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

Gor Oganesyan; Lara Nava; Giancarlo Ghirlanda; Annalisa Celotti

doi:10.3847/1538-4357/aa831e

paper

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

arXiv:1709.04689 · doi:10.3847/1538-4357/aa831e

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 $ν F_{ν}$ peak energy should display a power-law behavior with slope $α_{2} = - 3/2$ , which breaks to a higher value $α_{1} = - 2/3$ (i.e. to a harder spectral shape) at lower energies. Prompt emission spectral data (usually available down to $\sim 10 - 20$ 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 $α_{2} = - 3/2$ . 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 $α_{1}$ , a break energy $E_{break}$ , a second power law $α_{2}$ , and a peak energy $E_{peak}$ . We find $⟨ α_{1} ⟩ = - 0.66$ ( $σ = 0.35$ ), $⟨ lo g (E_{break} / keV)⟩ = 0.63$ ( $σ = 0.20$ ), $⟨ α_{2} ⟩ = - 1.46$ ( $σ = 0.31$ ), and $⟨ lo g (E_{peak} / keV)⟩ = 2.1$ ( $σ = 0.56$ ).The values $⟨ α_{1} ⟩$ and $⟨ α_{2} ⟩$ are very close to expectations from synchrotron radiation. In this context, $E_{break}$ corresponds to the cooling break frequency.

arXiv commentPublished in ApJ

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

Abstract

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