Dissecting the long-term emission behaviour of the BL Lac object Mrk 421

Understand

We report on long-term multi-wavelength monitoring of blazar Mrk 421 by the GASP-WEBT collaboration and Steward Observatory, and by the Swift and Fermi satellites.

• We study the source behaviour in the period 2007-2015, characterized by several extreme flares.
• The ratio between the optical, X-ray, and \gamma-ray fluxes is very variable.
• The \gamma-ray flux variations show fair correlation with the optical ones starting from 2012.