Presentation #100.56 in the session AGN.
Blazars are known for their variable emission across all wavelengths. A significant fraction of gamma-ray detected blazars have shown rapid variability at GeV and TeV energies during major flares. For five of them, variability time scales reached below ten minutes, which translates to emitting regions smaller than the light-crossing time of their central black holes. Such rapid variability challenges the established models that have been derived to explain the flaring behaviour of blazars. These extremely short variations have so far only been reported for the gamma-ray emission. We present X-ray timing results of Mrk 421 taken with XMM-Newton during a TeV flare detected by FACT. The accompanying very bright X-ray flare revealed rapid variations similar at time scales comparable to the fastest seen at gamma-ray energies. We will show the results of our X-ray data analysis and discuss the observed multi-band rapid variations in the context of currently existing models that involve kink instabilities, turbulence, or magnetic reconnection.