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Astron. Astrophys. 344, 807-816 (1999)

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Unusual radio variability in the BL Lacertae object 0235+164

A. Kraus 1, A. Quirrenbach 2, A.P. Lobanov 1, T.P. Krichbaum 1, M. Risse 1, P. Schneider 3, S.J. Qian 1,6, S.J. Wagner 4, A. Witzel 1, J.A. Zensus 1, J. Heidt 4, H. Bock 4, M. Aller 5 and H. Aller 5

1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
2 Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, Postfach 1603, D-85740 Garching, Germany
3 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, D-85740 Garching, Germany
4 Landessternwarte Heidelberg, Königstuhl, D-69117 Heidelberg, Germany
5 Astronomy Department, University of Michigan, 830 Dennison Building, Ann Arbor, MI 48109-1090, USA
6 Beijing Astronomical Observatory, Chinese Academy of Science, Beijing 100080, P.R. China

Received 7 December 1998 / Accepted 11 February 1999


We present radio observations at three frequencies and contemporaneous optical monitoring of the peculiar BL Lac object AO 0235+164. During a three-week campaign with the VLA we observed intraday variability in this source and found a distinct peak which can be identified throughout the radio frequencies and tentatively connected to the R-band variations. This event is characterized by unusual properties: its strength increases, and its duration decreases with wavelength, and it peaks earlier at 20 cm than at 3.6 and 6 cm. We discuss several generic models (a "standard" shock-in-jet model, a precessing beam, free-free-absorption in a foreground screen, interstellar scattering, and gravitational microlensing), and explore whether they can account for our observations. Most attempts at explaining the data on 0235+164 require an extremely small source size, which can be reconciled with the [FORMULA] K inverse Compton limit only when the Doppler factor of the bulk flow is of order 100. However, none of the models is completely satisfactory, and we suggest that the observed variability is due to a superposition of intrinsic and propagation effects.

Key words: galaxies: active – galaxies: BL Lacertae objects: individual: AO 0235+164 – radio continuum: galaxies

Present address: University of California San Diego, Dept. of Physics, Center for Astrophysics and Space Sciences, Mail Code 0424, La Jolla, CA 92093-0424, USA

Send offprint requests to: A. Witzel

© European Southern Observatory (ESO) 1999

Online publication: March 29, 1999