Long-Term Optical Variability of the Blazars PKS 2005−489 and PKS 2155−304

We present optical light curves for the period 1996–2000 of two of the brightest known EGRET BL Lac objects: PKS 2005-489 and PKS 2155-304, the latter also one of the few known TeV sources. For both objects, quiescent epochs of low-level variability were followed by active periods, without any indication of periodicity. In PKS 2005-489, several variability events with durations of about 20 days were observed. In PKS 2155-304, fast drops and subsequent rises in luminosity occurred in timescales of days. We propose an explanation in which a region moving along the relativistic jet is eclipsed by broad-line region clouds or star clusters in the host galaxy. We compare our light curves with contemporaneous X-ray observations from the All-Sky Monitor on board the Rossi X-Ray Timing Explorer. Correlations between optical and X-ray activity were not found in any of the sources at long timescales. However, in PKS 2005-489, possible correlations were observed in 1997 and 1998 at short timescales, with optical variability preceding X-rays by 30 days in 1997 and succeeding them by about 10 days in 1998. The analysis of the spectral energy distribution, using the optical data presented here and BeppoSAX contemporaneous observations obtained from the literature, shows only small shifts in the synchrotron peak as the X-ray flux density changes.