2017/09/14 by Gor Oganesyan, Lara Nava, Giancarlo Ghirlanda +1 · 1 citation
Physics and Astronomy · #astro-ph.HE
paper · pdf · doi:10.3847/1538-4357/aa831e
Published in ApJ
arxiv created 2017/09/14 · arxiv updated 2017/09/20
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 νFν 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 in reproducing 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\rm break, a second power law α2, and a peak energy E\rm peak. We find ⟨α1⟩=-0.66 ( \rm σ=0.35), ⟨ log (E\rm break/\rm keV)⟩=0.63 ( \rm σ=0.20), ⟨α2⟩=-1.46 (\rm σ=0.31), and ⟨ log (E\rm peak/\rm keV)⟩=2.1 ( \rm σ=0.56).The values ⟨α1⟩ and ⟨α2⟩ are very close to expectations from synchrotron radiation. In this context, E\rm break corresponds to the cooling break frequency.