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Astron. Astrophys. 362, 105-112 (2000)

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The soft X-Ray variability of PKS 2155-304

W. Brinkmann 1, M. Gliozzi 1, C.M. Urry 2, L. Maraschi 3 and R. Sambruna 4

1 Centre for Interdisciplinary Plasma Science (CIPS), Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, 85740 Garching, Germany
2 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
3 Osservatorio Astronomico di Brera, Via Brera, 28, 20121 Milano, Italy
4 George Mason University, Department of Physics and Astronomy and School of Computational Sciences, 4400 University Drive, Fairfax, VA 22030, USA

Received 25 May 2000 / Accepted 8 August 2000


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 [FORMULA] 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.

Key words: galaxies: active – galaxies: BL Lacertae objects: individual: PKS 2155-304 – galaxies: jets – X-rays: galaxies

Send offprint requests to: W. Brinkmann (wpb@mpe.mpg.de)

© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100