Light curves of Mkn 421 in April 2013: a) OMC dereddened (E_{B − V} = 0.014) photometry, corrected for the host galaxy and for a companion galaxy that is located in the OMC field of view; b) JEM-X count rates (resulting from the coaddition of the signal measured by the 2 detectors), in the bands 3.04–5.52 keV (light green), 5.52–10.24 keV (green), and 10.24–25.88 keV (dark green); c) IBIS/ISGRI count rates at 20–40 keV (light blue) and 40–100 keV (dark blue); d)Fermi-LAT fluxes binned with 12 h time-resolution. As INTEGRAL provides quasi-continuous datasets, we display them both as data points with a binning of 4.8 ks and as a curve representing a weighted smoothing of the data (see Sect. 3 for details). The shaded grey area in panels b) and c) represents the interval chosen for the cross-correlation analysis (see Fig. 5).