GRB 100906A light curves: unabsorbed 0.3–10 keV flux (black dots) observed by Swift/XRT, unfiltered MASTER band flux (blue crosses), R-band flux by the OSN (red dots) and R-band flux by the NOT (green triangles). The optical points are corrected for the Galactic extinction. The broken power-law fit is plotted over the 0.3–10 keV light curve in white and black, before and after the break time at 51 ks, respectively. Thin-line rectangles show the time intervals selected for spectral analysis.