Multiwavelength Monitoring of the BL Lacertae Object PKS 2155–304 in 1994 May. II. The IUE Campaign

PKS 2155-304, the brightest BL Lac object in the ultraviolet sky, was monitored with the International Ultraviolet Explorer satellite at ~1 hr time resolution for 10 nearly uninterrupted days in 1994 May. The campaign, which was coordinated with Extreme Ultraviolet Explorer, ROSAT, and ASCA monitoring, along with optical and radio observations from the ground, yielded the largest set of spectra and the richest short-timescale variability information ever gathered for a blazar at UV wavelengths. The source flared dramatically during the first day, with an increase by a factor of ~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 2 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 ΔL/Δt 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 1991 November, when the UV flux variations had completely different characteristics.