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Session 36 - Active Galactic Nuclei.
Display session, Tuesday, June 09
Atlas Ballroom,
In October 1992 we observed a number of Blazars in three radio bands with a five-antenna subarray of the VLA, to search for short timescale variations. The BL Lac object AO 0235+164 was observed six times per day for three weeks in the L, C, and X bands. Parallel monitoring in the visible wavelength range (R band) was conducted with four telescopes in Germany, France, Spain, and Mexico. Pronounced variability was found in all four wavelength bands. At least one major peak can be identified clearly throughout all three radio bands; this event can tentatively be identified with an optical maximum. Its properties in the radio regime are quite unusual: the lightcurve is broader at higher frequencies, with little delay between the maxima at the three radio bands. This is difficult to explain with the standard models of blazar variability. We examine different scenarios that might reproduce the salient features of these variations in AO 0235+164: gravitational microlensing, variable free-free absorption by a foreground screen, interstellar scattering, and variations of the Doppler factor in a pc-scale jet. While each one of theses mechanisms can explain some of the properties of the observed variability with plausible assumptions about the source geometry, none is fully satisfactory. The radio variability of AO 0235+164 may therefore be the superposition of two or more effects of comparable strength.
