Spectrum of GRB 100901A for two time intervals at t≲T₉₀. The optical flux density obtained by MASTER and corrected for the Galactic extinction A_V= 0.327 (NED; Schlegel et al. 1998), is shown by the single left point, whose horizontal bar corresponds to the MASTER unfiltered effective frequency interval. Spectra in 0.3–10 and 15–150 keV are made with the Swift BAT and XRT data. Best-fitting absorbed power laws are shown by the red lines. Their spectral parameters are described in Table 10 as Fit 100901.2 for 312–372 s and Fit 100901.3 for 387–467 s).