H. Rothermel

Calibration of the Energetic Gamma-Ray Experiment Telescope (EGRET) for the Compton Gamma-Ray Observatory

Calibration of the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory involves simulation, experimental calibration, and verification in flight. The principal properties of the instrument which have been determined as a function of energy and angle are the effective area, the angular resolution (point spread function), and the energy resolution (dispersion)

Detection of high-energy gamma radiation from quasar 3C 279 by the EGRET telescope on the Compton Gamma Ray Observatory

Intense gamma radiation has been observed from the direction of the quasar 3C 279 throughout the energy range from 30 MeV to over 5 GeV by the Energetic Gamma Ray Experiment Telescope (EGRET) during the period June 15-28, 1991. Its spectrum is well represented by a photon differential power-law exponent of 2.0 +/- 0.1, with a photon intensity above 100 MeV of (2.8 +/- 0.4) x 10 exp -6/sq cm s. For E is greater than 100 MeV, the 2-sigma upper limits were 1.0 x 10 exp -6/sq cm s in 1973 from the SAS 2 observations and 0.3 x 10 exp -6/sq cm s for the combined 1976, 1978, and 1980 COS B observations. Hence, there has been a large increase in high-energy gamma-ray intensity relative to the earlier times, as there has been in the radio, infrared, optical, and X-ray ranges. This source is the most distant and by far the most luminous gamma-ray source yet detected.

Observations of the Crab pulsar and nebula by the EGRET telescope on the Compton Gamma-Ray Observatory

The Crab pulsar and nebula were observed three times in 1991 April to June by the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory (CGRO): April 23 to May 7, May 16 to 30, and June 8 to 15. The results of analysis of the gamma-ray emission in the energy range from 50 MeV to more than 10 GeV are reported. The observed gamma-ray light curve exhibits two peaks separated in phase by 0.40 +/- 0.02, consistent with previous observations. The total pulsed emission from the Crab pulsar is found to be well represented by a power-law spectrum, softer than the spectrum measured by COS B (Clear et al., 1987). The interpulse emission has a harder spectrum than either of the pulses. The evidence for pulsed emission above 5 GeV in the EGRET data is not conclusive. Unpulsed emission in the energy range 50 MeV to 5 GeV was detected, with an indication of a hardening of the unpulsed spectrum above about 1 GeV. There was a significant change in the light curve over the 2 months of these observations, although the shape of the spectrum remained constant.

The project EGRET (energetic gamma-ray experiment telescope) on NASA's Gamma-Ray Observatory GRO

The Gamma Ray Observatory (GRO) is currently planned for a launch from the space shuttle in 1990. After the long hiatus in high-energy gamma-ray astronomy since the end of the COS-B mission in 1982, the Soviet missions Granat and Gamma-1 and the NASA mission GRO will resume observations in the energy range from below 100 keV and extending to above 10 GeV. GRO will carry four instruments designed to cover this range of over five decades in photon energy. It is planned to perform a complete sky survey above 1 MeV in the first year of the GRO mission. Data from this survey will be used to study galactic and extragalactic sources of gamma radiation as well as the galactic and extragalactic diffuse emissions. Additionally, measurements of gamma ray bursts will be performed. The angular and spectral resolution of the GRO instruments is significantly improved with respect to previous experiments. The sensitivity for point sources will be better by an order of magnitude, and the location of strong, high energy sources will be determined to about 0.1°–0.2°. After a brief description of the complement of GRO instruments, a detailed discussion of the high-energy telescope EGRET, its design and scientific objectives, is presented in this review.

Performance of the EGRET astronomical gamma ray telescope

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.<<ETX>>

Search for periodic gamma-ray emission from Cygnus X-3 by the EGRET telescope on the Compton Gamma-Ray Observatory

The Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory observed the Cygnus region of the Galaxy during the periods 1991 May 30-June 8 and 1991 August 8-15. We report on a periodicity analysis of the gamma rays originating from the direction of Cygnus X-3. This analysis showed no evidence of periodic modulation during these observations of the gamma-ray emission at the approximately 4.8 hr period observed at X-ray wavelengths.

Calibration of the Egret High-Energy Gamma-Ray Telescope in the Range 20-10,000 MeV with a Tunable Beam of Quasi-Monoenergetic Gamma Rays at SLAC

The EGRET telescope, one of the instruments to be carried on NASAs Gamma Ray Observatory, is designed to explore the high-energy portion (20 - 24,000 MeV) of the electromagnetic spectrum. A calibration source of ¿-rays over this wide energy range has been used at SLAC, together with an appropriate set of intensity-monitoring instruments, to conduct a calibration of the EGRET telescope over this energy range for the full telescope aperture. The calibration method and an overview of the results are described.

Characteristics of the telescope for high energy γ-ray astronomy selected for definition studies on the gamma ray observatory

The high energy γ-ray telescope selected for definition studies on the Gamma Ray Observatory provides a substantial improvement in observational capability over earlier instruments. It will have about 20 times more sensitivity, cover a much broader energy range, have considerably better energy resolution and provide a significantly improved angular resolution. The design and performance are described.

Results from the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Observatory

Abstract The Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory 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 having been detected at a level consistent with it having a cosmic ray density compatible with quasi-stable equilibrium. Sixteen Active Galactic Nuclei (AGNs) 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 AGNs.

High energy gamma‐ray emission from active galactic nuclei observed by the energetic gamma‐ray experiment telescope (EGRET)

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.