▪ Abstract A large collective effort to study the variability of active galactic nuclei (AGN) over the past decade has led to a number of fundamental results on radio-quiet AGN and blazars. In radio-quiet AGN, the ultraviolet (UV) bump in low-luminosity objects is thermal emission from a dense medium, very probably an accretion disk, irradiated by the variable X-ray source. The validity of this model for high-luminosity radio-quiet AGN is unclear because the relevant UV and X-ray observations are lacking. The broad-line gas kinematics appears to be dominated by virialized motions in the gravity field of a black hole, whose mass can be derived from the observed motions. The “accretion disk plus wind” model explains most of the variability (and other) data and appears to be the most appropriate model at present. Future investigations are outlined. In blazars, rapid variability at the highest energies (gamma-rays) implies that the whole continuum is relativistically boosted along the line of sight. The gener...

/pdf/variability-of-active-galactic-nuclei-5591oq6kth.pdf

Variability of active galactic nuclei

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 Diffuse Gamma-Ray Emission from the Galactic Plane

The cosmic-ray energy spectrum above ${10}^{18.5}\mathrm{eV}$ is reported using the updated data set of the Akeno Giant Air Shower Array from February 1990 to October 1997. The energy spectrum extends beyond ${10}^{20}\mathrm{eV}$ and the energy gap between the highest energy event and the others is being filled up with recently observed events. The spectral shape suggests the absence of the 2.7 K cutoff in the energy spectrum or a possible presence of a new component beyond the 2.7 K cutoff.

Extension of the Cosmic-Ray Energy Spectrum Beyond the Predicted Greisen-Zatsepin-Kuz'min Cutoff

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.

EGRET Observations of the Extragalactic Gamma Ray Emission

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

The Second 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.

/pdf/the-third-egret-catalog-of-high-energy-gamma-ray-sources-10vsl78sbe.pdf

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

Supplement to the Second EGRET Catalog of High-Energy Gamma-Ray Sources

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

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

Supplement to the Second EGRET Catalog of High-Energy Gamma-Ray Sources