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

Abstract

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 ν {F}<SUB>ν </SUB> peak energy should display a power-law behavior with slope {α }<SUB>2</SUB>=-3/2, which breaks to a higher value {α }<SUB>1</SUB>=-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 {α }<SUB>2</SUB>=-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 {α }<SUB>1</SUB>, a break energy {E}<SUB>{break</SUB>}, a second power law {α }<SUB>2</SUB>, and a peak energy {E}<SUB>{peak</SUB>}. We find < {α }<SUB>1</SUB>> =-0.66 (σ =0.35), < {log}({E}<SUB>{break</SUB>}/ {keV})> =0.63 (σ =0.20), < {α }<SUB>2</SUB>> =-1.46 (σ =0.31), and < {log}({E}<SUB>{peak</SUB>}/ {keV})> =2.1 (σ =0.56). The values < {α }<SUB>1</SUB>> and < {α }<SUB>2</SUB>> are very close to expectations from synchrotron radiation. In this context, {E}<SUB>{break</SUB>} corresponds to the cooling break frequency. The relatively small ratio {E}<SUB>{peak</SUB>}/{E}<SUB>{break</SUB>}∼ 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.