  Recent advances in fitting prompt emission spectra in gamma-ray bursts (GRBs)
are boosting our understanding of the still elusive origin of this radiation.
These progresses have been possible thanks to a more detailed analysis of the
low-energy part ($<$\,100\,keV) of the prompt spectrum, where the spectral
shape is sometimes found to deviate from a simple power-law shape. This
deviation is well described by a spectral break or, alternatively by the
addition of a thermal component. Spectral data extending down to less than
1\,keV are extremely relevant for these studies, but presently they are
available only for a small subsample of {\it Swift} GRBs observed by XRT (the
X-ray telescope, 0.3-10\,keV) during the prompt emission. The space mission
\th\ will allow a systematic study of prompt spectra from 0.3\,keV to several
MeV. We show that observations performed by \th\ will allow us to discriminate
between different models presently considered for GRB prompt studies, solving
the long-standing open issue about the nature of the prompt radiation, with
relevant consequences on the location of the emitting region, magnetic field
strength and presence of thermal components.
