Multiwavelength variability and correlation studies of Mrk 421 during historically low X-ray and γ-ray activity in 2015-2016

We report a characterization of the multi-band flux variability and correlations of the nearby (z=0.031) blazar Markarian 421 (Mrk 421) using datafrom Metsähovi, Swift, Fermi-LAT, MAGIC, FACT and other collaborations and instruments from November 2014 till June 2016. Mrk 421 did not show any prominent flaring activity, but exhibited periods of historically low activity above 1 TeV (F_{>1TeV}}<1.7 ×10^{-12} ph cm^{-2} s^{-1})
and in the 2-10 keV (X-ray) band (F_{2-10 keV}}< 3.6 ×10^{-11} erg cm^{-2} s^{-1}), during which the Swift-BAT data suggests an additional spectral component beyond the regular synchrotron emission. The highest flux
variability occurs in X-rays and very-high-energy (E > 0.1 TeV) γ-rays, which, despite the low activity, show a significant positive correlation with no time lag. The HR_{keV} and HR_{TeV} show the harder-when-brighter trend observed in many blazars, but the trend flattens at the highest fluxes, which suggests a change in the processes dominating the blazar variability. Enlarging our data set with data from years 2007 to 2014, we measured a positive correlation between the optical and the GeV emission over a range of about 60 days centered at time lag zero, and a positive correlation between the optical/GeV and the radio emission over a range of about 60 days centered at a time lag of 43^{+9}_{-6} days. This observation is consistent with the radio-bright zone being located about 0.2 parsec downstream from the optical/GeV emission regions of the jet. The flux distributions are better described with a LogNormal function in most of the energy bands probed,
indicating that the variability in Mrk 421 is likely produced by a multiplicative process.