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\title{MULTIWAVELENGTH MONITORING OF THE BL LACERTAE OBJECT
PKS~2155--304 IN MAY 1994. III. PROBING THE INNER JET THROUGH
MULTIWAVELENGTH CORRELATIONS}

\author{C.\ Megan Urry,\thanks{Space Telescope Science Institute,
3700 San Martin Drive, Baltimore, Maryland, 21218; electronic
mail: cmu@stsci.edu}
A.\ Treves,\thanks{SISSA/ISAS International School for Advanced Studies,
Trieste, Italy}\morethanks{Department of Physics, University of
Milan at Como, Via Lucini, I-22100 Como, Italy}
L.\ Maraschi,\thanks{Osservatorio Astronomico di Brera, via Brera 28,
I-20121 Milan, Italy}\morethanks{Department of Physics,
University of Milan, via Celoria 16, I-20133 Milan, Italy}
H.\ Marshall,\thanks{Eureka Scientific, Inc., 2452 Delmer St., Suite
100, Oakland, CA 94602}
T.\ Kii,\thanks{Institute for Space and Astronautical Science, 3-1-1
Yoshinodai, Sagamihara, Kanagawa 229, Japan}
G.\ Madejski,\thanks{Laboratory for High Energy Astrophysics, Code 666,
Goddard Space Flight Center, Greenbelt MD 20771}\\
\and
S.\ Penton,\thanks{Joint Institute for Laboratory Astrophysics,
University of Colorado, Campus Box 440, Boulder CO 80309-0440}
J.\ E.\ Pesce,\samethanks{1}
E. Pian,\samethanks{1}
A.\ Celotti,\samethanks{2}
R. Fujimoto,\samethanks{7}
F. Makino,\samethanks{7}
C. Otani,\samethanks{7}\\
\and
R.\ M.\ Sambruna,\samethanks{8}
K.\ Sasaki,\samethanks{7}
J.\ M.\ Shull,\samethanks{9}
P.\ Smith,\thanks{Steward Observatory, University of Arizona,
Tucson AZ 85721}
T.\ Takahashi,\samethanks{7}
M.\ Tashiro\/\thanks{Department of Physics, School of Science,
University of Tokyo, Bunkyo-ku, Tokyo 113, Japan}}

\pub{The Astrophysical Journal}
\recacc{9 July 1996}{2 April 1997}
\maketitle

\abstract{In May 1994 the BL~Lac object PKS~2155--304 was observed
continuously for $\sim10$ days with IUE and EUVE and for 2~days with
ASCA, as well as with ROSAT and with ground-based radio, infrared, and
optical telescopes. The light curves show a well-defined X-ray flare
followed by a broader, lower amplitude extreme ultraviolet (EUV) flare
$\sim1$~day later and a broad, low-amplitude UV flare $\sim$2~days later.
X-ray fluxes obtained at three well separated times the preceding week
indicate at least one previous flare of comparable amplitude or perhaps
ongoing stochastic X-ray variations, and additional rapid variability
was seen at the beginning of the IUE observation, when extremely sharp
changes in UV flux occurred. The X-ray flux observed with ASCA flared by
a factor of $\sim2$ in $\sim1/2$~day and decayed roughly as fast. In
contrast, the subsequent UV flare had an amplitude of only $\sim35$\%
and lasted longer than 2~days.

Assuming the X-ray, EUV, and UV events are associated, the lags, the
decrease of amplitude with wavelength, and the broadening of the
temporal profile with wavelength are all qualitatively as expected for
synchrotron emission from an inhomogeneous relativistic jet.
Due to the high quality of the data, we can rule out that the
observed flares were caused by either a Fermi-type shock acceleration
event or a pair cascade in a homogeneous synchrotron-emitting region.
A homogeneous region is still possible if there was an instantaneous
($t\ll$ hours) injection of high energy electrons that emit first at
X-ray energies. Alternatively, the data are consistent with a
compression wave or other disturbance crossing a region with stratified
particle energy distributions. This kind of situation is expected to
occur behind a shock front and/or in an inhomogeneous jet. The present
light curves are in sharp contrast to the multiwavelength variability
observed in November 1991, when the amplitude was wavelength independent
and the UV lagged the X-rays by less than $\sim3$~hours. This means that
the origin of rapid multiwavelength variability in this blazar is
complex, involving at least two different modes.}