MULTIWAVELENGTH MONITORING OF THE BL LACERTAE OBJECT PKS 2155-304 .1.THE IUE CAMPAIGN

Daily monitoring of PKS 2155-304 with the IUE satellite throughout 199 1 November has revealed dramatic, large-amplitude, rapid variations in the ultraviolet flux of this BL Lac object. Many smaller, rapid flare s are superposed on a general doubling of the intensity. During the 5d period when sampling was roughly continuous, the rapid flaring had an apparent quasi-periodic nature, with peaks repeating every approximat ely 0.d7. The short- and long-wavelength ultraviolet light curves are well correlated with each other, and with the optical light curve dedu ced from the Fine Error Sensor (FES) on IUE. The formal lag is zero, b ut the cross-correlation is asymmetric in the sense that the shorter w avelength emission leads the longer. The ultraviolet spectral shape va ries a small but significant amount. The correlation between spectral shape and intensity is complicated; an increase in intensity is associ ated with spectral hardening, but lags behind the spectral change by a pproximately 1 day. The sign of the correlation is consistent with the nonthermal acceleration processes expected in relativistic plasmas, s o that the present results are consistent with relativistic jet models , which can also account for quasi-periodic flaring. In contrast, curr ently proposed accretion disk models are strongly ruled out by the sim ultaneous optical and ultraviolet variability.