2000A&A...357...84Q

An unusual event of rapid flux density variations (with a timescale of ∼4 days) observed at 1.49, 4.86 and 8.44GHz in the highly variable BL Lac object AO 0235+164 is analysed. The distinct properties of the burst are: (1) the flux density reached its maximum at 1.49GHz earlier than at 4.86 and 8.44GHz; (2) the duration of the burst decreased with decreasing frequency; (3) the variability amplitude increased with decreasing frequency. This behaviour is different from that of normal radio outbursts in blazars and cannot be explained easily with known models. We investigate the spectral evolution of the burst and suggest that the burst may be caused by relativistic aberration effects when a thin-shock moves with a large Lorentz factor (Γ>25) along slightly curved magnetic field lines in the jet.