Extended Data Fig. 4: Fermi/GBM spectra of two one-minute accumulations during the 2017 September 10 flare and their spectral fits.

Panels (a, b) show background-subtracted counts spectrum (black points) near the peak of the MeV electron emission (a), and at a time when there is still strong power-law electron emission, but the emission from the MeV-peaked component is weak (b). Vertical error bars correspond to the 1σ count uncertainty estimated from Poisson statistics, while horizontal bars indicate the width of each energy bin. The spectral fits are composed from three components: (i) power-law extension of the standard HXR component (red); (ii) the standard OSPEX template of combined spectrum of broad and narrow nuclear de-excitation lines (green); and (iii) thin-target bremsstrahlung component from the new MeV population modeled with a broken power-law energy spectrum. Gray lines on panels (a, b) show the sum of all three fitting components. (c) The electron energy spectrum (blue lines) needed to generate thin-target bremsstrahlung shown in blue in panels (a, b). The light gray polygons show 1σ uncertainties in the slopes of this spectrum reported in⁹. For reference, the red line shows the normalized spectrum of electrons responsible for the standard power-law X-ray / γ-ray component shown in red in panels (a, b).