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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 $ u F_ u$ 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 $sim10-20,$keV) are consistent with one single power-law behavior below the peak, with typical slope $langlealpharangle=-1$, 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 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 $alpha_1$, a break energy $E_{rm break}$, a second power law $alpha_2$, and a peak energy $E_{rm peak}$. We find $langlealpha_1rangle=-0.66$ ($ rm sigma=0.35$), $langle log (E_{rm break}/rm keV)rangle=0.63$ ($ rm sigma=0.20$), $langlealpha_2rangle=-1.46$ ($rm sigma=0.31$), and $langle log (E_{rm peak}/rm keV)rangle=2.1$ ($ rm sigma=0.56$).The values $langlealpha_1rangle$ and $langlealpha_2rangle$ are very close to expectations from synchrotron radiation. In this context, $E_{rm break}$ corresponds to the cooling break frequency.
We aim to obtain a measure of the curvature of time-resolved spectra that can be compared directly to theory. This tests the ability of models such as synchrotron emission to explain the peaks or breaks of GBM prompt emission spectra. We take the bur
Gamma-ray Bursts (GRBs) prompt emission spectra are often fitted with the empirical Band function, namely two power laws smoothly connected. The typical slope of the low energy (sub-MeV) power law is $alpha_{B}simeq -1$. In a small fraction of long G
The prompt emission of most gamma-ray bursts (GRBs) typically exhibits a non-thermal Band component. The synchrotron radiation in the popular internal shock model is generally put forward to explain such a non-thermal component. However, the low-ener
The long-lasting tension between the observed spectra of gamma ray bursts (GRBs) and the predicted synchrotron emission spectrum might be solved if electrons do not completely cool. Evidence for incomplete cooling was recently found in Swift GRBs wit
Gamma-ray Burst (GRB) collimation has been inferred with the observations of achromatic steepening in GRB light curves, known as jet breaks. Identifying a jet break from a GRB afterglow lightcurve allows a measurement of the jet opening angle and tru