V. Connaughton

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.

Discovery of Gamma-Ray Emission above 350 GeV from the BL Lacertae Object 1ES 2344+514

We present the discovery of gamma-ray emission greater than 350 GeV from the BL Lacertae (BL Lac) object 1ES 2344+514 with the Whipple Observatory 10 m gamma-ray telescope. This is the third BL Lac object detected at very high energies (VHE, E > 300 GeV), the other two being Markarian 421 (Mrk 421) and Mrk 501. These three active galactic nuclei are all X-ray selected and have the lowest known redshifts of any BL Lac objects currently identified with declination greater than 0?. The evidence for emission from 1ES 2344+514 comes mostly from an apparent flare on 1995 December 20 (UT) during which a 6 ? excess was detected with an average flux of I(>350 GeV) = 6.6 ? 1.9 ? 10-11 photons cm-2 s-1. This is approximately 63% of the VHE emission from the Crab Nebula, the standard candle in this field. Observations taken between 1995 October and 1996 January, excluding the night of the flare, yield a 4 ? detection indicating a flux level of I(>350 GeV) = 1.1 ? 0.4 ? 10-11 photons cm-2 s-1, or about 11% of the VHE Crab Nebula flux. Observations taken between 1996 September and 1997 January on this object did not yield a significant detection of a steady flux or any evidence of flaring activity. The 99.9% confidence level upper limit from these observations is I(>350 GeV) < 8.2 ? 10-12 photons cm-2 s-1, 8% of the Crab Nebula flux. The low baseline emission level and variation in the nightly and yearly flux of 1ES 2344+514 are the same as the VHE emission characteristics of Mrk 421 and Mrk 501.We present the discovery of >350 GeV gamma-ray emission from the BL Lacertae object 1ES 2344+514 with the Whipple Observatory 10m gamma-ray telescope. This is the third BL Lac object detected at gamma-ray energies above 300 Gev, the other two being Markarian 421 (Mrk 421) and Mrk501. These three active galactic nuclei are all X-ray selected and have the lowest known redshifts of any BL Lac objects currently identified. The evidence for emission derives primarily from an apparent flare on December 20, 1995 when a 6 sigma excess was detected with a flux approximately 63% of the very high energy gamma-ray emission from the Crab Nebula, the standard candle for TeV gamma-ray sources. Excluding the flare, observations between October 1995 and January 1996 yield a 4 sigma detection corresponding to 11% of the VHE Crab Nebula flux. Observations spanning September 1996 to January 1997 failed to yield a significant detection of a steady flux or any flaring. For this period, the 99.9% confidence level upper limit is <8% of the Crab Nebula. The low baseline emission level and variations in nightly and yearly flux of 1ES 22344+514 are the same as the VHE emission characteristics of Mrk 421 and Mrk 501

TeV Gamma-Ray Observations of the Galactic Center

We report a possible detection of TeV gamma rays from the Galactic center by the Whipple 10 m gamma-ray telescope. Twenty-six hours of data were taken over an extended period from 1995 through 2003 resulting in a total significance of 3.7 σ. The measured excess corresponds to an integral flux of 1.6 × 10-8 ± 0.5 × 10-8 (stat) ± 0.3 × 10-8 (sys) photons m-2 s-1 above an energy of 2.8 TeV, roughly 40% of the flux from the Crab Nebula at this energy. The 95% confidence region has an angular extent of about 15 and includes the position of Sgr A*. The detection is consistent with a point source and shows no evidence of variability.

Multiwavelength observations of Markarian 501 during the 1997 high state

During the observation period 1997, the nearby blazar Mrk 501 showed extremely strong emission and high variability. We examine multiwavelength aspects of this event using radio, optical, soft and hard X-ray, and TeV data. We concentrate on the medium-timescale variability of the broadband spectra, averaged over weekly intervals. We confirm the previously found correlation between soft and hard X-ray emission and the emission at TeV energies, while the source shows only minor variability at radio and optical wavelengths. The nonlinear correlation between hard X-ray and TeV fluxes is consistent with a simple analytic estimate based on an SSC model in which Klein-Nishina effects are important for the highest-energy electrons in the jet, and flux variations are caused by variations of the electron density and/or the spectral index of the electron injection spectrum. The time-averaged spectra are fitted with a synchrotron self-Compton (SSC) dominated leptonic jet model, using the full Klein-Nishina cross section and following the self-consistent evolution of relativistic particles along the jet, accounting for γγ absorption and pair production within the source as well as due to the intergalactic infrared background radiation. The contribution from external inverse-Compton scattering is tightly constrained by the low maximum EGRET flux and found to be negligible at TeV energies. We find that high levels of the X-ray and TeV fluxes can be explained by a hardening of the energy spectra of electrons injected at the base of the jet, in remarkable contrast to the trend found for γ-ray flares of the flat-spectrum radio quasar PKS 0528+134.

Evidence for an early high-energy afterglow observed with BATSE from GRB 980923

In this Letter, we present the first evidence in the BATSE data for a prompt high-energy (25-300 keV) afterglow component from a γ-ray burst, GRB 980923. The event consists of rapid variability lasting ~40 s followed by a smooth power-law emission tail lasting ~400 s. An abrupt change in spectral shape is found when the tail becomes noticeable. Our analysis reveals that the spectral evolution in the tail of the burst mimics that of a cooling synchrotron spectrum, similar to the spectral evolution of the low-energy afterglows for γ-ray bursts. This evidence for a separate emission component is consistent with the internal-external shock scenario in the relativistic fireball picture. In particular, it illustrates that the external shocks can be generated during the γ-ray emission phase, as in the case of GRB 990123.

A new analysis method for reconstructing the arrival direction of TeV gamma rays using a single imaging atmospheric Cherenkov telescope

Abstract We present a method of atmospheric Cherenkov imaging which reconstructs the unique arrival direction of TeV gamma rays using a single telescope. The method is derived empirically and utilizes several features of gamma-ray-induced air showers which determine, to a precision of 0.12°, the arrival direction of photons on an event-by-event basis. Data from the Whipple Observatorys 10 m gamma-ray telescope are utilized to test selection methods based on source location. The results compare these selection methods with traditional techniques and three different camera fields of view. The method will be discussed in the context of a search for a gamma-ray signal from a point source located anywhere within the field of view and from regions of extended emission.

The GRB coordinates network (GCN): A status report

The GRB Coordinates Network (GCN) continues to deliver locations of GRBs to instruments and observers in real-time (a few seconds)—while the burst is still bursting—so that they can make multi-band simultaneous follow-up observations. This was routine during the GRO-BATSE years and has resumed with HETE. This goal was realized with the optical detection of the burst counterpart for GRB990123 by the ROTSE instrument [1]. A brief review of the function and capabilities of the GCN system is given. Complementing the real-time location Notices, the GCN Circulars allow the follow-up observers to share the results of their observations rapidly with the community. A status report on recent improvements to the GCN system and a list of future improvements is given.

EXTENDED POWER-LAW DECAYS IN BATSE GAMMA-RAY BURSTS: SIGNATURES OF EXTERNAL SHOCKS?

The connection between gamma-ray bursts (GRBs) and their afterglows is currently not well understood. Afterglow models of synchrotron emission generated by external shocks in the GRB fireball model predict emission detectable in the gamma-ray regime (e25 keV). In this paper, we present a temporal and spectral analysis of a subset of BATSE GRBs with smooth extended emission tails to search for signatures of the ‘‘ early high-energy afterglow,’’ i.e., afterglow emission that initially begins in the gamma-ray phase and subsequently evolves into X-ray, UV, optical, and radio emission as the blast wave is decelerated by the ambient medium. From a sample of 40 GRBs we find that the temporal decays are best described with a power law

A search for TeV counterparts to batse gamma-ray bursts

Intense effort has gone into the observation of optical, radio, and X-ray gamma-ray burst (GRB) counterparts, either simultaneous to the burst or as quasi-steady lingering remnants. Here we report on a similar study at higher energies of 250 GeV and above using ground-based telescopes. The recent technical advances represented by the atmospheric Cherenkov imaging technique (Cawley & Weekes 1995) have opened up the field of gamma-ray astronomy above 250 GeV and raised the possibility that these techniques can be used with excellent fluence sensitivity in exploring the GRB phenomenon. Observations by the Whipple collaboration of nine BATSE positions, one acquired within 2 minutes of the reported BATSE burst time, using coordinates distributed through the BATSE Coordinates Distribution Network (BACODINE) are reported. No evidence of TeV emission is found, and upper limits to the high-energy delayed or extended emission of observed candidates are calculated.

The Structure and Evolution of LOCBURST: The BATSE Burst Location Algorithm

The gamma-ray burst (GRB) location algorithm used to produce the BATSE GRB locations is described. The general flow of control of the current location algorithm is presented, and the significant properties of the various physical inputs required are identified. The development of the burst location algorithm during the releases of the BATSE 1B, 2B, and 3B GRB catalogs is presented so that the reasons for the differences in the positions and error estimates between the catalogs can be understood. In particular, differences between the 2B and 3B locations are discussed for events that have moved significantly and the reasons for the changes explained. The locations of bursts located independently by the interplanetary network (IPN) are used to illustrate the effect on burst location accuracy of various components of the algorithm. IPN data and locations from other gamma-ray instruments are used to calculate estimates of the systematic errors on BATSE burst locations.