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\title{\bf MULTIWAVELENGTH MONITORING OF THE BL~LACERTAE\\
OBJECT PKS 2155--304 IN MAY 1994. \\
I.  THE GROUND-BASED CAMPAIGN}

\author{Joseph E. Pesce\altaffilmark{1}, 
C. Megan Urry\altaffilmark{1},
Laura Maraschi\altaffilmark{2},  
Aldo Treves\altaffilmark{3}, \\
Paola Grandi\altaffilmark{4}, 
Ronald I. Kollgaard\altaffilmark{5}$^{,}$\altaffilmark{6}, 
Elena Pian\altaffilmark{1},
Paul S. Smith\altaffilmark{7}$^{,}$\altaffilmark{8},  \\
Hugh D. Aller\altaffilmark{9},
Margo F. Aller\altaffilmark{9}, 
Aaron J. Barth\altaffilmark{10},  
David A. H. Buckley\altaffilmark{11},  \\
Elvira Covino\altaffilmark{12}, 
Alexei V. Filippenko\altaffilmark{10}, 
Eric J. Hooper\altaffilmark{7}, 
Michael D. Joner\altaffilmark{13}$^{,}$\altaffilmark{14}, \\
Lucyna Kedziora-Chudczer\altaffilmark{15},
David Kilkenny\altaffilmark{11},  
Lewis B. G. Knee\altaffilmark{16}$^{,}$\altaffilmark{17}, \\
Michael Kunkel\altaffilmark{18},  
Andrew C. Layden\altaffilmark{19}$^{,}$\altaffilmark{20}, 
Antonio M\'ario Magalh\~aes\altaffilmark{21}, \\
Fred Marang\altaffilmark{11},  
Vera E. Margoniner\altaffilmark{21}, 
Christopher Palma$^{\rm 5,}$\altaffilmark{22}, \\
Antonio Pereyra\altaffilmark{21}, 
Claudia V. Rodrigues\altaffilmark{21}$^{,}$\altaffilmark{23},
Andries Schutte\altaffilmark{24}$^{,}$\altaffilmark{25}, \\
Michael L. Sitko\altaffilmark{26}, 
Merja Tornikoski\altaffilmark{27},
Johan van der Walt\altaffilmark{28},  \\
Francois van Wyk\altaffilmark{11},
Patricia A. Whitelock\altaffilmark{11}, 
Scott J. Wolk\altaffilmark{14}$^{,}$\altaffilmark{29}
}
\altaffiltext{1}{Space Telescope Science Institute, 3700 San Martin Drive, 
  Baltimore, MD 21218.  The Space Telescope Science Institute is operated by 
  the Association of Universities for Research in Astronomy, Inc., under 
  contract with the National Aeronautics and Space Administration.}
\altaffiltext{2}{Osservatorio Astronomico di Brera, via Brera 28, I-20121 
  Milan, Italy.} 
\altaffiltext{3}{SISSA/ISAS, strada Costiera 11, I-34014 Trieste, Italy.}
\altaffiltext{4}{IAS/CNR, via Enrico Fermi 23, CP67, I-00044 Frascati, Italy.}
\altaffiltext{5}{Department of Astron. and Astrophys., Penn State Univ., 
  University Park, PA 16802.} 
\altaffiltext{6}{Present address: Fermi National Accelerator Laboratory, 
  Box 500, Batavia, IL, 60510.}
\altaffiltext{7}{Steward Observatory, University of Arizona, Tucson, AZ 85721.} 
\altaffiltext{8}{Present address:  NOAO/KPNO, P.O. Box 26732, Tucson, AZ 85726-6732.}
\altaffiltext{9}{Department of Astronomy, University of Michigan, Ann Arbor, 
  MI 48109-1090.} 
\altaffiltext{10}{Department of Astronomy, University of California, 
  Berkeley, CA 94720-3411.} 
\altaffiltext{11}{SAAO, P.O. Box 9, 7935 
  Observatory, Western Cape, South Africa.} 
\altaffiltext{12}{Osservatorio Astronomico di Capodimonte, via Moiariello 16,
  I-80131 Naples, Italy.} 
\altaffiltext{13}{Dept. of Physics and Astronomy, FB, Brigham Young 
  University, Provo, UT 84602.} 
\altaffiltext{14}{Visiting Astronomer, Cerro Tololo Inter-American 
 Observatory, La Serena, Chile. CTIO is operated by Association of 
  Universities for Research in Astronomy, Inc., under contract
  with the National Science Foundation.}
\altaffiltext{15}{Australia Telescope National Facility, P.O. Box 76, Epping
  NSW 2121, Australia.}
\altaffiltext{16}{Swedish-ESO Submillimetre Telescope, ESO, Casilla 19001, 
  Santiago 19, Chile.} 
\altaffiltext{17}{Also with the Onsala Space Observatory, S-43992 Onsala, 
  Sweden.}
\altaffiltext{18}{Max-Planck-Institut f\"ur Astronomie, K\"onigstuhl 17, 
  69117 Heidelberg, Germany.} 
\altaffiltext{19}{Cerro Tololo Inter-American Observatory,
  Casilla 603, La Serena, Chile.}
\altaffiltext{20}{Present address: McMaster University, Dept. of Physics and Astronomy, 
  Hamilton, ON L8S 4M1 Canada.} 
\altaffiltext{21}{Instituto Astronomico e Geof\'isico, Universidade de S\~ao 
  Paulo, Caixa Postal 9638, S\~ao Paulo SP 01065-970, Brazil.} 
\altaffiltext{22}{Present address:  Department of Astronomy, University of 
  Virginia, PO Box 3818, Charlottesville, VA 22903-0818.}
\altaffiltext{23}{Present address:  Instituto Nacional de Pesquisas 
 Espaciais-INPE Divis\~ao de Astrof\'\i sica-DAS, Caixa Postal 515,
 S\~ao Jos\'e dos Campos, SP 12201-970, Brazil.}
\altaffiltext{24}{Department of Physics, University of Zululand, Private 
  Bag X1001, Kwa-Dlangezwa 3886, South Africa.} 
\altaffiltext{25}{Present address:  Siemens Telecommunications, 270 Maggs 
  Street, Waltloo, Pretoria, South Africa.}
\altaffiltext{26}{Department of Physics, University of Cincinnati, Cincinnati, 
  OH 45221-0011.} 
\altaffiltext{27}{Metsahovi Radio Research Station, Metsahovintie 114, 
  FIN-02540 Finland.} 
\altaffiltext{28}{Space Research Unit, Potchefstroom University, 
 Potchefstroom 2520, South Africa.} 
\altaffiltext{29}{SUNY, Stony Brook, New York 11794-2100.}

\abstract{
Optical, near-infrared, and radio observations of the BL~Lac object 
PKS~2155--304 were obtained simultaneously with a continuous UV/EUV/X-ray 
monitoring
campaign in 1994 May.  Further optical observations were gathered throughout
most of 1994. The radio, millimeter, and near-infrared data show no strong
correlations with the higher energies.  The optical light curves exhibit
flickering of 0.2--0.3~mag on timescales of 1--2~days, superimposed on longer
timescale variations. Rapid variations of $\sim$0.01~mag min$^{-1}$, which,
if real, are the fastest seen to date for any BL~Lac object.  Small 
(0.2--0.3~mag) increases in the $V$ and $R$~bands occur simultaneously with a 
flare seen at higher energies. All optical wavebands ({\sl UBVRI\/}) track 
each other well over the period of observation with no detectable delay.  For 
most of the period the average colors remain relatively constant, although 
there is a tendency for the colors (in particular $B-V$) to vary more when the 
source fades.  In polarized light, PKS~2155--304 showed strong color 
dependence (polarization increases toward the blue, $P_U/P_I = 1.31$) and the 
highest optical polarization ($U = 14.3$\%) ever observed for this source. The 
polarization variations trace the flares seen in the ultraviolet flux.  For 
the fastest variability timescale observed, we estimate a central black hole 
mass of $\lesssim1.5 \times 10^9 (\frac{\delta}{10})~M_{\sun}$, consistent 
with UV and X-ray constraints and smaller than previously calculated for this 
object. }