We present the results of a program to monitor the structure of the radio emission in 42 γ-ray bright blazars (31 quasars and 11 BL Lac objects) with the VLBA at 43, 22, and occasionally 15 and 8.4 GHz, over the period from November 1993 to July 1997. We determine proper motions in 33 sources and find that the apparent superluminal motions in γ-ray sources are much faster than for the general population of bright compact radio sources. This follows the strong dependence of the γ-ray flux on the level of relativistic beaming for both external-radiation Compton and synchrotron self-Compton emission. There is a positive correlation (correlation coefficient r=0.45) between the flux density of the VLBI core and the γ-ray flux and a moderate correlation (partial correlation coefficient r=0.31) between γ-ray apparent luminosity and superluminal velocities of jet components, as expected if the γ-ray emission originates in a very compact region of the relativistic jet and is highly beamed. In 43% of the sources the jet bends by more than 20 • on parsec scales, which is consistent with amplification by projection effects of modest actual changes in position angle.In 27 of the sources in the sample there is at least one non-core component that appears to be stationary during our observations. Different characteristics of stationary features close to and farther from the core lead us to suggest two different classes of stationary components: those within about 2 milliarcseconds (mas) of the core, probably associated with standing hydrodynamical compressions, and those farther down the jet, which tend to be associated with bends in the jet.

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 Ñare bright enough to be detected as a source, the Large Magellanic Cloud, Ðve pulsars, one probable radio galaxy detection (Cen A), and 66 high-conÐdence identiÐcations of blazars (BL Lac objects, Ñat-spectrum radio quasars, or unidentiÐed Ñat-spectrum radio sources). In addition, 27 lower conÐdence potential blazar identiÐcations are noted. Finally, the catalog contains 170 sources not yet identiÐed Ðrmly with known objects, although potential identiÐcations have been suggested for a number of those. A Ðgure 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.

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We examine the coincidence of times of high γ-ray flux and ejections of superluminal components from the core in EGRET blazars based on a VLBA monitoring program at 22 and 43 GHz from November 1993 to July 1997. In 23 cases of γ-ray flares for which sufficient VLBA data exist, 10 of the flares (in 8 objects) fall within 1σ uncertainties of the extrapolated epoch of zero separation from the core of a superluminal radio component. In each of two sources (0528+134 and 1730−130) two successive γ-ray flares were followed by the appearance of new superluminal components. We carried out statistical simulations which show that if the number of coincidences ≥ 10 the radio and γ-ray events are associated -2with each other at >99.999% confidence. Our analysis of the observed behavior, including variability of the polarized radio flux, of the sources before, during, and after the γ-ray flares suggests that the γ-ray events occur in the superluminal radio knots. This implies that the γ-ray flares are caused by inverse Compton scattering by relativistic electrons in the parsec-scale regions of the jet rather than closer to the central engine.

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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 eleven 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 + 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.Subject headings: quasars: individual (3C 279)

paper we present UBVRI light curves obtained by the WEBT from 1994 to 2002, including the last, extended BL Lac 2001 campaign. A total of about 7500 optical observations performed by 31 telescopes from Japan to Mexico have been collected, to be added to the ∼15 600 observations of the BL Lac Campaign 2000. All these data allow one to follow the source optical emission behaviour with unprecedented detail. The analysis of the colour indices reveals that the flux variability can be interpreted in terms of two components: longer-term variations occurring on a fewday time scale appear as mildly-chromatic events, while a strong bluer-when-brighter chromatism characterizes very fast (intraday) flares. By decoupling the two components, we quantify the degree of chromatism inferring that longer-term flux changes imply moving along a ∼0.1 bluerwhen-brighter slope in the B − R versus R plane; a steeper slope of ∼0.4 would distinguish the shorter-term variations. This means that, when considering the long-term trend, the B-band flux level is related to the R-band one according to a power law of index ∼1.1. Doppler factor variations on a "convex" spectrum could be the mechanism accounting for both the long-term variations and their slight chromatism.

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