  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
$\sim10-20\,$keV) are consistent with one single power-law behavior below the
peak, with typical slope $\langle\alpha\rangle=-1$, 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 $\langle\alpha_1\rangle=-0.66$ ($ \rm \sigma=0.35$),
$\langle \log (E_{\rm break}/\rm keV)\rangle=0.63$ ($ \rm \sigma=0.20$),
$\langle\alpha_2\rangle=-1.46$ ($\rm \sigma=0.31$), and $\langle \log (E_{\rm
peak}/\rm keV)\rangle=2.1$ ($ \rm \sigma=0.56$).The values
$\langle\alpha_1\rangle$ and $\langle\alpha_2\rangle$ are very close to
expectations from synchrotron radiation. In this context, $E_{\rm break}$
corresponds to the cooling break frequency.
