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It is a well-known fact that the high-energy gamma-ray telescopes in conventional designs are inherently limited in their spatial resolving powers. EGRET can locate a point source to 5 to 10 arcmin accuracy, while GLAST can achieve source location capabilities to better than 0.5 arcmin. Although there is still room for improvement in the angular resolution of the conventional high-energy gamma-ray telescopes, other means can also be explored at this time. Coded masks have been successfully used for X-ray and low-energy gamma-ray telescopes. Such devices have also been considered for high-energy gamma-ray telescopes from time to time. In this paper we investigate the feasibility of using coded masks to achieve much higher source location capabilities for a high-energy gamma-ray space telescope. We discuss the merits and the difficulties of using coded masks to achieve this goal. In particular, we look into the issues of size, weight, field of view, and the trade-off between the angular resolution and the f...

It has long been recognized that the high-latitude EGRET sources can be identified with blazars of significant radio emission. Many aspects of the relation between high-energy gamma-ray emission and radio emission of EGRET-detected blazars remain uncertain. In this paper, we use the results of the recently published Third EGRET Source Catalog to examine in more detail to what extent the EGRET flux and the radio flux are correlated. In particular we examine the correlation (or the lack of it) in flux level, spectral shape, temporal variation, and detection limit. Many significant previous studies in these areas are also evaluated.

The compact, radio-loud quasar PKS 0528+134 has been one of the brightest active galactic nuclei (AGN) detected by EGRET. The flux history of the source including data up to March 1997, as well as the spectra measured during the flaring and non-flaring states is presented. Multiwavelength observations of PKS 0528+134 are also presented. A detailed relativistic SSC jet model agrees well with the multiwavelength spectrum; the data, however are insufficient to discriminate between this and other emission models.

Source distribution studies only provide meaningful information if the basic populations are carefully determined, especially if the statistics are limited. This necessitates restrictions from the approach of using the complete EGRET source catalog data. Considering various peculiarities of the catalogs a more uniform data set is defined for further source distribution studies. Here, we focus on the temporal and spectral characteristic of the unidentified EGRET high latitude sources (|b|>10°). Variability is addressed on the basis of flux histories with timescales of individual viewing periods, and spectra are determined for all sources based on the enhanced statistics from EGRET observations of Phase 1 to 4. The derived features are used to discuss whether the unidentified high latitude sources show similarities or differences from classes of already identified objects, or if no conclusive answer can be given.

PKS 2005−489, at z=0.071, is one of the closest BL Lacertae objects, bright in radio to X-ray wavebands. It has been in EGRET’s field of view a total of six times within an aspect angle of 30° from Phase 1 to Phase 3 (1991 May to 1994 October), none afterwards. But the total EGRET exposure to this region of the sky was still rather low. PKS 2005−489 was listed in the First EGRET Catalog at the significance TS=4.3, in the context of the maximum likelihood analysis adopted by the EGRET team as part of the standard EGRET software package. Later when the EGRET data were reanalyzed with revised software and improved diffuse model to generate the Second EGRET Catalog, the detection significance of PKS 2005−489 was found to be at TS=3.8, slightly below the TS=4.0 cut line to qualify for an entry in the Second EGRET Catalog. The status of EGRET results on PKS 2005−489 at this time is presented in this paper.

Several known low- and high-mass X-ray binaries with orbital periods between tens of minutes and several days were examined in EGRET data. Instrument thresholds for orbital modulation were calculated, and appropriate observations were analyzed with new software which uses maximum likelihood methods for signal detection, fully accounting for variations in EGRET exposure, as well as the structured gamma-ray background and the energy-dependent point-spread function. While some sources were not detected by EGRET at all, several were marginally detected with less than 5 σ significance. However, due to significant background and low statistics, the observations were unable to constrain the modulation fraction of any of the sources.

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.

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.

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 ...

The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory has detected 16 active galactic nuclei in high energy gamma rays thus far during the first year in orbit. Of these, the majority are quasars and four are usually classified as BL Lacertae objects. No Seyferts have been seen. Of the AGN’s seen, 3C 279 had the highest flux when first observed, but it is variable and was at a significantly lower level when observed later. There is a wide range of distances among the observed set, with four quasars having known z values in excess of 1.0. One BL Lac has a z of 0.94. If the photon spectra are represented by power laws in energy, those which have been determined have spectral indices ranging from about −1.7 to −2.4. In some cases, there is a suggestion of a steepening at the highest energies observed.

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.

A critical question in the study of the dynamics of our own galaxy is whether the cosmic rays are galactic or universal. Diffuse gamma ray emission from the Magellanic Clouds allows us to carry out studies of the cosmic ray distribution in these external galaxies and in turn permit us to answer the question on the origin of cosmic rays in our own galaxy. These gama rays are believed to be produced primarily through the interaction of cosmic rays with interstellar matter. Hence, combined with a knowledge of the interstellar matter distribution, they can provide a direct measure of the cosmic ray density. Results of a model calculation assuming quasi‐stable equilibrium between cosmic rays, magnetic fields, and interstellar gas, are presented and compared with recent data from the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Observatory. On the important question of whether the origin of the bulk of cosmic rays is galactic or extragalactic, a definitive test of the universality of cosmic rays can be accomplished through the measurement of the high energy gamma ray emission from the SMC and the LMC. The results obtained from the EGRET on the Magellanic Clouds clearly resolves this debate in favor of a galactic origin.

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.