Astron. Astrophys. 325, 27-32
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Optical and UV observations
of the BL Lacertae object
J. Heidt1, S.J. Wagner1, and U. Wilhelm-Erkens1
Königstuhl, D-69117 Heidelberg, Germany
Received 20 January 1997 / Accepted 10 March 1997
The BL Lac object PKS 2155-304 has been monitored in two
subsequent observing campaigns within three weeks in June 1990 and
during another run in September 1994 at optical wavelengths.
During the first part PKS 2155-304 showed night-to-night variations of a
few percent, whereas in the second part no variability could be detected.
During the third run PKS 2155-304 showed variations similar to those in the
first campaign. We did not detect any variations on time-scales shorter
than one day.
The object was in a bright state during the observations
in June 1990 (mV ~ 12.7) and close to its historical
in September 1994 (mV ~ 12.3). The observations
imply that even objects with a high duty cycle and a well known history of
rapid variability may be quiescent over longer periods.
The power density spectra are not constant at high frequencies
(time-scales shorter than a few days) and are not
related to the average brightness level at a particular epoch.
Comparison with IUE spectra in June 1990 show that PKS 2155-304
was also very bright in the UV. During that period we found an indication
that the optical-UV spectral index steepens, when the source became brighter,
whereas usually the opposite is observed.
Key words: BL Lacertae objects: general -
BL Lacertae objects: individual PKS 2155-304
*Based on observations
collected at the European Southern Observatory, La Silla, Chile
during program 53.B-0036.
Send offprint requests to: J. Heidt, (email@example.com)
© European Southern Observatory (ESO) 1997