Observational evidences of multiple shock waves in X-ray selected BL Lacerate objects

The results of optical R-band photometry of three X-ray selected BL Lacertae objects 1ES 0502+675, 1ES 0806+524 and 1ES 1959+650 from the Einstein Slew Survey are presented. The observations are performed during 1997–2006 with the 70-cm meniscus type telescope of Abastumani Astrophysical Observatory. The objects show clear long-term variability with time scales of 1–3 years without evident periodicity. The main results consist in the discovery of multiple-peak structures on historical light curves as predicted theoretically on the basis of the assumption that long-term variability of blazars are triggered by ultrarelativistic shock waves propagating through their jets. They are direct evidence of the existence of reverse shocks besides the main one arising by a single disturbance. Two-peak maxima are found for 1ES 0502+675 and 1ES 0806+524. The more complicated structure shows 1ES 199+650 - a maximum with four consequent peaks. The relative strengths of the main and reverse shocks are mainly equal according to the shape of the respective peaks. The brightness dip between them is on average 60% less than in the case of consequent main shocks. Optical maximum epochs, covered well by the observations, show that the main shocks are either not always accompanied by reverse ones or the later are not always strong enough to be discovered by the observations.