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The Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory has been operating for over 7 yr since its launch in 1991 April. This span of time far exceeds the design lifetime of 2 yr. As the instrument has aged, several changes have occurred owing to spark chamber gas exchanges as well as some hardware degradation and failures, all of which have an influence on the instrument sensitivity. This paper describes postlaunch measurements and analysis that are done to calibrate the instrument response functions. The updated instrument characteristics are incorporated into the analysis software.

The COS B source 2CG 135+01 has been observed by the EGRET instrument on 10 different occasions during the first ~52 months of the Compton Gamma Ray Observatory mission. The source is detected in all but one of the observations. For that one, the exposure was inadequate. The only likely source that is spatially coincident with the gamma-ray position is the radio source GT 0236+610/LS I +61°303. However, there is no compelling evidence for time variations in the gamma-ray emission associated with the radio outbursts from GT 0236+610. Spectral determinations on a timescale of a few days also give no strong evidence for a spectral variation associated with the radio emission of GT 0236+610. Such fluctuations might be expected based on models involving a compact object in an elliptical binary orbit about a massive star. The search for correlations simultaneous with the 8.4 GHz radio outbursts were supported by coordinated observations with the Madrid Deep Space Network during one of the exposures and by Green Bank Interferometer observations on two others. Although there is some possible variability in the gamma-ray flux, it is not clear that it is related to the radio phasing.

The high-energy diffuse gamma-ray emission from the Galactic plane, |b| ≤ 10°, is studied using observations from the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory. The spatial distribution of the diffuse emission has been determined for four broad energy ranges after removing the contribution from point sources detected with greater than 5 σ significance. The longitude and latitude distributions of the intensity, averaged over 4° latitude ranges and 10° longitude ranges, respectively, are shown for the four energy ranges. Spectra of the diffuse emission in 11 energy bands, covering the energy range 30 MeV to 30 GeV, were determined for 10° × 4° (l × b) bins after correcting for the finite EGRET angular resolution. The average spectrum from the direction of the inner Galaxy is shown for 29 energy bands, covering the energy range 30 MeV to 50 GeV. At latitudes |b| > 2°, corresponding to gamma rays emitted within about 3 kpc of the Sun, there is no significant variation in the spectra with Galactic longitude. Comparison of the spectra from the Galactic plane (|b| < 2°) reveals no significant variation with Galactic longitude below about 4 GeV, which suggests that the cosmic-ray electron to proton ratio does not vary significantly throughout the Galaxy. Above 4 GeV, however, there is weak (about 3 σ) evidence for variation of the Galactic plane (|b| < 2°) spectrum with longitude. The spectrum is softer in the direction of the outer Galaxy by about E compared to the spectrum from the inner Galaxy. This variation of the diffuse gamma-ray emission hints at a variation of the cosmic-ray proton spectrum with Galactic radius, which might be expected if cosmic rays are accelerated primarily in the inner Galaxy and then propagate to the outer Galaxy or if the high-energy cosmic rays are confined less well in the outer Galaxy. The spatial and spectral distributions of the diffuse emission are compared with a model calculation of this emission based on dynamic balance and realistic interstellar matter and photon distributions. The spatial comparison is used to establish the value of the molecular mass calibrating ratio N(H2)/WCO and the cosmic-ray/matter coupling scale r0, which are the only adjustable parameters of the model. Comparisons with the observations indicates N(H2)/WCO = (1.56 ± 0.05) × 1020 mol cm-2 (K km s-1)-1 and r0 = (1.76 ± 0.2) kpc. The spatial agreement between this model and the observation is very good. However, above about 1 GeV the integral intensity predicted by the model is about 60% less than the observed intensity. Although the explanation of this excess is unclear, uncertainties in the neutral pion production function or variations in the cosmic-ray spectrum with Galactic radius may partially account for the underprediction. A small medium-latitude (2° < |b| < 10°) excess in the direction of the inner Galaxy exists and may indicate that the low-energy photon density used in the model is too low.

We present evidence for the first detection of gamma rays from the extragalactic object BL Lacertae. Observations taken with EGRET on the Compton Gamma Ray Observatory between 1995 January 24 and 1995 February 14 indicate a 4.4 σ excess from the direction of BL Lacertae. The corresponding flux is (40 ± 12) × 10-8 photons cm-2 s-1 above 100 MeV. The combination of all previous observations where BL Lacertae was in EGRET's field of view result in a 2.4 σ excess and a corresponding 95% confidence upper limit of 14 × 10-8 photons cm-2 s-1, indicating that its gamma-ray emission is variable, at least on timescales of several months. Observations of BL Lacertae between 22 and 375 GHz were also taken between 1995 January 24 and 1995 February 14, and the flux levels for those measurements are similar to the historical average values for this object. A deep exposure on BL Lacertae with the Whipple Observatory 10 m gamma-ray telescope shows no evidence of emission above 350 GeV during a period 9 months after the EGRET observations. The 99.9% confidence flux upper limit derived from these observations is 0.53 × 10-11 photons cm-2 s-1, which implies a large reduction in the gamma-ray emission of BL Lacertae between EGRET and Whipple Observatory energies. This reduction should result from processes intrinsic to BL Lacertae because it is near enough to Earth that intergalactic background IR fields should not significantly reduce the flux of gamma rays to which the Whipple Observatory telescope is sensitive.

0!rQ1The Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory observed the Cygnus region in 14 different viewing periods during 1991 May to 1994 July. We present here our results on unpulsed and pulsed emissions of gamma rays at E > 50 MeV from Cyg X-3. While we detect a gamma-ray source consistent with the position of Cyg X-3, there was no evidence for the characteristic ~4.79 hr periodicity observed in X-rays and infrared.

In the course of the Compton Gamma Ray Observatory mission, the EGRET telescope has detected 14 BL Lacertae objects in high-energy gamma rays through Phase 4. According to the BL Lac sample in Ciliegi, Bassani, & Caroli, two of these 14 sources are X-ray selected (XBL) and 10 are radio-selected (RBL); two are not listed in this sample. An examination of the radio fluxes and the X-ray fluxes for the BL Lac objects detected by EGRET reveals that it seems a strong radio flux must be present if high-energy gamma rays are to be detected in a BL Lac object, while detection is independent of the X-ray flux of the source. This result is in agreement with the picture of the coaligned radio jet and X-ray jet model of the broadband properties of BL Lac objects. For the BL Lac objects listed in Ciliegi et al. that are not detected by EGRET, co-adding of the EGRET data for the undetected RBLs in such a way that the source positions coincide with each other yields a flux excess at a 3.5 σ level, but similar co-adding of the EGRET data for the undetected XBLs yields no flux excess.

The Energetic Gamma-Ray Experiment Telescope (EGRET) has observed gamma-rays bursts with the highest energy gamma-rays and the longest high energy emission to date. EGRET measures the high energy gamma-rays with its large NaI scintillator (1 to 200 MeV) and its spark chamber (30 MeV to 30 GeV). The spark chamber also measures time and arrival directions of individual photons allowing locations for the energetic bursts to be determined. Since the Compton Gamma Ray Observatory launch in 1991, EGRET has observed five bursts in the spark chamber with several having gamma-ray energies grater than 1 GeV. The recording breaking burst, GRB940217, had gamma-rays up to 18 GeV and lasted over 5000 seconds. The results for the energetic bursts are presented. The high energies observed from these gamma-ray bursts set constraints for the burst distances.

The energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO) has the capability of exploring the high energy plasma gamma ray range from approximately 30 MeV to 30 GeV with a sensitivity considerably greater than earlier gamma-ray satellite telescopes. The dominant radiation from the high energy gamma ray sky is the diffuse radiation along the galactic plane. Thus far, five pulsars have been detected and their properties measured. The high energy gamma rays appear to be an increasing fraction of the total emitted electromagnetic radiation as the age of the pulsar increases up to one million years. Observations of one solar flare event showed that there was strong evidence for long-term trapping of relativistic solar nuclei in the solar vicinity for at least 10 hours. Regarding the gamma ray bursts of unknown origin, high energy gamma rays have been seen following a burst for over an hour, with energies in the GeV range and even higher in the case of two individual gamma rays. The observation of the Small Magellanic Cloud appears to answer finally the long open question of whether the bulk of the cosmic rays are galactic or pervade some much larger volume on the side of their being galactic. Over 30 Active Galactic Nuclei (AGN's) have been seen in high energy gamma rays with high probability thus far including quasars and BL Lac objects, but no Seyfert galaxies. Time variations have been detected in many of these AGN's.

The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory has detected more than 20 Active Galactic Nuclei (AGN) at photon energies above 30 MeV.

The Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO) covers the high energy gamma ray energy range, approximately 30 MeV to 30 GeV, with a sensitivity considerably greater than earlier high energy gamma-ray satellites. Thus far, 4 pulsars have been detected and their properties measured, including in 3 cases the energy spectrum as a function of phase. The details of the galactic plane are being mapped and a spectra of the center region has been obtained in good agreement with that expected from cosmic ray interactions. The Magellanic clouds have been examined with the Large Magellanic Cloud (LMC) having been detected at a level consistent with it having a cosmic ray density compatible with quasi-stable equilibrium. Sixteen Active Galactic Nuclei (AGN's) have been seen thus far with a high degree of certainty including 12 quasars and 4 BL Lac objects, but no Seyferts. Time variation has been detected in some of these AGN's

A calculation method of estimating instrumental background in high-energy gamma -ray telescopes, using the hadronic Monte Carlo code FLUKA87, is presented. The method is applied to the SAS-2 and EGRET telescope designs and is also used to explore the level of background to be expected for alternative configurations of the proposed GRITS telescope, which adapts the external fuel tank of a Space Shuttle as a gamma -ray telescope with a very large collecting area. The background produced in proton-beam tests of EGRET is much less than the predicted level. This discrepancy appears to be due to the FLUKA87 inability to transport evaporation nucleons. It is predicted that the background in EGRET will be no more than 4-10 % of the extragalactic diffuse gamma radiation. >

The attraction of high energy gamma-ray astronomy lies in this radiation relating directly to those processes in astrophysical situations which deviate most from thermo-dynamic equilibrium. Some examples of these phenomena which are known to or expected to emit gamma rays are cosmic rays as they interact in intergalactic space, the high energy particles in the magnetic fields of neutron stars, the death of a black hole, the explosion and residual of a supernova, lumps of Weakly Interacting Massive Particles, energetic solar particles interacting near the sun, and very high energy particles in the extreme conditions associated with active galaxies. Although the intensities are known to be low as seen near the earth, a partially compensating characteristic is that the very penetrating nature of high energy gamma rays increases the probability that they can escape from their origin and reach the solar system.

In order to continue the achievements in high-energy (10 MeV-100-GeV) gamma-ray astronomy made with the Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton Gamma Ray Observatory (CGRO), a next-generation high-energy gamma-ray telescope with a large increase in sensitivity coupled with improved angular resolution will be required. This telescope is envisioned as a 2-m*2-m active area telescope using drift chambers for the imaging detector. The four major components of the instrument are the anticoincidence shield, the track imaging system, the coincidence/time-of-flight system, and the energy measurement system. The authors discuss the design goals and challenges for the four subsystems and the techniques being used to achieve them, as well as the design and performance of high-speed electronics that are being developed specifically for this application. >

On April 5, 1991, the Space Shuttle Atlantis carried the Compton Gamma Ray Observatory into orbit and the satellite was deployed on April 7. The Energetic Gamma Ray Experiment Telescope (EGRET) instrument was activated on April 15, and the first month of operations was devoted to verification of the instrument performance. Measurements made during that month and in the subsequent sky survey phase have verified that the instrument time resolution, angular resolution, and gamma ray detection efficiency are all within nominal limits. >

Observations of the Small Magellanic Cloud using the Energetic Gamma Ray Experiment Telescope on the Compton Observatory yield an upper limit (95% confidence) for gamma-ray emission g100 MeV of 0.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$ photon/${\mathrm{cm}}^{2}$ s. The expected flux if the cosmic rays are universal rather than galactic in origin, is (2.4-0.5)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$ photon/${\mathrm{cm}}^{2}$ s, only a third of which arise from cosmic ray electron interactions. Hence, the bulk of the cosmic ray energy density is almost certainly not metagalactic, and therefore galactic in origin.

We present the results of the multi-wavelength campaigns on 3C 273 in 1993-1995. During the observations in late 1993 this quasar showed an increase of its flux for energies greater equal 100 MeV from about 2.1 x 10^-7 photons/cm^2/s to approximately 5.6 x 10^-7 photons/cm^2/s during a radio outburst at 14.5, 22 and 37 GHz. However, no one-to-one correlation of the gamma-ray radiation with any frequency could be found. The photon spectral index of the high energy spectrum changed from (3.20 +- 0.54) to (2.20 +- 0.22) in the sense that the spectrum flattened when the gamma-ray flux increased. Fits of the three most prominent models (synchrotron self-Comptonization, external inverse Comptonization and the proton initiated cascade model) for the explanation of the high gamma-ray emission of active galactic nuclei were performed to the multi-wavelength spectrum of 3C 273. All three models are able to represent the basic features of the multi-wavelength spectrum. Although there are some differences the data are still not decisive enough to discriminate between the models., 63 pages including 9 eps figures and 9 tables, LateX, uses aaspp4.sty and tighten.sty, accepted for publication in ApJ

We report CGRO, RXTE, ASCA, ROSAT, IUE, HST and ground-based observations of a large flare in 3C 279 in February 1996. X-rays and γ-rays peaked simultaneously (within one day). We show simultaneous spectral energy distributions prior to and near the flare peak. The γ-ray flare was the brightest ever observed in this source.

The blazar 3C 279, one of the brightest identified extragalactic objects in the gamma-ray sky, underwent a flare of a factor 10 amplitude in gamma-rays towards the end of a 3-week pointing by CGRO, in 1996 January-February. The flare peak represents the highest gamma-ray intensity ever recorded for this object. During the high state, extremely rapid gamma-ray variability was seen. Coordinated multifrequency observations were carried out with RXTE, ASCA, ROSAT and IUE and from many ground-based observatories, covering most accessible wavelengths. The well-sampled, simultaneous RXTE light curve shows an outburst of lower amplitude (factor of ~3) well correlated with the gamma-ray flare without any apparent lag. The optical-UV light curves, which are not well sampled during the high energy flare, exhibit more modest variations (factor of ~2) and a lower degree of correlation. The flux at millimetric wavelengths was near an historical maximum during the gamma-ray flare peak, with suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The gamma-rays vary by more than the square of the observed IR-optical flux change, which poses some problems for specific blazar emission models. The synchrotron-self Compton model would require that the largest synchrotron variability occurred in the mostly unobserved sub-mm/far-infrared region. Alternatively, a large variation in the external photon field could occur over a time scale of few days. This occurs naturally in the ``mirror'' model, wherein the flaring region in the jet photoionizes nearby broad-emission-line clouds, which in turn provide soft photons that are Comptonized to gamma-ray energies., Comment: 18 pages, Latex, 2 tables, 3 postscript figures, to appear in The Astrophysical Journal

The all-sky survey in high-energy gamma rays (E$>$30 MeV) carried out by the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma-Ray Observatory provides a unique opportunity to examine in detail the diffuse gamma-ray emission. The observed diffuse emission has a Galactic component arising from cosmic-ray interactions with the local interstellar gas and radiation as well an almost uniformly distributed component that is generally believed to originate outside the Galaxy. Through a careful study and removal of the Galactic diffuse emission, the flux, spectrum and uniformity of the extragalactic emission is deduced. The analysis indicates that the extragalactic emission is well described by a power law photon spectrum with an index of -(2.10+-0.03) in the 30 MeV to 100 GeV energy range. No large scale spatial anisotropy or changes in the energy spectrum are observed in the deduced extragalactic emission. The most likely explanation for the origin of this extragalactic high-energy gamma-ray emission is that it arises primarily from unresolved gamma-ray-emitting blazars., 19 pages latex, 10 figures, accepted for publication in ApJ

The number of positive gamma-ray source detections by the Compton Gamma-Ray Observatory has been large, and the significance of the results has been substantial, as is described in other reports at this conference. In addition, searches for gamma rays were made that have led to null results, but that nonetheless deserve attention because of their important scientific implications. In the gamma-ray observations there has been as yet no indication of antimatter in the universe other than that produced in cosmic-ray interactions. The upper limit for microsecond bursts has implications for attempts at unified theories of physics. The search for signatures from supersymmetric particles has not revealed any positive evidence. The measurements of the high-energy gamma-ray emission from blazars and its interpretation in terms of the relativistic particles and their origin has been one of the highlights of the EGRET results. However, there are a few flat-spectrum radio quasars that are very strong in the radio reg...

The exciting results from the highly successful Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton Gamma-Ray Observatory (CGRO) has contributed significantly to increasing our understanding of high energy gamma-ray astronomy. A follow-on mission to EGRET is needed to continue these scientific advances as well as to address the several new scientific questions raised by EGRET. Here we describe the work being done on the development of the Advanced Gamma-Ray Astronomy Telescope Experiment (AGATE), visualized as the successor to EGRET. In order to achieve the scientific goals, AGATE will have higher sensitivity than EGRET in the energy range 30 MeV to 30 GeV, larger effective area, better angular resolution, and an extended low and high energy range. In its design, AGATE will follow the tradition of the earlier gamma-ray telescopes, SAS-2, COS B, and EGRET, and will have the same four basic components of an anticoincidence system, directional coincidence system, track imaging, and energy measurement systems. However, due to its much larger size, AGATE will use drift chambers as its track imaging system rather than the spark chambers used by EGRET. Drift chambers are an obvious choice as they have less deadtime per event, better spatial resolution, and are relatively easy and inexpensive to build. Drift chambers have low power requirements, so that many layers of drift chambers can be included. To test the feasibility of using drift chambers, we have constructed a prototype instrument consisting of a stack of sixteen 1/2m × 1/2m drift chambers and have measured the spatial resolution using atmospheric muons. The results on the drift chamber performance in the laboratory are presented here.

For the period from launch in April 1991 to present, EGRET had reported on the detection of seven energetic gamma-ray bursts. A systematic analysis of the Total Absorption Shower Counter (TASC) data has been made for bursts within the EGRET field of view. Spectral indexes are given for these bursts detected by EGRET and upper limits are given for bursts detected by COMPTEL and not by EGRET.

In the mid 1970’s Hawking (1) and Page and Hawking (2), investigated theoretically the possibility of detecting high-energy gamma rays produced by the quantum-mechanical decay of a small black hole created in the early universe. They concluded that, at the end of the life of the small black hole, it would radiate a burst of gamma rays peaked near 250 MeV with a total energy of about 1034 ergs in the order of a microsecond or less if certain details of the theory were true. The characteristics of a black hole are determined by laws of physics beyond the range of current particle accelerators; hence, the search for these short bursts of high-energy gamma rays provides at least the possibility of detecting directly the gamma rays from such bursts, and a search of the EGRET data has led to an upper limit below 5×10−2 black hole decays per pc3 yr−1, placing constraints on this and other theories predicting microsecond high-energy gamma-ray bursts.

Burst spectra over a restricted energy range look like a power law plus some curvature. Such spectra can be well fit by most models or mechanisms, and hence reveal little of the underlying physics. With the launch of the Compton Gamma Ray Observatory, we can construct composite spectra from tens of keV to tens of GeV. This wide spectral range yields burst spectra that can sharply confront models. We have constructed wide-range composite spectra for several bursts, two of which are presented here: GRB910601 and GRB910814. The energy ranges are 28 keV to 10 MeV and 100 keV to 200 MeV. These spectra were constructed from spectra by BATSE, EGRET, COMPTEL, and OSSE. The spectral shape of GRB910601 shows a continuous turnover between 100 keV and 1 MeV, with peak νFν at 500 keV. The spectral shape of GRB910814 is dominated by a sharp and strong spectral break near 2 MeV. In addition to these two broad spectra, we have derived an ‘average’ spectral shape for 20 bright bursts from 40 keV to 1500 keV. This average spectra will be useful for studies of the red shifting of average spectra.Burst spectra over a restricted energy range look like a power law plus some curvature. Such spectra can be well fit by most models or mechanisms, and hence reveal little of the underlying physics. With the launch of the Compton Gamma Ray Observatory, we can construct composite spectra from tens of keV to tens of GeV. This wide spectral range yields burst spectra that can sharply confront models. We have constructed wide-range composite spectra for several bursts, two of which are presented here: GRB910601 and GRB910814. The energy ranges are 28 keV to 10 MeV and 100 keV to 200 MeV. These spectra were constructed from spectra by BATSE, EGRET, COMPTEL, and OSSE. The spectral shape of GRB910601 shows a continuous turnover between 100 keV and 1 MeV, with peak νFν at 500 keV. The spectral shape of GRB910814 is dominated by a sharp and strong spectral break near 2 MeV. In addition to these two broad spectra, we have derived an ‘average’ spectral shape for 20 bright bursts from 40 keV to 1500 keV. This average ...

We report simultaneous observations of the optically violent variable (OVV) quasar PKS 1406-076 in the optical and gamma-ray regime. Gamma-ray emission is detected with very high significance. Both the gamma-ray and optical properties of PKS 1406-076 are typical for the group of gamma-bright active galactic nuclei detected by EGRET. During 1993 January the source was unusually bright in both frequency ranges. During this period we observed a rapid, symmetrical flare in the optical wavelength range which lasted for about 4 days. EGRET detected a similar rapid flare peaking about 22 hr after the optical outburst. Because such rapid flares are rare events, it is likely that the outbursts in the two frequency regimes are correlated. Correlated outbursts with similar profiles and a lag between the optical and gamma-ray maxima of 22 hr are inconsistent with simple versions of models suggested for the emission of high-energy radiation from blazars.

The EGRET instrument aboard the Compton Gamma Ray Observatory (CGRO) has completed the first all-sky survey in high-energy gamma rays and has repeatedly viewed selected portions of the sky. Analysis of the region with galactic latitude $+10\arcdeg < b < +40\arcdeg$ indicates the presence of nineteen point sources, including nine which can be identified as active galactic nuclei, some of which have been reported previously, as well as ten other sources with no definite counterparts. Using the combined exposures from Phase 1 and Phase 2 of the CGRO viewing program, the spectra, time variability, and positions of all detected sources in this region are determined. It is tentatively suggested that one of the unidentified sources might be associated with the radio galaxy Centaurus A., Comment: 18 pages, 3 Postscript figures (one of 3 pages) Uses aaspp4.sty

We detected a sixth high-energy gamma-ray pulsar, PSR B1951+32, pulsating in gamma rays at E ≥ 100 MeV with the same 39.5 ms periodicity as in radio, using the data obtained during 1991 May to 1994 July by the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory. Although seen only as a weak source amidst the high background of galactic disk emission, the pulsation in high-energy gamma rays is clearly seen. The pulsed radiation has a photon spectral index of -1.74 ± 0.11. There is no evidence as yet for unpulsed emission from the object. The pulsar appears to have an efficiency of ~0.004 for converting its rotational energy loss into gamma rays at E ≥ 100 MeV.

The EGRET instrument on the Compton Gamma Ray Observatory, which detects photons in the energy range 30 MeV to 30 GeV, has detected about 40 sources of gamma rays (>100 MeV) at high galactic latitudes. Most of these appear to be associated with bright, flat-spectrum radio sources, many of which have been previously classified as optically violent variable (OVV) quasars or BL Lac objects (together often referred to as blazars). The remaining sources are presently unidentified, although some of them are positionally compatible with flat-spectrum radio sources which are somewhat less bright and less well-studied.

The Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Observatory has increased our understanding of active galactic nuclei, pulsars, diffuse emission, solar flares and gamma ray bursts. In order to continue these advances in high energy gamma-ray astronomy, development of a follow-on telescope to EGRET is in progress. The new experiment, named the Advanced Gamma-Ray Astronomy Telescope Experiment (AGATE), has the same basic components which have made EGRET a successful, low background telescope for 30 MeV to 30 GeV gamma rays; however, the sensitivity has been increased by an order of magnitude, the energy range extended to higher energies, and the angular resolution improved.

Gamma Ray Bursts (GRBs) have puzzled astronomers since their discovery more than 20 years ago. As no counterparts at wavelengths other than X- and γ-rays have yet been found the identification of the sources is still missing. Theoretical explanations range from colliding comets (1993) and merging neutron stars (1982) to more exotic objects, such as superconducting cosmic strings (1988). Data accumulated until now still do not discriminate between these models, although results from the BATSE (Burst and Transient Source Experiment) instrument aboard the Compton Gamma Ray Observatory ( CGRO ) strongly favor extragalactic models. The Energetic Gamma Ray Experiment Telescope ( EGRET ) aboard CGRO has s ofar detected photons from 5 GRBs with its spark chamber. These are the highest energy γ-rays associated with GRBs to date. In this work we review previously published data and summarize the properties of these events. Elsewhere we present possible constraints from the data on the models proposed to explain GRBs.

The development of large area xenon drift chambers as imaging systems for the advanced Gamma-Ray Astronomy Telescope Experiment (AGATE), sensitive in the energy range 20 MeV - 100 GeV, is presented here. AGATE is visualized as the successor to the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory, and will add to the wide range of important results currently being obtained by EGRET. Experiments were carried out with a laboratory prototype consisting of a stack of sixteen 1/2m X 1/2m active area drift chambers using both xenon and argon gas mixtures. The spatial resolution of the drift chamber stack was measured with a multi-wire readout plane using atmospheric muons traversing the active volume. A spatial resolution of about 0.23 mm was measured with drift chambers using xenon- methane gas mixtures. The experiments with the argon-isobutane gas mixtures yielded a spatial resolution of about 0.14 mm.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper provides a summary of the data reduction and analysis procedures used by the Energetic Gamma Ray Experimental Telescope (EGRET) team. The steps are summarized beginning with the extensive calibration, proceeding to the basic data reduction, and finishing with the data analysis techniques, including a brief description of the source determination, the intensity calculation, the spectral evaluation, the pulsar program, and time variations. References are provided for the detailed discussions of the major portions of the work. The types of data and the software available to Guest Investigators are also reviewed with guides to more detailed descriptions. In this way, the overview is given in one place with references for those wishing to see parts in detail.

EGRET onboard the Compton Gamma Ray Observatory observed four energetic X‐type solar flares during June 1991. Two of these flares were in the EGRET spark chamber field of view and were observed to have high energy gamma ray emission lasting hours after the impulsive phase of the flare. Measurements of all four flares were obtained by the EGRET large NaI spectrometer.

The EGRET experiment on board the Compton Gamma Ray Observatory observed energetic gamma rays from the impulsive solar flares on June 30 and July 2, 1991. The June 20 and July 2 spectra were measured by the large NaI spectrometer in EGRET and extended up to nearly ∼50 MeV and ∼10 MeV respecetively before appearing to cut off. These spectra can be fit by a power law model or an isothermal plasma model.

The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Observatory has detected, with a high degree of certainty, more than 20 active galactic nuclei (AGN) in high energy gamma‐ray (E≳100 MeV). All of these sources appear to be members of the blazar class of AGN, exhibiting one or more of the characteristics of this source class (flat radio spectrum, strong variability, optical polarization).Variability on time scales from days to months or longer is a characteristic of the gamma‐ray emission from most of the blazars detected by EGRET. In this paper the EGRET observations of variability are summarized and some of the implications for emission models are discussed. In particular, the EGRET observations and consideration of gamma‐ray absorption via photon‐photon pair production strongly suggest that the gamma radiation from AGN is beamed toward us. This is consistent with the interpretation of blazars as objects in which the non‐thermal radiation arises predominantly from a ‘‘jet’’ of relativis...

The EGRET instrument aboard the Compton Gamma‐Ray Observatory has detected gamma‐ray emission from many Active Galactic Nuclei during the Phase I & II observation periods. With the continuing discovery of gamma‐ray loud AGN, it becomes increasingly important to characterize the luminosity and density distribution of these objects in order to understand how the gamma‐ray characteristics are related to properties observed at radio, optical and X‐ray wavelengths. The 〈V/Vmax〉 test is applied to the population of gamma‐ray loud Active Galactic Nuclei which have been observed by the EGRET instrument during the Phase I all‐sky survey. Evidence for luminosity evolution is presented, and estimates of the degree of evolution are made given standard assumptions about the cosmology and the form of the luminosity evolution. The effects of observational biases due to gamma‐ray flux‐detection limits and optical identifications are considered.

GRB 910601 was well observed by all three broad‐field experiments on the COMPTON Observatory; it was also at a known position within the narrow aperture of the OSSE instrument. This has permitted us to compare spectra observed from all four of the COMPTON Observatory instruments. The burst lasted for about 40 s and was observed into the MeV region by all of the detectors; it was not detected at energies above 10 MeV. Time‐integrated spectra from COMPTEL, EGRET, and OSSE are in good agreement. Spectra from both the BATSE large area and spectroscopy detectors during the early part of the burst are also in good agreement with the OSSE spectrum.

EGRET has imaged three gamma‐ray bursts at energies above 30 MeV. The bursts are GRB930131, GRB910503, and GRB910601—listed in order of decreasing intensity. GeV emission has been detected in two bursts which increases by an order of magnitude the maximum energy gamma rays observed from a gamma‐ray burst. The high energy emission is a significant contributor to the fluence of these burst, and the duration of the high energy emission is at least as long and in some cases longer than the emission observed by BATSE. The high energy gamma rays are also used to determine the most probable direction of the burst.

The EGRET phase 1 & 2 observations of Geminga have been used to determine an optimum contemporary ephemeris using the ‘‘Z2n timing method.’’ The residuals for this linear ephemeris over 2.1 years are less than 5 ms, suggesting that Geminga has not glitched. A position estimate is found to be consistent with the G‘ optical counterpart. The comparison of f between this ephemeris and that of COS‐B yields an estimate of f which is an order of magnitude smaller than that determined from the COS‐B data. It is estimated that the anticipated EGRET phase 3 Geminga exposure will allow for the comparison of Geminga phase between EGRET, COS‐B, and SAS‐2.

The EGRET gamma‐ray telescope has detected a persistent source of high energy gamma rays which has not been identified. It was detected in seven different observations in 1991–1993. Variations in the spectrum, combined with a galactic latitude of +25°, make it likely that this is a previously unknown blazar.

During Phase 1 and Phase 2 of the Compton Observatory’s viewing program, Mrk 421 was in EGRET’s field of view a total of 6 times. This BL Lac object was positively detected by EGRET at 5 σ level and with a photon flux of (1.4±0.3)×10−7 cm−2 s−1 for E≳100 MeV when it was first covered by EGRET in 1991 June 28–July 12. The photon spectrum of this first observation can be represented by a single power law with a photon spectral index of −1.96±0.14. During the 5 subsequent observations, this source was covered by EGRET either at large viewing angles with respect to the instrument axis or with short exposure times. But the source was always visible to the EGRET telescope at about the same flux level as during the first observation except one viewing period, VP222, 1993 May 24–31, for which only an upper limit can be obtained for the photon flux. This upper limit is also consistent with the detected flux levels of the other observations. The average photon floux for E≳100 MeV as calculated from the combined data of all these 6 observations is (1.7±0.3)×10−7 cm−2 s−1 and the corresponding photon spectral index for the combined data is −1.71±0.15. A summary of these results are given in this paper.

In order to continue the achievements in high energy (10 MeV - 100 GeV) gamma-ray astronomy made with the Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton Gamma Ray Observatory (CGRO), a 'next generation' high energy gamma- ray telescope with a large increase in sensitivity coupled with improved angular resolution will be required. This 'next generation' telescope is envisioned as a 2 m X 2 m active area telescope using drift chambers for the imaging detector. The four major components of the instrument are the anticoincidence shield, the track imaging system, the coincidence/time-of- flight system and the energy measurement system. In this paper we discuss the design goals and challenges for the four subsystems and the techniques we are utilizing to achieve them as well as the design and performance of high speed electronics that we have developed specifically for this application.