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 $\nu F_{\nu }$ peak energy should display a power-law behavior with slope ${\alpha }_2=-3/2$, which breaks to a higher value ${\alpha }_1=-2/3$ (i.e., to a harder spectral shape) at lower energies. Prompt emission spectral data (usually available down to $\sim 10\text{--}20$ keV) are consistent with one single power-law behavior below the peak, with typical slope $⟨\alpha ⟩=-1$, higher than (and then inconsistent with) the expected value ${\alpha }_2=-3/2$. 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 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 ${\alpha }_1$, a break energy $E_{break}$, a second power law ${\alpha }_2$, and a peak energy $E_{peak}$. We find $⟨{\alpha }_1⟩=-0.66$ ($\sigma =0.35$), $⟨log\left(E_{break}/keV\right)⟩=0.63$ ($\sigma =0.20$), $⟨{\alpha }_2⟩=-1.46$ ($\sigma =0.31$), and $⟨log\left(E_{peak}/keV\right)⟩=2.1$ ($\sigma =0.56$). The values $⟨{\alpha }_1⟩$ and $⟨{\alpha }_2⟩$ are very close to expectations from synchrotron radiation. In this context, $E_{break}$ corresponds to the cooling break frequency. The relatively small ratio $E_{peak}/E_{break}\sim 30$ 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.