A SURVEY OF ULTRAVIOLET VARIABILITY IN BLAZARS

This is the first comprehensive, consistent survey of the ultraviolet variability properties of a large sample of blazars. All of the 14 blazars with five or more observations in a single IUE camera showed significant variability. The strength of the ultraviolet variability is positively correlated with both the apparent ultraviolet luminosity and the degree of optical polarization of the source. This is the first clear indication that the ultraviolet continua of blazars are beamed. A search for rapid variability yielded nine "events" with doubling time scales of 0.5-7 days. These rapid flux variations and correlations argue for the standard synchrotron self-Compton model, and against the accretion disk model, which would predict slower variations and the opposite sense of correlation. Other analyses were less decisive. Short wavelength variability was stronger than that at long wavelengths. Of the two blazars with at least 35 observations in each band, one, Mkn 421, shows a correlation between flux and spectral index, while the other, PKS 2155-304, does not. Both sources show correlations between changes in flux density and spectral index, but the physical meaning, if any, is unclear. These data are too sparsely and unevenly sampled for more ambitious time-series analyses to yield meaningful results.