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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.

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 has detected more than 20 Active Galactic Nuclei (AGN) at photon energies above 30 MeV.

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. >

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. >

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 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.

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.

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.

The EGRET instrument has detected gamma‐ray bursts in its large‐volume NaI spectrometer, anticoincidence shield and imaging spark chamber. Its spectral range from 1 MEV to greater than 200 MeV overlaps with and extends beyond those of other instruments on the Compton Gamma Ray Observatory. For intense high‐energy burst in the telescope field of view, detection of gamma rays in the spark chamber telescope allows the location of a burst to be determined. Only six bursts, three of which were strong bursts with gamma‐ray energies greater than 10 MeV, have been observed to date as a result of BATSE triggers. Interest in spectral breaks at lower energies led to a careful examination of the EGRET sensitivity in the overlap region with BATSE. The low‐energy sensitivity was studied using the detailed CGRO mass model; the resulting sensitivity values are presented as a function of burst direction relative to EGRET.

The Compton Observatory was pointed at Cygnus twice during the full sky survey (5/30–6/8/91 & 8/8–8/15/91). The COS‐B sources 2CG 075+00 and 2CG 078+01 are detected by EGRET, and smaller error boxes have been obtained, but the sources are not yet identified at other wavelengths. An excess of counts is noted at the position of Cygnus X‐3 consistent with a point source. The corresponding flux is 8±2×10−7 cm−2 s−1 (E≳0.1 GeV)—a factor of 5 less than the flux reported by the SAS‐2 group for Cygnus X‐3. The observed intensities for the individual periods were consistent with no change in intensity between the first and second observations (the second observation began two weeks after the beginning of the July 1991 radio flare). More work is required to refine and verify the galactic diffuse model (which comprises at least 10 times more flux than Cygnus X‐3 in this direction) before a definitive Cygnus X‐ray point‐source result will be obtained. The 4.8 hr X‐ray periodicity of Cygnus X‐3 is not detected in exta...

The Energetic Gamma Ray Experimental Telescope (EGRET) instrument detected the gamma ray bursts on June 1, 1991 (GRB 910601) and August 14, 1991 (GRB 910814). Significant time structures down to 0.256 s were resolved by the by the anti‐coincidence dome. Spectra accumulated by the NaI(Tl) crystal are consistent with a power law model with evidence of spectral evolution. Four gamma ray events within 5 degrees from the direction of GRB 910601 were found in the spark chamber, when 1.5 could be expected from the detection rate, but they arrived only after the main burst emission had decayed. When GRB 910814 arrived, the spark chamber was disabled because the earth’s atmosphere was in its field of view.

Ever since the detection of the Crab and Vela pulsars as high‐energy gamma‐ray sources, there has been speculation that many pulsars might be gamma‐ray emitters, some of which might be detectable with more sensitive instruments. Based on theoretical considerations, most interest has focused on young pulsars where a relatively high fraction of the spin down energy might be expected to be carried away in gamma rays. Millisecond pulsars have also been suggested as possible detectable gamma‐ray sources. Consequently, the Compton Gamma Ray Observatory was designed with the ability to time tag gamma rays to an accuracy of a tenth of a millisecond in absolute time. This is needed in view of the long integration times required because of the low expected fluxes. In addition, a cooperative effort has been organized with radio astronomers who are making contemporary measurements of the ephemerides of a relatively large fraction of known pulsars which might be expected to be candidate gamma‐ray ‘‘pulsars’’ based on ...

Most of the active galactic nuclei (AGN) detected by EGRET are known or suspected to be superluminal sources. We report here the results of the observations of the blazars 3C 345, BL Lac and 3C 371, which have been selected from the list of known and possible superluminal sources. For none of them we found a signal in high energy γ‐rays. The 2σ upper limits for the γ‐ray fluxes for energies above 100 MeV (equivalent to 95% confidence) are presented.

One of the primary goals of the EGRET mission on the Compton Observatory has always been the study of the diffuse gamma ray emission of both galactic and extra‐galactic origin. The first 18 months of the mission, following the initial activation period, is being devoted to the all‐sky survey with the goal of obtaining a nearly uniform exposure. In anticipation of the improved capabilities of the EGRET instrument relative to the earlier SAS‐2 and COS‐B missions, a new model of the galactic diffuse gamma ray emission was developed that incorporates recent surveys of the matter distribution and which permits a variety of assumptions regarding galactic cosmic ray distribution to be tested. This paper summarizes the model calculation and gives a preliminary comparison of the model with EGRET observations.

The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray observatory has observed energetic gamma ray bursts and flares. On May 3, 1991, EGRET detected a gamma ray burst both in the energy measuring NaI (Tl) scintillator and independently in the spark chamber imaging assembly. The NaI spectra were accumulated by a special BURST mode of EGRET. The spectra were measured over a range from 1 to 200 MeV, in three sequential spectra of 1, 2, and 4 seconds. During the peak of the burst, six individual gamma rays were detected in the spark chamber, allowing a determination of the burst arrival direction. The intense flares of June were also detected. A solar flare on June 4 was observed to last for several minutes and for a brief time, less than a minute, had significant emission of gamma rays exceeding 150 MeV.

EGRET detected an unidentified source, GRO J0744+54, at a significance of 6.5 sigma, during its observations from 1993 June 28 to July 12. The source was seen again in the following 2 week viewing period and was weakly evident in the earlier phase 1 of the EGRET observations. Considering the variability of its gamma-ray flux, and its location at high Galactic latitude, GRO J0744+54 is likely to be a previously undetected blazar. Its most likely identification is with the radio source 87GB 073840.5+545138. A second source, GRO J0957+65, was seen by EGRET during the same two viewing periods at a combined significance of 5.7 sigma. The most probable counterpart of GRO J0957+65 is the BL Lacertae object 0954+658. The spectra, time variability, and positions of the two sources are presented. Multiwavelength observations of 0954+658 are also presented.

During the Compton Observatory's viewing programs Phase 1 (1991 April to 1992 November, also known as the All-Sky Survey) and Phase 2 (1992 November to 1993 September), the BL Lac object 0716+714 was in the field of view of the EGRET telescope a total of six times, three times in Phase 1 and three more times in Phase 2, while the BL Lac object 0521-365 was in the field of view of EGRET only once in Phase 1. The source 0716+714 was detected in high-energy gamma rays by EGRET at a flux level of (2.0 +/- 0.4) x 10(exp -7) photons/sq cm/s for E greater than 100 MeV with a 6 sigma significance when it was first observed by EGRET in 1992 January 10 to 23. The corresponding spectral slope of the photon number distribution is determined to be -2.04 +/- 0.33. The gamma-ray flux of 0716+714 showed considerable time variability in subsequent EGRET observations. But the spectral slope stayed about the same within the statistical uncertainties of the EGRET data. The average spectral slope of the four viewing periods during which the photon flux of 0716+714 stayed above the EGRET detection threshold is found to be -1.85 +/- 0.20 from the combined data. The source 0521+365 was detected by EGRET in 1992 May 14 to June 4 at a flux level of (1.8 +/- 0.5) x 10(exp -7) photons/sq cm/s for E greater than 100 MeV with a 4 sigma significance. The corresponding spectral slope of the photon number distribution is found to be 2.16 +/- 0.36. Details of the observations of these two BL Lac objects with the EGRET telescope are presented.

As part of its ongoing survey of the high-energy gamma-ray sky, the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory has searched for emission from spin-powered pulsars, five of which have now been detected in the energy range 30 MeV less than or = E less than or = 20 GeV. A systematic study of the all-sky survey has found no additional pulsed gamma-ray sources. The pulsar detections, coupled with the upper limits on pulsed gamma radiation from other radio pulsars, indicate that the simplest models of gamma-ray pulsars are incomplete.

Hawking (1974) and Page & Hawking (1976) 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 very 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 10(exp 34) ergs in the order of a microsecond or less. 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 being the first test of this region of physics. The Compton Observatory Energetic Gamma-Ray Experiment Telescope (EGRET) has the capability of detecting directly the gamma rays from such bursts at a much fainter level than SAS 2, and a search of the EGRET data has led to an upper limit of 5 x 10(exp -2) black hole decays per cu pc per yr, placing constraints on this and other theories predicting microsecond high-energy gamma-ray bursts.