AO 0235+164: rapid flux variations caused by relativistic aberration effects

An unusual event of rapid flux density variations (with a timescale of simi lar to 4 days) observed at 1.49, 4.86 and 8.44 GHz in the highly variable B L Lac object AO 0235+164 is analysed. The distinct properties of the burst are: (I) the flux density reached its maximum at 1.49 GHz earlier than at 4 .86 and 8.44 GHz; (2) the duration of the burst decreased with decreasing f requency; (3) the variability amplitude increased with decreasing frequency . This behaviour is different from that of normal radio outbursts in blazar s and cannot be explained easily with known models. We investigate the spec tral evolution of the burst and suggest that the burst may be caused by rel ativistic aberration effects when a thin-shock moves with a large Lorentz f actor (Gamma greater than or similar to 25) along slightly curved magnetic field lines in the jet.