Top: background-subtracted and exposure-corrected light curves of J1023 obtained with the XMM–Newton EPIC cameras (0.3–10 keV; black data), NuSTAR FPMA + FPMB (3–79 keV; red data) and Swift UVOT (UV M2 filter; blue data) during the time interval covered by XMM–Newton. For plotting purposes, light curves are shown with a binning time of 50 s and the vertical axis is plotted in logarithmic scale. Bottom: zoom of the XMM–Newton EPIC and NuSTAR FPMA + FPMB light curves around the longest and brightest flaring episode detected in the data. Light curves are shown with a binning time of 10 s and the vertical axis is plotted in linear scale.