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Astron. Astrophys. 355, 880-884 (2000)

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1. Introduction

BL Lac objects are a special subclass of active galactic nuclei (AGNs) showing some extreme properties: rapid and large variability, high and variable polarization, no or only weak emission lines in its classical definition.

BL Lac objects are variable not only in the optical band, but also in radio, infrared, X-ray, and even [FORMULA]-ray bands. Some BL Lac objects show that the spectral index changes with the brightness of the source (Bertaud et al. 1973; Brown et al. 1989; Fan 1993), generally, the spectrum flattens when the source brightens, but a different phenomenon has also been found (Fan et al. 1999).

The nature of AGNs is still an open problem; the study of AGN variability can yield valuable information about their nature, and the implications for quasar modeling are extremely important (see Fan et al. 1998a).

PKS 2155-304, the prototype of the X-ray selected BL Lac objects and TeV [FORMULA]-ray emitter (Chadwick et al. 1999), is one of the brightest and the best studied objects. Its spectrum from [FORMULA] to [FORMULA] appears blue (B-V[FORMULA]0.1) and featureless (Wade et al. 1979). A 0.17 redshift was claimed from the potentially detected weak [O III] emission feature (Charles et al. 1979), which was not detected in Miller & McAlister (1983) observations. Later, a redshift of 0.117 was obtained from several discrete absorption features (Bowyer et al. 1984). PKS 2155-304 varies at all observation frequencies and is one of the most extensively studied objects for both space-based observations in UV and X-ray bands (Treves et al. 1989; Urry et al. 1993; Pian et al. 1996; Giommi et al. 1998) and multiwavelength observations (Pesce et al. 1997). Variation over a time scale of one day was observed (Miller & Carini 1991) and that over a time scale of as short as 15 minutes is also reported by Paltani et al. (1997) in the optical band. Different brightness-dependent spectrum properties are found (see Miller & McAlister 1983; Smith & Sitko 1991; Urry et al. 1993; Courvoisier et al. 1995; Xie et al. 1996; Zhang & Xie 1996; Paltani et al. 1997).

In this paper, we will investigate the periodicity in the light curve and discuss the variation as well. The paper has been arranged as follows: In Sect. 2, the variations are presented and the periodicities are searched, in Sect. 3, some discussion and a brief conclusion are given.

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© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000