  PKS 2155-304, the brightest BL Lac object in the ultraviolet sky, was
monitored with the IUE satellite at ~1 hour time-resolution for ten nearly
uninterrupted days in May 1994. The campaign, which was coordinated with EUVE,
ROSAT, and ASCA monitoring, along with optical and radio observations from the
ground, yielded the largest set of spectra and the richest short time scale
variability information ever gathered for a blazar at UV wavelengths. The
source flared dramatically during the first day, with an increase by a factor
~2.2 in an hour and a half. In subsequent days, the flux maintained a nearly
constant level for ~5 days, then flared with ~35% amplitude for two days. The
same variability was seen in both short- and long-wavelength IUE light curves,
with zero formal lag (~<2 hr), except during the rapid initial flare, when the
variations were not resolved. Spectral index variations were small and not
clearly correlated with flux. The flux variability observed in the present
monitoring is so rapid that for the first time, based on the UV emission alone,
the traditional Delta L/Delta t limit indicating relativistic beaming is
exceeded. The most rapid variations, under the likely assumption of synchrotron
radiation, lead to a lower limit of 1 G on the magnetic field strength in the
UV emitting region. These results are compared with earlier intensive
monitoring of PKS 2155-304 with IUE in November 1991, when the UV flux
variations had completely different characteristics.
