On the difference in short timescale light variation between the high and low states of BL Lac objects

Combining our data for three BL Lac objects obtained in 1993–1995 with other authors' we note that activity is stronger and short timescale variations more frequent during the high than the low state. We propose that the relativistic jet in the AGN may change its orientation with time, and the high state corresponds to smaller angles of inclination to the line of sight, and the low, to larger angles. This explains the difference in behaviour as well as the new phenomenon discovered in the monitoring by Takalo et al.