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Paper: ROSAT Observations of PKS 2155-304
Volume: 159, BL Lac Phenomenon
Page: 168
Authors: Brinkmann, W.; Siebert, J.
Abstract: The BL Lac object PKS 2155-304 has been monitored extensively in May 1996 and November 1996 during a multi-wavelength campaign with the ROSAT HRI. The monitoring was very 'dense', mostly a pointing for every satellite orbit. Thus, we could study the intensity variations of the BL Lac on all time scales up to days. The light-curves can be interpreted as the superposition of individual, single 'shots' occurring on time scales of less than one day and long term smooth intensity variations. The short-term variability with time scales of several minutes cannot be uniquely attributed to source intrinsic intensity changes as systematic effects related to the orbital period of the satellite seem to be present as well. A comparison with the only other 'dense' ROSAT observation with the PSPC in November 1991 indicates a long term persistence of the character of these daily variations. Over all, the soft X-ray flux from the source, observed by ROSAT repeatedly since 1990, varies by a factor of two with respect to the average over the years.
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