Fig. 2: Illustration of the spectral evolution caused by a shift of the spectrum towards lower energies.
From: Spectral index-flux relation for investigating the origins of steep decay in γ-ray bursts
The transition of the spectral peak through the XRT band explains the observed spectral softening. The spectra in (b) colored in blue, green, and red correspond to the three temporal bins shown in (a) with the same colors. The inset in (b) shows how the local spectral slope evolves as observed in the XRT band. Since in the b we plot the flux density, the local slope in the XRT band is given by 1 − α, where α is the photon index. Both the x and y axes in (a) and (b) have arbitrary units.
