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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 campaign, 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.
Radio-to-TeV observations of the bright nearby (z=0.034) blazar Markarian 501 (Mrk 501), performed from December 2012 to April 2018, are used to study the emission mechanisms in its relativistic jet. We examined the multi-wavelength variability and t
The MAGIC telescopes observed S2 0109+22 in 2015 July during its flaring activity in high energy gamma rays observed by Fermi-LAT. We analyse the MAGIC data to characterise the very high energy (VHE) gamma-ray emission of S2 0109+22, which belongs to
Flux distribution is an important tool to understand the variability processes in active galactic nuclei. We now have available a great deal of observational evidences pointing towards the presence of log-normal components in the high energy light cu
We examine the 2008-2016 $gamma$-ray and optical light curves of three bright BL Lac objects, 0716+714, MRK 421, BL Lac, which exhibit large structured variability. We searched for periodicities by using a fully Bayesian approach. For two out of thre
A flare from the TeV blazar Mrk 421, occurring in March 2010, was observed for 13 consecutive days from radio to very high energy (VHE, E > 100 GeV) gamma-rays with MAGIC, VERITAS, Whipple, FermiLAT, MAXI, RXTE, Swift, GASP-WEBT, and several optical