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Detection of Low-energy Breaks in Gamma-Ray Burst Prompt Emission Spectra

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 peak energy should display a power-law behavior with slope , which breaks to a higher value (i.e., to a harder spectral shape) at lower energies. Prompt emission spectral data (usually available down to keV) are consistent with one single power-law behavior below the peak, with typical slope , higher than (and then inconsistent with) the expected value . 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 , a break energy , a second power law , and a peak energy . We find ( ), ( ), ( ), and ( ). The values and are very close to expectations from synchrotron radiation. In this context, corresponds to the cooling break frequency. The relatively small ratio 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.