Infrared , radio and optical variability of the BL Lacertae object 2007 + 777 ?

We present infrared observations at wavelengths of 60 and 100 μm and contemporaneous radio (five frequencies) and optical (R band) monitoring of the BL Lac object 2007+777. During a three-week campaign with the ISO1 spacecraft, the VLA2, the Effelsberg 100 m radio telescope, and optical telescopes in Heidelberg and on Calar Alto, Spain3, we observed variability on time scales of a few days at 100 μm, at radio, and at optical frequencies. We discuss briefly several generic models which could account for our observations. The attempt at explaining the results by an intrinsic mechanism requires an extremely small source angular size of the order of a few μas4, which can be reconciled with the 1012 K inverse Compton limit only when the Doppler factor of the bulk flow is about 40. The general behavior of the variability of the BL Lac object 2007+777 found in this IR-radio campaign follows the expectations of a shock-in-jet model, and we argue that this is the most likely explanation of our observations.