2017ApJ...846..137O

The radiative process responsible for gamma-ray burst (GRB) prompt emission has not been identified yet. If dominated by fast-cooling synchrotron radiation, the part of the spectrum immediately below the νF_ν peak energy should display a power-law behavior with slope α₂=-3/2, which breaks to a higher value α₁=-2/3 (i.e., to a harder spectral shape) at lower energies. Prompt emission spectral data (usually available down to ∼10–20 keV) are consistent with one single power-law behavior below the peak, with typical slope <α>=-1, higher than (and then inconsistent with) the expected value α₂=-3/2. To better characterize the spectral shape at low energy, we analyzed 14 GRBs for which the Swift X-ray Telescope started observations during the prompt. When available, Fermi-GBM observations have been included in the analysis. For 67% of the spectra, models that usually give a satisfactory description of the prompt (e.g., the Band model) fail to reproduce the 0.5-1000 keV spectra: low-energy data outline the presence of a spectral break around a few keV. We then introduce an empirical fitting function that includes a low-energy power law α₁, a break energy E_break, a second power law α₂, and a peak energy E_peak. We find <α₁>=-0.66 (σ =0.35), <log(E_break/ keV)>=0.63 (σ =0.20), <α₂>=-1.46 (σ =0.31), and <log(E_peak/ keV)>=2.1 (σ =0.56). The values <α₁>and <α₂>are very close to expectations from synchrotron radiation. In this context, E_break corresponds to the cooling break frequency. The relatively small ratio E_peak/E_break∼30 suggests a regime of moderately fast cooling, which might solve the long-lasting problem of the apparent inconsistency between measured and predicted low-energy spectral index.