9\times 10^{52}$ erg and $8.2 \times 10^{52}$ erg, respectively. We confirm the tight correlation between \eprest and \eiso (Amati relation) and the one between \eprest and the 1-s peak luminosity ($L_p$) (Yonetoku relation). Additionally, we observe a parameter reconstruction effect, i.e. the low-energy power law index $\alpha$ gets softer when \ep is located at the lower end of the detector energy range. Moreover, we do not find any significant cosmic evolution of neither \eprest nor \tninetyrest., Comment: accepted by A&A

The Gamma-ray Burst Monitor (GBM) on the Fermi Gamma-ray Space Telescope detected 12 intense terrestrial gamma ray flashes (TGFs) during its first year of observation. Typical maximum energies for most of the TGFs are similar to 30 MeV, with one TGF having a 38 MeV photon; two of the TGFs are softer and longer than the others. After correcting for instrumental effects, a representative bright TGF is found to have a fluence of similar to 0.7 photons cm (2). Pulses are either symmetrical or have faster risetimes than fall times; they are well fit with Gaussian or lognormal functions. The fastest risetime observed was 7 mu s, constraining the source radius to be less than about 2 km from the velocity of light. TGFs with multiple pulses separated in time have been known since their discovery; the GBM sample also includes clear cases of partially overlapping pulses. Four TGFs are associated with lightning locations from the World Wide Lightning Location Network. With the several mu s absolute time accuracy of GBM, the time order can be confidently identified: one TGF occurred before the lightning, two were simultaneous, and one TGF occurred after the lightning.

This work reports on the use of scintillating optical fibers in two experiments being prepared for space flight. The Advanced Composition Explorer (ACE) [1], to be launched in August 1997, includes the Cosmic Ray Isotope Spectrometer (CRIS) which uses a Scintillating Optical Fiber Trajectory detector (SOFT) for trajectory information. The Scintillating Fiber Telescope for Energetic Radiation (SIFTER) is currently being developed as a high energy gamma ray telescope for a future space mission. It uses scintillating optical fibers distributed throughout the converter material to allow detailed study of the development of the electromagnetic cascade produced by gamma rays.

We present an overview of the Long Duration Exposure Facility (LDEF) induced activation measurements. The LDEF, which was gravity-gradient stabilized, was exposed to the low Earth orbit (LEO) radiation environment over a 5.8 year period. Retrieved activation samples and structural components from the spacecraft were analyzed with low and ultra-low background HPGe gamma spectrometry at several national facilities. This allowed a very sensitive measurement of long-lived radionuclides produced by proton- and neutron-induced reactions in the time-dependent, non-isotropic LEO environment. A summary of major findings from this study is given that consists of directionally dependent activation, depth profiles, thermal neutron activation, and surface beryllium-7 deposition from the upper atmosphere. We also describe a database of these measurements that has been prepared for use in testing radiation environmental models and spacecraft design.

The Gamma-ray Burst duration distribution is bi-modal with a minimum around 2 s suggesting that there are two separate populations. The GRB time profiles, believed to be the result of internal shocks, reflect the activity of the central engine. A systematic study of the GRB temporal profiles was carried out with a view to understanding the possible differences in the properties of the central engine in the two populations of GRBs. We analyse the temporal profiles of gamma-ray bursts detected by the Gamma-ray Burst Monitor onboard the Fermi Gamma-ray Space Telescope. We deconvolve these profiles into pulses superposed on a quadratic background. The basic pulse shapes are fitted to lognormal functions. It is found that the distributions of rise as well as decay times, their full width at half maximum and the time intervals between successive pulses for long and short GRBs are distinct and each follows a lognormal distribution. We identify several differences in the temporal properties of long and short bursts which in turn reflect the differences in the activities of central engines of the two types of GRBs.

Using our new 3‐D relativistic particle‐in‐cell (PIC) code, we investigated long‐term particle acceleration associated with a relativistic electron‐positron jet propagating in an unmagnetized ambient electron‐positron plasma. The simulations were performed using a much longer simulation system than our previous simulations in order to investigate the full nonlinear stage of the Weibel instability and its particle acceleration mechanism. Cold jet electrons are thermalized and ambient electrons are accelerated in the resulting shocks. Acceleration of ambient electrons leads to a maximum ambient electron density three times larger than the original value as predicted by hydrodynamic compression. Behind the bow shock, in the jet shock, strong electromagnetic fields are generated. These fields may lead to time dependent afterglow emission. In order to go beyond the standard synchrotron model used in astrophysical objects we have used PIC simulations and calculated radiation based on first principles. We calcul...

The Energetic X-ray Imaging Survey Telescope (EXIST) is designed to i) use the birth of stellar mass black holes, as revealed by cosmic Gamma-Ray Bursts (GRBs), as probes of the very first stars and galaxies to exist in the Universe. Both their extreme luminosity (~104 times larger than the most luminous quasars) and their hard X-ray detectability over the full sky with wide-field imaging make them ideal "back-lights" to measure cosmic structure with X-ray, optical and near-IR (nIR) spectra over many sight lines to high redshift. The full-sky imaging detection and rapid followup narrow-field imaging and spectroscopy allow two additional primary science objectives: ii) novel surveys of supermassive black holes (SMBHs) accreting as very luminous but rare quasars, which can trace the birth and growth of the first SMBHs as well as quiescent SMBHs (non-accreting) which reveal their presence by X-ray flares from the tidal disruption of passing field stars; and iii) a multiwavelength Time Domain Astrophysics (TDA) survey to measure the temporal variability and physics of a wide range of objects, from birth to death of stars and from the thermal to non-thermal Universe. These science objectives are achieved with the telescopes and mission as proposed for EXIST described here., 19 pages, 13 figures, 6 tables. Invited talk at SPIE Conference "Astronomical Telescopes and Instrumentation 2010"; to appear in Proceedings SPIE (2010)

Using our new 3-D relativistic electromagnetic particle (REMP) code parallelized with MPI, we investigated long-term particle acceleration associated with a relativistic electron-positron jet propagating in an unmagnetized ambient electron-positron plasma. We have also performed simulations with electron-ion jets. The simulations were performed using a much longer simulation system than our previous simulations in order to investigate the full nonlinear stage of the Weibel instability for electron-positron jets and its particle acceleration mechanism. Cold jet electrons are thermalized and ambient electrons are accelerated in the resulting shocks for both cases. Acceleration of ambient electrons leads to a maximum ambient electron density three times larger than the original value for pair plasmas. Behind the bow shock in the jet shock strong electromagnetic fields are generated. These fields may lead to time dependent afterglow emission. We calculated radiation from electrons propagating in a uniform parallel magnetic field to verify the technique. We also used the new technique to calculate emission from electrons based on simulations with a small system with two different cases for Lorentz factors (15 and 100). We obtained spectra which are consistent with those generated from electrons propagating in turbulent magnetic fields with red noise. This turbulent magnetic field is similar to the magnetic field generated at an early nonlinear stage of the Weibel instability.

Using our new 3-D relativistic electromagnetic particle (REMP) code parallelized with MPI, we have investigated long-term particle acceleration associated with an relativistic electron-positron jet propagating in an unmagnetized ambient electron-positron plasma. The simulations have been performed using a much longer simulation system than our previous simulations in order to investigate the full nonlinear stage of the Weibel instability and its particle acceleration mechanism. Cold jet electrons are thermalized and ambient electrons are accelerated in the resulting shocks. The acceleration of ambient electrons leads to a maximum ambient electron density three times larger than the original value. Behind the bow shock in the jet shock strong electromagnetic fields are generated. These fields may lead to the afterglow emission. We have calculated the time evolution of the spectrum from two electrons propagating in a uniform parallel magnetic field to verify the technique., Comment: 3 pages, 2 figures, submitted for the Proceedings of The Sixth Huntsville Gamma-Ray Burst Symposium 2008, Huntsville, AL, October 20-23, 2008

One of the scientific objectives of NASA’s Fermi Gamma‐ray Space Telescope is the study of Gamma‐Ray Bursts (GRBs). The Fermi Gamma‐Ray Burst Monitor (GBM) was designed to detect and localize bursts for the Fermi mission. By means of an array of 12 NaI(Tl) (8 keV to 1 MeV) and two BGO (0.2 to 40 MeV) scintillation detectors, GBM extends the energy range (20 MeV to >300 GeV ) of Fermi’s main instrument, the Large Area Telescope (LAT), into the traditional range of current GRB databases. The physical detector response of the GBM instrument to GRBs is determined with the help of Monte Carlo simulations, which are supported and verified by on‐ground individual detector calibration measurements. We present the principal instrument properties, which have been determined as a function of energy and angle, including the channel‐energy relation, the energy resolution and the effective area.

The Fermi Gamma‐ray Space Telescope launched on June 11, 2008 carries two experiments onboard—the Large Area Telescope (LAT) and the Gamma‐ray Burst Monitor (GBM). The primary mission of the GBM instrument is to support the LAT in observing γ‐ray bursts (GRBs) by providing low‐energy measurements with high temporal and spectral resolution as well as rapid burst locations over a large field‐of‐view (⩾8 sr). The GBM will complement the LAT measurements by observing GRBs in the energy range 8 keV to 40 MeV, the region of the spectral turnover in most GRBs. The GBM detector signals are processed by the onboard digital processing unit (DPU). We describe some of the hardware features of the DPU and its expected limitations during intense triggers.

Nonthermal radiation observed from astrophysical systems containing relativistic jets and shocks e.g. gamma-ray bursts (GRBs) active galactic nuclei (AGNs) and microquasars commonly exhibit power-law emission spectra. Recent PIC simulations of relativistic electron-ion (or electron-positron) jets injected into a stationary medium show that particle acceleration occurs within the downstream jet. In collisionless relativistic shocks particle (electron, positron and ion) acceleration is due to plasma waves and their associated instabilities (e.g. the Weibel (filamentation) instability) created in the shock region. The simulations show that the Weibel instability is responsible for generating and amplifying highly non-uniform small-scale magnetic fields. These fields contribute to the electron's transverse deflection behind the jet head. The resulting ``jitter'' radiation from deflected electrons has different properties compared to synchrotron radiation which assumes a uniform magnetic field. Jitter radiation may be important for understanding the complex time evolution and/or spectra in gamma-ray bursts, relativistic jets in general and supernova remnants., 19 pages,7 figures, contributed talk at Seventh European Workshop on Collisionless Shocks, Paris, 7- 9 November 2007. High resolution version can be obtained at http://gammaray.nsstc.nasa.gov/~nishikawa/shockws07.pdf

The GLAST Burst Monitor (GBM) is designed to provide wide field of view observations of gamma‐ray bursts and other fast transient sources in the energy range 10 keV to 30 MeV. The GBM is composed of several unshielded and uncollimated scintillation detectors (twelve NaI and two BGO) that are widely dispersed about the GLAST spacecraft. As a result, reconstructing source locations, energy spectra, and temporal properties from GBM data requires detailed knowledge of the detectors' response to both direct radiation as well as that scattered from the spacecraft and Earth's atmosphere. This full GBM instrument response will be captured in the form of a response function database that is derived from computer modeling and simulation. The simulation system is based on the GEANT4 Monte Carlo radiation transport simulation toolset.

Nonthermal radiation observed from astrophysical systems containing relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active galactic nuclei (AGNs), and microquasars commonly exhibit power-law emission spectra. Recent PIC simulations of relativistic electron-ion (or electron-positron) jets injected into a stationary medium show that particle acceleration occurs within the downstream jet. In collisionless, relativistic shocks, particle (electron, positron, and ion) acceleration is due to plasma waves and their associated instabilities (e.g., the Weibel (filamentation) instability) created in the shock region. The simulations show that the Weibel instability is responsible for generating and amplifying highly non-uniform, small-scale magnetic fields. These fields contribute to the electron's transverse deflection behind the jet head. The resulting "jitter" radiation from deflected electrons has different properties compared to synchrotron radiation, which assumes a uniform magnetic field. Jitter radiation may be important for understanding the complex time evolution and/or spectra in gamma-ray bursts, relativistic jets in general, and supernova remnants., Comment: : 4 pages, 1 figure and 1 table, typos are corrected, submitted for the Proceedings of The 4th Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, July 7-11, 2008, in Heidelberg, Germany

We present a summary of our deep optical photometric surveys of five small ( 25°). All of the localizations contain galaxies ranging in brightness from V∼18 to our survey limits at about V∼23. Without model-dependent assumptions of acceptable GRB distances, these galaxies, which are several magnitudes brighter than the galaxy associated with the GRB970228 optical transient, are viable GRB host candidates. We find a QSO candidate in only one of the localizations, and are therefore unable to support the hypothesis, based on larger IPN1 localizations, that QSO/AGNs or blue galaxies may be associated with GRBs [1,2]. We examine possible reasons for this negative result.

In this paper we present the results of a search for GeV Gamma Ray Bursts made by the INCA experiment during the first 9 months of operation. INCA, an air shower array located at Mount Chacaltaya (Bolivia) at 5200 m a.s.l., has been searching for GRBs since December 1996. Up to August, 1997, 34 GRBs detected by BATSE occurred in the field of view of the experiment. For any burst, the counting rate of the array in the 2 hours interval around the burst trigger time has been studied. No significant excess has been observed. Assuming for the bursts a power low energy spectrum extending up to 1 TeV with a slope α=−2 and a duration of 10 s, the obtained 1 GeV-1 TeV energy fluence upper limits range from 7.9 10−5 erg cm−2 to 3.5 10−3 erg cm−2 depending on the event zenith angles.

We present the R_c band light curve of the optical transient (OT) associated with GRB970228, based on re-evaluation of existing photometry. Data obtained until April 1997 suggested a slowing down of the decay of the optical brightness. However, the HST observations in September 1997 show that the light curve of the point source is well represented by a single power law, with a ``dip'', about a week after the burst occured. The exponent of the power law decay is $\alpha$ = --1.10 $\pm$ 0.04. As the point source weakened it also became redder., Comment: 5 pages, latex, to appear in Gamma-Ray Bursts, 4-th Huntsville Symposium, eds Meegan, Preece, Koshut

The Compton Gamma Ray Observatory and its complement of four scientific instruments has completed over six years of outstanding on-orbit operation. In this paper we summarize the capabilities of the four CGRO experiments. The enhanced capabilities that have been achieved since the beginning of the mission are discussed. Recently, the spacecraft has been re-boosted to a higher orbit that could extend the mission lifetime for another ten years. This will enable continued monitoring of the hard X-ray sky by BATSE, continued mapping of the Galaxy across the gamma ray spectrum, the capability to observe rare events such as extragalactic supernovae, and solar flare observations during the next solar maximum by all instruments. The instrumental capabilities for an extended mission are discussed.

We report on the on-going project to search for serendipitous time-correlated optical photographic observations of γ-ray bursts (GRBs). The regular photography at 12 observatories is used to look for plates which have been exposed simultaneously with a GRB detected by BATSE and contain its position. The results of this search for GRBs detected during the previous 4 yrs of BATSE operation are presented.

STARE (Survey for Transient Astronomical Radio Emission) is a program designed to detect transient radio emission, including that which might accompany GRBs. STARE differs from other attempts to detect radio counterparts to GRBs in that it covers a large fraction of the sky nearly 24 hours per day. Thus it will be sensitive to radio emission which arrives after, during, or even before the GRB itself. The STARE project as proposed would consist of two phases: I) Total power radiometers at geographically separated locations monitor the sky, allowing quick and easy measurements, but providing low sensitivity and low angular resolution. II) Geographically separated correlating arrays replace the radiometers, providing better sensitivity and higher angular resolution. The first phase is currently underway: one radiometer is operating in Hancock, NH and two more are under construction for other sites. With one site operational, it is impossible to reject local interference, but we can still place upper limits on radio emission from GRBs which occur when the radio record is quiet. When the second and third sites are operational, we will be able to reject local interference events by requiring coincidence among sites, and infer positions by comparing arrival times among sites. Multisite detections will allow meaningful comparisons of our data with the GRB record.

X-ray counterpart searches of GRBs are of key importance in solving the mystery of their origin. Possible X-ray counterparts of the May 1, 1992 and the Nov. 19, 1978 GRBs were observed with ASCA. In this report, we mention only GRB920501. Its 0.5–10.0 keV spectrum with ASCA appears hard and was heavily absorbed at low energies. The amount of low-energy absorption is consistent with the galactic value in this direction. This suggests that the X-ray source is quite distant, and cannot be a field star or a supernova remnant in the galactic disk.

We introduce new parameters for characterizing gamma ray burst time profiles. The method is based on estimating the average parameters of gamma-ray bursts (e.g., pulse duration, duration between pulses, hardness ratio) for corresponding flux intervals. The profiles of all bursts are normalized to unity at their count rate peaks, then the parameters are calculated for each particular dimensionless flux interval. In this paper we discuss application of the method to bursts of the 3B Catalog.

The comparison of gamma-ray burst observations from Ulysses and from CGRO BATSE has provided several hundred source regions that consist of large-diameter but narrow annuli. Although awkward for use in most applications, the one-dimensional accuracies of these ring segments can be exploited in the search for classical repeaters (as evidenced by the necessary crossing of multiple annuli at the same locations). The catalog of Ulysses-to-BATSE source loci (1) and the BATSE 3B catalog (2) have been used to search for source rings from the interplanetary network that intersect within or close to their own 3B source fields, with calculations done varying the BATSE field size. The regions determined in this manner are consistent with randomicity or can be accounted for as systematic to this technique. Thus, no evidence for the repetition of classical GRBs has been found. Extensions of this work could be possible using event catalogues from other missions, e.g., GGS-Wind, and from the earlier networks.

Intense effort has gone into the observation of optical, radio and X-ray GRB counterparts, either simultaneous to the burst or as quasi-steady remnants. Here we report on a similar study at higher energies of 250 GeV and above using ground-based telescopes. Imaging atmospheric Cherenkov telescopes have achieved great sensitivity and now complement observations by orbiting telescopes such as CGRO. Previous studies of bursts by the Whipple Collaboration (4) combined with recent improvements to the telescope, indicate that sensitivity to a fluence of 6×10−9 erg-cm−2 can be achieved. Observations by the Whipple Collaboration of nine BATSE positions, one within 2 minutes of the BATSE burst, using coordinates distributed through BACODINE will be reported. Analysis techniques will be described and an upper limit to the high-energy delayed or extended emission of observed candidates will be calculated.

Between 1992 October 4 and 1993 August 1, 11 GRBs were concurrently observed by CGRO, Mars Observer (MO), and Ulysses. Unfortunately, for all but one of the events MO was in a low-time-resolution mode. Arrival time analysis was used to compute error boxes whose larger dimensions are typically of the same order as the diameters of the BATSE-only boxes and whose smaller dimensions are in the arc minute range. By and large, our arrival-time results are consistent with the BATSE RMS errors stated in the 3B Catalog. That is, the mean uncertainty in the locations of the stronger BATSE events appears to be about 2 degrees.

The BATSE/COMPTEL/NMSU Rapid Response Network (BCN network) was formed in 1993 to provide rapid follow-up of well localized γ-ray bursts imaged by the Compton Gamma-Ray Observatory. To date, about twenty burst fields have been imaged. COMPTEL localizations were available for about half of these GRBs. BATSE/BACODINE error boxes were used for the remaining bursts. The principle contributor to the optical follow-up effort has been the U.S. Air Force GEODSS system. This network consists of three sites located at Socorro, NM, Maui, HI, and Deigo Garcia in the Indian Ocean. Each site possesses two or three 40 inch Schmidt-like telescopes that can reach 17th magnitude in less than 1 second of exposure time. The remaining BCN network consists of ≈22 professional observatories located around the world spanning both northern and southern hemispheres. It has deeply imaged recent GRB error boxes at both radio and optical wavelengths within 0.03 days of the detection of a GRB. No GRB counterparts have been identified.

The RA, Dec coordinates of gamma-ray burst (GRB) sources observed with BATSE on GRO are automatically determined and distributed in real time to members of the global scientific community. These data are now being used by 27 operations to enable searches for associated transients in a variety of other wavelength or energy regimes to identify the burst source objects. The minimum total delay time from the onset of a GRB to the receipt of its coordinates by distant experimenters can be under 4 sec, less than the duration of a typical GRB, and the maximum total delay is 7 sec, or longer, depending on the distribution method. Recent improvements to the BACODINE system and some future improvements are discussed. A summary of the follow-up observations made by the BACODINE sites is given.

We apply peak aligned averaging to BATSE 16 channel 2.048 s accumulation time spectra from the brightest LAD. We present average peak count and photon spectra. These signatures are used to compare bright and dim GRBs.

I would like to thank the participants in this round-table discussion for their contributions and insights. Clearly, there are regions of overlap between the variable star observers and those looking for counterparts to GRBs. The experiments that search for counterparts to GRBs, independent of energy, have been designed to look for variability in celestial sources, and thus are “tuned” to the characteristics of “flare” and “flash” sources: these experiments may be able to contribute to the understanding of variable stars of many classes. In particular, the wide-field experiments described here should be able to provide information about the rate of rare, high-amplitude flares.

The basic properties of γ-ray bursts are reviewed. Many of these properties have been known for over two decades, although new and more detailed observations have been made by Compton Observatory in the past three years. The new observation with the greatest impact has been the observed isotropic distribution of bursts along with a deficiency of weak bursts. This is not compatible with any known Galactic population of objects.

The interplanetary gamma‐ray burst network of the Ulysses, Compton‐GRO, and Pioneer‐Venus Orbiter missions has made source localizations with fractional‐arc‐minute precision for a number of events, and with auxiliary data, will provide useful annular‐segment loci for many more. These studies have, thus far, yielded one possible counterpart, a Rosat x‐ray association with the 92 May 1 burst. Similar to the historic 1978 November 19 burst/Einstein association, this possibility gives hope that network studies will provide a fundamental source clue for ‘classical’ bursts, just as a second supernova remnant in a network‐defined source field has done for sgr events.

Previous experiments have reported emission features in gamma‐ray burst spectra in the region of 400 keV to 500 keV. These lines have been interpreted as gravitationally red‐shifted 511 keV annihilation lines produced in the vicinity of neutron stars (Barat 1984, Mazets 1980, Mitrofanov 1984, Nolan 1984). We describe a method to aid in the search for transient emission and absorption features in gamma‐ray burst spectra. The process involves fitting a 3‐dimensional continuum model to spectra in time‐energy space. Significant spectral features form easily recognizable islands in the contour plot of residuals versus energy and time.

We report on the setup and operation of an instrument to search for optical counterparts of gamma‐ray bursts. This instrument consists of a wide angle camera mounted on an automated, rapidly slewing telescope located at the NASA/GSFC Optical Test Site. The observations are triggered by a phone line data transmission of burst coordinates from the BACODINE network. The camera used is a 1‐inch format SIT video camera with a zoom lens providing a 5–20 degree adjustable field of view. Observations of the burst field are recorded on video tape and subsequently digitized and analyzed.

We have searched for gamma‐ray bursts during stellar flares using Ulysses and BATSE/CGRO data. A total of five stellar flares were identified (two from AD Leo and three from AU Mic), but neither BATSE nor Ulysses observed any gamma‐ray bursts which could be attributed to them. Using the BATSE trigger threshold, and the known distances to these flare stars, upper limits to the gamma‐ray luminosity were obtained. The conditions under which stellar flare gamma‐ray bursts could be detected by BATSE as weak events were studied. In particular, we found that if weak events (10−7 ergs cm−2 s−1) are to be explained by flares as recently suggested, the stellar X‐ray emission (≳25 keV) must be comparable to its optical emission, and the ratio of Lx/Lopt must be higher than that for solar flares by at least 4 orders of magnitude. The stellar flare logN‐logS and spatial distributions are studied and their implications for gamma‐ray bursts are discussed.

We have searched for gravitational-lens induced echoes between gamma-ray bursts (GRBs) in NASA's orbiting {\it Compton} Gamma Ray Observatory's Burst and Transient Source Experiment (BATSE) data. The search was conducted in two phases. In the first phase we compared all GRBs in a brightness complete sample of the first 260 GRBs with recorded angular positions having at least 5 \% chance of being coincident from their combined positional error. In the second phase, we compared all GRB light curves of the first 611 GRBs with recorded angular positions having at least 55 \% chance of being coincident from their combined positional error. No unambiguous gravitational lens candidate pairs were found in either phase, although a ``library of close calls" was accumulated for future reference. This result neither excludes nor significantly constrains a cosmological origin for GRBs., Comment: in press: ApJ, Plain TeX, 14 pages, 17 figures (available by FAX)

First results are presented from the analysis of 260 BATSE gamma‐ray bursts (GRBs) using a method whereby all events are synchronized and averaged around the bins which are the brightest time intervals of each of them. For the averaged time history, a difference is found between the rise front and the back slope, and good evidence is found for the presence of hard‐to‐soft spectral evolution. We compare sub‐sets of ‘‘strong’’ and ‘‘weak’’ events and find no evidence for time dilation in weak GRBs, as would be expected in cosmological GRB models. On the other hand, for the strong events the averaged hardness ratio is found to be larger than for the weak GRBs.

One of the primary objectives of BATSE is to determine the direction to each observed gamma‐ray burst, allowing the angular distribution of the burst sources to be studied. Most of the BATSE burst data types are not suitable for this purpose, since they are produced by combining rates from several detectors (details of the BATSE experiment and the various datatypes can be found elsewhere). After the loss of the CGRO tape recorders in March, 1992, the availability of data with rates from individual detectors has often been limited by telemetry gaps. Consequently, many burst directions are derived using the MAXBC data, which are almost always available, but have significant limitations. MAXBC data contain the maximum background‐subtracted count rate (counts per 64 ms) in the energy range of ∼50–300 keV, averaged over one second, for each of the eight Large Area Detectors. It is important to evaluate the locations computed using MAXBC data because these locations will appear in the BATSE 2B Catalog when no other data are available. We report on the comparison of burst locations computed using MAXBC data with locations derived from the Third Interplanetary Network, hereafter known as IPN. We report on the comparison of MAXBC locations of hard solar flares with the known position of the Sun. We also report on the comparison of burst locations computed using MAXBC data with those locations given in the first BATSE catalog, for which the burst data are essentially complete.

The large errors in the locations of gamma‐ray bursts observed by the Bursts and Transient Source Experiment (BATSE) forces one to adopt statistical comparisons of the burst catalog to models of burst isotropy to determine if bursts are repeating or clustered. In the first BATSE catalog, a nearest neighbor analysis finds a deviation from isotropy. In a recent article it was shown that known instrumental effects cannot produce significant small scale anisotropies. It was also shown that burst repeater models can produce the observe anisotropies. In this paper we examine in more detail repeater burst models. We also show that nearest and farthest neighbor analyses of more recent bursts fail to find significant small scale clustering.

Gamma ray bursts (GRB) of durations less than about 2 sec, which presumably form a separate class, constitute nearly 26% of the bursts detected by the burst and transient source experiment (BATSE) on board the Compton Gamma Ray Observatory. A very high time resolution study of these bursts is undertaken. A few parameters to describe the complexity, its temporal and spectral evolution have been defined and computed for each burst. Results of analysis of 37 short bursts show that more complex bursts have higher average hardness ratio. It is also found that bursts with relatively higher hardness ratio exhibit sharper temporal features at higher energies. A systematic search for photon bunching during these bursts show no positive evidence. We also find that short bursts show a wide variation in spectral evolution similar to that of longer bursts.

Many transient sources have been detected and monitored by the Burst and Transient Source Experiment (BATSE) since launch using the Earth occultation technique. These include x‐ray novae, pulsars, and other galactic sources. We present preliminary results of the temporal and spectral characteristics for seven transients which are potentially black hole candidates due to their hard spectra, lack of pulsations, and the appearance of their light curves. The seven transients are: GRO J0422+32 (X‐Ray Nova Persei 1992), GS/GRS 1124−68 (Nova Muscae 1991), GX 339−4, 4U 1543−47, GRS 1915+105, GRO J1719−24=GRS 1716−249 (X‐Ray Nova Ophiuchi 1993) and GRS 1009−45 (X‐Ray Nova Velorum 1993). We search for signatures in their spectra and light curves which are found to be common to black hole candidates. Generally, these objects appear to divide into two classes based on their light curves: the x‐ray nova‐type transients with fast rise and approximately exponential decay, and the slow rising, long duration bright transients.

Among the gamma ray bursts observed by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory we define a sub‐class of bursts based on similar morphology: a sharp rise followed by a longer decay time. About 7% of all the Gamma Ray Bursts observed by BATSE fall in this sub‐class. We study the spectral evolution of these bursts by fitting models to time segmented burst spectra and find no clear distinction between the spectral evolutionary properties of this sub‐class and those of other bursts. Further we study the high time resolution spectral evolution of this sub‐class of GRB’s using their spectral hardness ratios. A majority of the bursts show hardness ratio leading the counting rate and also display a continuous hard to soft evolution. The time lag between the counting rate and the hardness ratio is found to be directly correlated with the rise time of the counting rate profile. We also find for the first time, evidence for spectral variation in a time scale of 64 ms.

The transient hard x‐ray source and burster GX 354‐0 (MXB 1728‐34) has been detected several times since April 1991 using the newly‐developed occultation transform imaging to process data from the BATSE large area detectors (20‐1800 keV) on the COMPTON Observatory. One period corresponds to observations of an outburst seen by GRANAT/SIGMA in February 1992 (Goldwurm et al. 1992). This exploratory effort uses the occultation imaging technique to guide the identification of bright galactic center region sources within a few‐degree field. Then, using this information, temporal and spectral information is extracted with the standard occultation software.

BATSE detection of a new hard x‐ray pulsar is reported. The source was first observed on 14 Jul 1993, reached a maximum intensity on 25 Jul 1993, then declined smoothly until it fell below the detection threshold on 16 Aug 1993. The BATSE location was sufficiently well determined to permit OSSE, ASCA, & ROSAT observations, leading to an improved source location. The observed period at the solar system barycenter was 93.548±0.002s, 93.5665±0.0005s, and 93.541±0.004 on 15 Jul 1993, 23 Jul 1993, and 10 Aug 1993, respectively. The source is detected between ∼20 keV and 160 keV, with a spectrum fit by an optically thin thermal bremsstrahlung form, having a kT of 25.4±2.1 keV during the rise to maximum intensity, decreasing monotonically during the remainder of the outburst to 17.1±2.5 keV on 5 Aug 1993. The pulse profile in this energy range has a single broad peak, with the maximum occurring later at higher energies. The pulse profile, intensity history, and spectral behavior observed by BATSE during the outb...