Fractional rms of the optical LCO g′ (green dots) and i′ (black dots) data vs. the Swift/XRT fractional rms amplitude (%) in the 2.0−10.0 keV energy band. The first three panels show the Swift/XRT rms obtained from 5 s-, 20 s-, and 60 s-binned light curves, respectively. The rightmost panel shows the Swift/XRT broadband-frequency rms 0.01−64 Hz (i.e. 0.02–100 s) on the x-axis. Horizontal and vertical error bars represent one-sigma uncertainties calculated analytically for each respective method.