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 thebrightest known EGRET BL Lac objects: PKS 2005-489 and PKS 2155-304, the latteralso one of the few known TeV sources. For both objects, quiescent epochs oflow level of variability were followed by active periods, without anyindication of periodicity. In PKS 2005-489, several variability events withduration of about 20 days were observed. In PKS 2155-304 fast drops andsubsequent rises in luminosity occurred in time scales of days. We proposed anexplanation in which a region moving along the relativistic jet is eclipsed bybroad line region clouds or star clusters in the host galaxy. We compare ourlight curves with contemporaneous X-ray observations from All-Sky Monitor/RXTE.Correlations between optical and X-ray activity were not found in any of thesources at long time scales. However in PKS 2005-489 possible correlations wereobserved in 1997 and 1998 at short time scales, with optical variabilitypreceding X-rays by 30 days in 1997 and succeeding them by about 10 days in1998. The analysis of the SED, using the optical data presented here andBeppoSAX contemporaneous observations obtained from the literature, shows onlysmall shifts in the synchrotron peak as the X-ray flux density changes.Comment: 23 pages,12 figures, accepted for publication in A