Coordinated Multiwavelength Observations of BL Lacertae in 2000

BL Lacertae was the target of an extensive multiwavelength monitoringcampaign in the second half of 2000. Simultaneous or quasi-simultaneousobservations were taken at radio (UMRAO and Metsaehovi) and optical(WEBTcollaboration) frequencies, in X-rays (BeppoSAX and RXTE), and at VHEgamma-rays (HEGRA). The WEBT optical campaign achieved an unprecedented timecoverage, virtually continuous over several 10 - 20 hour segments. It revealedintraday variability on time scales of ~ 1.5 hours and evidence for spectralhardening associated with increasing optical flux. During the campaign, BLLacertae underwent a major transition from a rather quiescent state prior toSeptember 2000, to a flaring state for the rest of the year. This was alsoevident in the X-ray activity of the source. BeppoSAX observations on July26/27 revealed a rather low X-ray flux and a hard spectrum, while a BeppoSAXpointing on Oct. 31 - Nov. 2, 2000, indicated significant variability on timescales of < a few hours, and provided evidence for the synchrotron spectrumextending out to ~ 10 keV during that time. During the July 26/27 observation,there is a tantalizing, though not statistically significant, indication of atime delay of ~ 4 - 5 hr between the BeppoSAX and the R-band light curve. Also,a low-significance detection of a time delay of 15 d between the 14.5 GHz andthe 22 GHz radio light curves is reported. Several independent methods toestimate the co-moving magnetic field in the source are presented, suggesting avalue of ~ 2 e_B^{2/7} G, where e_B is the magnetic-field equipartition factorw.r.t. the electron energy density in the jet.Comment: Accepted for publication in Ap