Abstract:

The BL Lac objects Mrk 421 and Mrk 501 are objects of a large multi-year multiinstrument campaign including VLBA, F-GAMMA, GASP-WEBT, Swift, RXTE, Fermi-LAT, MAGIC and VERITAS, among other instruments and collaborations. This extensive radio to TeV gamma-ray dataset provides an unprecedented temporal and energy coverage, which allows for detailed studies of the evolution of their broad-band spectral energy distribution that cannot be achieved on any other BL Lac object. We report on the temporal variability and spectral correlations from the 4.5 months-long campaigns of 2009. We show that Mrk 421 and Mrk 501 have differences in their broad band variability, suggesting different mechanisms for the production of the high-energy emission. In addition, we also report on two events of high activity in very high energies (VHE) of Mrk 501 during May 2009, one of which was not accompanied by enhanced optical/X-ray activity but showed a clear correlation with an increase in the polarized optical flux and a rotation of the polarization vector. This is the first time that such type of flaring activity is observed in a high-synchrotron-peaked BL Lac object. © 2012 American Institute of Physics.