Extreme intranight variability in the BL Lacertae object AO 0235+164

Abstract

We present results of two-colour photometry with high time resolution of the violently variable BL Lac object AO 0235+164. We have found extreme intranight variability with amplitudes of ~ 100% over time scales of 24 hours. Changes of 0.5 magnitudes in both R and V bands were measured within a single night, and variations up to 1.2 magnitudes occurred from night to night. A complete outburst with an amplitude ~ 30% was observed during one of the nights, while the spectrum remained unchanged. This seems to support an origin based on a thin relativistic shock propagating in such a way that it changes the viewing angle, as recently suggested by Kraus et al. (1999) and Qian et al. (2000).