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\title{MULTIWAVELENGTH MONITORING OF THE BL LACERTAE OBJECT\\
~\\
PKS~2155--304 IN MAY 1994. II.~THE IUE CAMPAIGN}

\author{Elena Pian,\thanks{Space Telescope Science Institute, 3700 San Martin 
Drive, Baltimore, MD 21218.}\morethanks{Guest Observer with the International 
Ultraviolet Explorer.}
C.\ Megan Urry,\samethanks{1}\moresamethanks{2}
Aldo Treves,\thanks{Dpt.\ of Physics, University of Milan, via Celoria 16, 
I-20133 Milan, Italy.}\morethanks{Scuola Internazionale Superiore di Studi 
Avanzati, via Beirut 2-4, I-34014 Trieste, Italy.}
Laura Maraschi,\thanks{Osservatorio Astronomico di Brera, via Brera 28, I-20121 
Milano, Italy.}\moresamethanks{2}\\
\and
Steve Penton,\thanks{University of Colorado, JILA, Campus Box 440, 
Boulder, CO 80309-0440.}
J.~Michael Shull,\samethanks{6}
Joseph E.\ Pesce,\samethanks{1}\moresamethanks{2} 
Paola Grandi,\thanks{IAS/CNR, via Enrico Fermi 23, CP67, I-00044 
Frascati, Italy.}\\
\and
Tsuneo Kii,\thanks{Institute for Space and Astronautical Science, 3-1-1 
Yoshinodai, Sagamihara, Kanagawa 229, Japan.}
Ron I.\ Kollgaard,\thanks{Department of Astronomy and Astrophysics, The 
Pennsylvania State University, University Park, PA 16802.}
Greg Madejski,\thanks{Laboratory for High Energy Astrophysics, Goddard 
Space Flight Center, Greenbelt, MD 20771.}
Herman Marshall,\thanks{Eureka Scientific, Inc., 2452 Delmer St., Suite 
100, Oakland, CA 94602}\\
\and
Willem Wamsteker,\thanks{ESA IUE Observatory, P.O.\ Box 50727, 28080 
Madrid, Spain.}
Annalisa Celotti,\samethanks{4}\morethanks{Institute of Astronomy, Madingley 
Road, Cambridge, CB3 0HA, United Kingdom.}
Thierry J.-L. Courvoisier,\thanks{INTEGRAL Science Data Centre, 16 
Chemin d'Ecogia, CH-1290 Versoix, Switzerland.}\\
\and
Renato Falomo,\thanks{Osservatorio Astronomico di Padova, via 
dell'Osservatorio~5, I-35122 Padova, Italy.}
Henner H.\ Fink,\thanks{Max Planck-Institute f\"ur Extraterrestrische Physik,
Giessenbachstrasse, 85740 Garching bei M\"unchen, Germany.}
Ian M.\ George,\samethanks{10}
Gabriele Ghisellini\/\thanks{Osservatorio Astronomico di Brera, via Bianchi~46,
I-22055 Merate, Italy.}}

\pub{The Astrophysical Journal}
\recacc{8 July 1996}{2 August 1996}

\maketitle

\abstract{
PKS~2155--304, the brightest BL~Lac object in the ultraviolet sky, was monitored
with the IUE satellite at $\sim1$~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 $\sim2.2$ in an hour and a half. In subsequent days, the 
flux maintained a nearly constant level for $\sim5$ days, then flared with 
$\sim35$\% amplitude for two days. The same variability was seen in both
short- and long-wavelength IUE light curves, with zero formal lag 
($\simlt2$~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. }