Multifrequency observations of BL Lacertae

Abstract

We present 20 years of optical, infrared, and radio monitoring data for BL Lac as well as four simultaneous multifrequency spectra covering the 10 9 -10 18 Hz range. Although there is no time delay between the optical and infrared variability, the high-frequency radio variations precede lower frequency variations, but only by weeks. The optical variability precedes the radio variability by a few years. The structure function for the radio variations is nearly that of shot noise for time scales less than 600 days, in contrast to the optical variation, which is similar to flicker noise. These results indicate that, although there are fundamental differences between the optical and radio emitting regions, they are related, possibly by the propagation of shocks between regions.