The TeV BL Lac object Markarian 501 has been the subject of a 4.5 month multi-instrument campaign conducted in 2009, which provided an excellent temporal and energy coverage from radio to very high energy gamma rays (>100 GeV, VHE). During the campai
gn, Mrk 501 was mostly in a comparably low state, but for two flares at VHE with very different characteristics. While the second flare seems to be correlated with a moderate increase in the X-ray flux, the first one is most likely accompanied by a shift of the synchrotron bump towards higher energies. Moreover, the first flare occurs during an abrupt change in the polarized optical flux, and was preceded by a rotation of the electric field vector position angle. This is the first time that such behavior is observed in a high-frequency-peaked BL Lac object, while similar events have been seen in the low-frequency peaked BL Lac object BL Lacertae and the flat spectrum radio quasar PKS 1510-089, hence suggesting that similar physical processes occur in the jets of different blazar subclasses.
