Multi-wavelength variability of Mrk 421

We used 5.5 years of unbiased observing campaign data, obtained using the FACT telescope and the Fermi LAT detector at TeV and GeV energies, the longest and densest so far, together with contemporaneous multi-wavelength observations, to characterise the variability of Mrk 421 and to constrain the underlying physical mechanisms. We studied and correlated light curves obtained by ten different instruments and found two significant results.

The TeV and X-ray light curves are very well correlated with a lag of <0.6 days. The GeV and radio (15GHz band) light curves are widely and strongly correlated. Variations of the GeV light curve lead those in the radio.