2000A&A...362..105B

The BL Lac object PKS 2155-304 was studied in two very deep ROSAT HRI observations in May and in November, 1996 during a multi-wavelength monitoring campaign. The observed intensity variations by more than a factor of two in May and by about 20% in November can be separated into slow flux changes with time scales comparable to the observation intervals and superposed, low-amplitude flares with durations of approximately one day. A comparison with all previous ROSAT observations of PKS 2155-304 confirms the persistence of these phenomena. While over several years the historical amplitudes of variability have spanned a range of a factor of ≲4.5, PKS 2155-304 varies nearly as much over much shorter periods, thus showing that the dominant time scales in this blazar are truly quite short. The close correspondence between the light curves of November 1996 and November 1991 strongly argues against a microlensing interpretation of these achromatic variations. BL Lac objects might be the first active galactic nuclei where systematic intrinsic temporal variations, like the precession of the jets, can be observed in X-rays given a sufficiently long observational coverage.