D. J. Thompson

The Third EGRET Catalog of High-Energy Gamma-Ray Sources

The third catalog of high-energy gamma-ray sources detected by the EGRET telescope on the Compton Gamma Ray Observatory includes data from 1991 April 22 to 1995 October 3 (cycles 1, 2, 3, and 4 of the mission). In addition to including more data than the second EGRET catalog and its supplement, this catalog uses completely reprocessed data (to correct a number of mostly minimal errors and problems). The 271 sources (E > 100 MeV) in the catalog include the single 1991 solar flare bright enough to be detected as a source, the Large Magellanic Cloud, five pulsars, one probable radio galaxy detection (Cen A), and 66 high-confidence identifications of blazars (BL Lac objects, flat-spectrum radio quasars, or unidentified flat-spectrum radio sources). In addition, 27 lower confidence potential blazar identifications are noted. Finally, the catalog contains 170 sources not yet identified firmly with known objects, although potential identifications have been suggested for a number of those. A figure is presented that gives approximate upper limits for gamma-ray sources at any point in the sky, as well as information about sources listed in the second catalog and its supplement, that do not appear in this catalog.

EGRET Observations of the Diffuse Gamma-Ray Emission from the Galactic Plane

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.

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.

EGRET Observations of High-Energy Gamma-Ray Emission from Blazars: An Update

The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO) has so far detected 51 blazars during phases 1, 2, and 3, and cycle 4 of the CGRO mission. We present here a summary of these observations, including flux variations and spectra of the blazars. The high luminosities and time variations seen in the gamma-ray data indicate that gamma rays are an important component of the relativistic jet thought to characterize blazars.

Multiwavelength Observations of a Dramatic High-Energy Flare in the Blazar 3C 279

The blazar 3C 279, one of the brightest identified extragalactic objects in the γ-ray sky, underwent a large (factor of ~10 in amplitude) flare in γ-rays toward the end of a 3 week pointing by Compton Gamma Ray Observatory (CGRO), in 1996 January-February. The flare peak represents the highest γ-ray intensity ever recorded for this object. During the high state, extremely rapid γ-ray variability was seen, including an increase of a factor of 2.6 in ~8 hr, which strengthens the case for relativistic beaming. Coordinated multifrequency observations were carried out with Rossi X-Ray Timing Explorer (RXTE), Advanced Satellite for Cosmology and Astrophysics (ASCA; or, Astro-D), Roentgen Satellite (ROSAT), and International Ultraviolet Explorer (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 γ-ray flare without any lag larger than the temporal resolution of ~1 day. 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 a historical maximum during the γ-ray flare peak, and there is a suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The γ-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 (SSC) model would require that the largest synchrotron variability occurred in the mostly unobserved submillimeter/far-infrared region. Alternatively, a large variation in the external photon field could occur over a timescale of a 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 external photons that are Comptonized to γ-ray energies.

Multiepoch multiwavelength spectra and models for blazar 3C 279

Of the blazars detected by EGRET in GeV γ-rays, 3C 279 is not only the best observed by EGRET but also one of the best monitored at lower frequencies. We have assembled 11 spectra, from GHz radio through GeV γ-rays, from the time intervals of EGRET observations. Although some of the data have appeared in previous publications, most are new, including data taken during the high states in early 1999 and early 2000. All of the spectra show substantial γ-ray contribution to the total luminosity of the object; in a high state, the γ-ray luminosity dominates over that at all other frequencies by a factor of more than 10. There is no clear pattern of time correlation; different bands do not always rise and fall together, even in the optical, X-ray, and γ-ray bands. The spectra are modeled using a leptonic jet, with combined synchrotron self-Compton plus external Compton γ-ray production. Spectral variability of 3C 279 is consistent with variations of the bulk Lorentz factor of the jet, accompanied by changes in the spectral shape of the electron distribution. Our modeling results are consistent with the UV spectrum of 3C 279 being dominated by accretion disk radiation during times of low γ-ray intensity.

Gamma Radiation from PSR B1055–52

The telescopes on the Compton Gamma Ray Observatory (CGRO) have observed PSR B1055-52 a number of times between 1991 and 1998. From these data a more detailed picture of the gamma radiation from this source has been developed, showing several characteristics that distinguish this pulsar: the light curve is complex; there is no detectable unpulsed emission; the energy spectrum is flat, with no evidence of a sharp high-energy cutoff up to greater than 4 GeV. Comparisons of the gamma-ray data with observations at longer wavelengths show that no two of the known gamma-ray pulsars have quite the same characteristics; this diversity makes interpretation in terms of theoretical models difficult.

Phase-resolved Studies of the High-Energy Gamma-Ray Emission from the Crab, Geminga, and Vela Pulsars

Using the first 3.5 years of observations from the Energetic Gamma Ray Telescope (EGRET) on board the Compton Gamma Ray Observatory, phase-resolved analyses are performed on the emission from the three brightest high-energy γ-ray pulsars: Crab, Geminga, and Vela. For each pulsar, it is found that there is detectable high-energy γ-ray emission above the galactic diffuse background throughout much of the pulsar rotation cycle. A hardness ratio is introduced to characterize the evolution of the spectral index as a function of pulsar phase. While the hardest emission from the Crab and Vela pulsars comes from the bridge region between the two γ-ray peaks, the hardest emission from Geminga corresponds to the second γ-ray peak. For all three pulsars, phase-resolved spectra of the pulse profile components reveal that although there is a large variation in the spectral index over the pulsar phase interval, the high-energy spectral turnover, if any, occurs at roughly the same energy in each component. The high-energy γ-ray emission from the Crab complex appears to include an unpulsed ultrasoft component of spectral index ~-4.3, which dominates the total emission below 100 MeV. This component is consistent with the expected emission from the tail end of the Crab Nebula synchrotron emission.

EGRET Observations of the Gamma‐Ray Source 2CG 135+01

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.

Variability of CGRO/EGRET Gamma-Ray Sources

We have developed a method for quantifying the flux variability of EGRET high-energy gamma-ray sources. We apply this method to all sources in the Second EGRET Catalog except for the one solar flare. Allowing for a small systematic uncertainty, the phase-averaged flux densities of the pulsars are consistent with being nonvariable. Many identified active galactic nuclei are variable, as expected, and it is likely that the apparent nonvariability of some identified active galactic nuclei results from decreased sensitivity to variability at low fluxes and low latitudes. Populations of both variable and nonvariable unidentified sources are found to be in excess at low Galactic latitudes. While low-flux, nonvariable, unidentified sources could result from errors in the Galactic diffuse model, some higher flux, nonvariable, unidentified sources are likely to be Galactic pulsars. The excess of variable, unidentified sources at low latitudes suggests that either pulsars can produce variable gamma rays under special circumstances, or that a new class of Galactic gamma-ray sources exists.