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We have begun to examine the EGRET database for short term variations in the fluxes of blazars and unidentified sources at high Galactic latitude. We find that several AGN show previously unreported variability. Such variations are consistent with inverse Compton scattering processes in a shock propagating through a relativistic jet., Comment: 5 pages tex; 4 figures; to appear in Proceedings of the 4th Compton Symposium

We report preliminary results of a multi-frequency campaign on the TeV candidate blazar W Comae (z=0.102). Flux limits by Whipple and HEGRA show that the TeV flux must be considerably below an E^-2 extrapolation of the EGRET flux. In the framework of proton-initiated cascade models, this implies a moderate amount of gamma-ray attenuation due to pair production in collisions with low-energy photons of the extragalactic infrared background. In a simple SSC model, the gamma-ray spectrum cuts off intrinsically just below TeV without requiring external absorption., Comment: 6 pages, Latex using crckapb.sty, to appear in the proceedings of the conference on Blazars, Black Holes, and Jets, Girona, Spain, 9-12 September 1996, eds. M. Kidger and J.A. de Diego, Astrophysics and Space Science Series

We present the results of the multi-wavelength campaigns on 3C 273 in 1993-1995. During the observations in late 1993 this quasar showed an increase of its flux for energies greater equal 100 MeV from about 2.1 x 10^-7 photons/cm^2/s to approximately 5.6 x 10^-7 photons/cm^2/s during a radio outburst at 14.5, 22 and 37 GHz. However, no one-to-one correlation of the gamma-ray radiation with any frequency could be found. The photon spectral index of the high energy spectrum changed from (3.20 +- 0.54) to (2.20 +- 0.22) in the sense that the spectrum flattened when the gamma-ray flux increased. Fits of the three most prominent models (synchrotron self-Comptonization, external inverse Comptonization and the proton initiated cascade model) for the explanation of the high gamma-ray emission of active galactic nuclei were performed to the multi-wavelength spectrum of 3C 273. All three models are able to represent the basic features of the multi-wavelength spectrum. Although there are some differences the data are still not decisive enough to discriminate between the models., Comment: 63 pages including 9 eps figures and 9 tables, LateX, uses aaspp4.sty and tighten.sty, accepted for publication in ApJ

The possibility of a correlation between the radio (cm)- and $\gamma$-ray luminosity of variable AGN seen by EGRET is investigated. We performed Monte-Carlo simulations of typical data sets and applied different correlation techniques (partial correlation analysis, $\chi^2$-test applied on flux-flux relations) in view of a truncation bias caused by sensitivity limits of the surveys. For K-corrected flux densities, we find that with the least squares method only a linear correlation can be recovered. Partial correlation analysis on the other side provides a robust tool to detect correlations even in flux-limited samples if intrinsic scatter does not exceed $\sim 40$ \% of the original $\gamma$-ray luminosity. The analysis presented in this paper takes into account redshift bias and truncation effects simultaneously which was never considered in earlier papers. Applying this analysis to simultaneously observed radio- and $\gamma$-ray data, no correlation is found. However, an artificial correlation appears when using the mean flux. This is probably due to the reduction of the dynamical range in the flux-flux relation. Furthermore, we show that comparing the emission in both spectral bands at a high activity state leads to no convincing correlation. In conclusion, we can not confirm a correlation between radio and $\gamma$-ray luminosities of AGN which is claimed in previous works., Comment: 8 pages, 4 figures, uuencoded + gzipped, latex. Submitted to A&A

Simultaneous observations of 3C 279 at radio, millimeter, near-infrared, optical, ultraviolet (with IUE) and X-ray (with ROSAT) wavelengths were obtained in 1992 December-1993 January, during a three week pointing at the source by the Compton Gamma Ray Observatory. The blazar was in a quiescent or 'low' state during this period. Comparing the multiwavelength energy distribution to that from 1991 June, when 3C 279 was in its brightest recorded gamma-ray state, we find the following: (1) 3C 279 faded dramatically at all frequencies above 10(exp 14) Hz, while the flux variations at low frequencies (radio to millimeter wavelengths) were minor. (2) The near-infrared-optical-ultraviolet spectral shape was softer (steeper) in the quiescent state, and the X-ray spectra also appear softer, although the spectral indix measured by ROSAT refer to a lower energy band than that measured earlier with Ginga. (3) The ratio of the gamma-ray luminosity to that across all other frequencies decreased from a value of approximately equal to 10 in the flaring state to a value approximately equal to 1 in the quiescent state. These findings imply that the production of gamma-rays is closely related to the optical-ultraviolet continuum, in agreement with models where gamma-rays are produced through inverse Compton (IC) scattering by relativistic electrons emitting the synchrotron continuum. The observed nonlinear relation between the synchrotron and IC requires both a change in the electron spectrum and an associated change in the seed photons.

The quasar 3C 273 was detected by COS-B in the 1970's. EGRET observations of this sky region in June and October 1991 revealed a flux from 3C 273 lower than that measured by COS-B. The flux observed by EGRET in the June period is approximately 3 x 10 exp -7/sq cm s for energies greater than 100 MeV. During the October observation it appears to be even lower. For the first observation a preliminary spectrum which has a photon index of 2.4 has been derived.