Thomas D. Willis

EGRET observations of the extragalactic gamma-ray emission

The all-sky survey in high-energy gamma rays (E > 30 MeV) carried out by 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 as 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 are 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.

In-Flight Calibration of EGRET on the Compton Gamma-Ray Observatory

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.

EGRET Observations of Gamma Rays from Point Sources with Galactic Latitude +10 degrees < B < +40 degrees

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

Comparison of X-Ray- and Radio-selected BL Lacertae Objects in High-Energy Gamma-Ray Observations

+10\arcdeg < b < +40\arcdeg

EGRET observations of active galactic nuclei

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.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

The unidentified gamma ray source GRO J1837+59

+10\arcdeg<b<+40\arcdeg

Variability of EGRET‐detected active galaxies and acceleration mechanisms

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.

The luminosity function of EGRET‐detected gamma‐ray loud active galactic nuclei

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.

Timing the geminga pulsar with high-energy gamma rays

The full‐sky survey completed by EGRET during Phase 1 of the CGRO Mission resulted in 21 strong detections of active galactic nuclei (AGNs); 17 additional AGNs were marginally detected. All of these objects are radio bright (≳≊1 Jy near 5 GHz), and have flat radio spectra between 2 and 5 GHz. Many have been previously classified as having some combination of ‘‘superluminal’’ VLBI activity, large and rapid time variations, high polarization, etc. Seven strong and 30 marginal sources were detected for which no convincing identification has been found. Several AGNs were detected during the early portions of Phase 2 which had not been detectable during Phase 1.A number of the sources were sufficiently bright to provide quite accurate energy spectra; all are found to be compatible with power laws over most of the EGRET energy range, although several are also compatible with a knee or roll‐off in the 2–10 GeV range. When compared with fluxes measured in lower frequency bands, the detected energy budget appears ...

The Likelihood Analysis of EGRET Data

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.