Monitoring of AO 0235+164 during a faint state^*

¹ Tuorla Observatory, Tuorla, FIN-21500 Piikkiö, Finland
² Metsähovi Radio Station, Metsähovi, Kylmälä, Finland
³ Osservatorio Astronomico di Torino, Strada Osservatorio 20, I-10025 Pino Torinese, Italy
⁴ 1655, Main St. Stittsville, Ont, K2S 1N6, Canada
⁵ Cattedra di Astrofisica, Universita di Perugia, Italy

Send offprint request to: L.O. Takalo

Received: 11 June 1997
Accepted: 31 October 1997

Abstract

We present the results of the optical and radio monitoring of the BL Lac object AO 0235+164 during a faint state. In both optical and radio bands the source has been observed at the faintest ever recorded levels, reaching and Jy. In the optical bands we still see variability with amplitudes up to 1.5 magnitudes on timescales from days to weeks. The radio variability is less dramatic, but in general follows the optical behaviour. A correlation between general trends in the optical and radio behaviour of AO 0235+164 may be recognized in the data from the present monitoring as well as in the historical light curves, suggesting a "base” mechanism responsible for the emission at both frequencies. A good candidate is a synchrotron process in the relativistic jet. Optical flares with no corresponding radio counterparts have been observed too. These events may be interpreted in terms of microlensing by a foreground galaxy.