Swift 0.3−10.0 keV light curve of AT2018cow. The times of the NuSTAR and XMM-Newton observations are highlighted with red and blue vertical lines, respectively. The six epochs we use to group the observations and the joint analysis in this paper are marked at the top of the figure. See Table B.1 for details of the observations in each epoch. We note that, at E4, the NuSTAR and XMM-Newton observations do not overlap in time.