Gamma-ray burst prompt emission: new insights into spectral characterization

The origin of gamma-ray burst (GRB) prompt emission is a debated issue, strictly connected to fundamental open problems such as jet composition, energy dissipation and radiation mechanisms. The radiative processes responsible for the prompt radiation are remaining uncertain. The typical observed prompt emission spectrum in the energy power spectrum representation consists of two power-laws smoothly connected at a peak energy. Its non-thermal spectral shape calls for synchrotron or inverse Compton radiation, but the hardness characterizing the low-energy part of the spectra is inconsistent with these processes. The unestablished nature of the prompt radiation has strong repercussions on our understanding of the GRB phenomenon, preventing us from constraining macro- and micro-physical properties of the source. While the prompt emission is usually observed only between 10 keV and 1 MeV, in this thesis I extend the energy range for prompt studies down to soft X-rays for those cases where Swift/XRT (0.3-10 keV) started observations during the prompt phase (34 GRBs). My analysis revealed for the first time that prompt spectra often ( 65%) display spectral break at a few keV. Below the break, the spectrum is well described by a power-law with hard photon index (-2/3). The overall shape is consistent with synchrotron radiation, where the break energy corresponds to the cooling break. I added, when available, simultaneous optical observations, providing an additional and independent test on the presence of the low-energy break. In the synchrotron scenario, the small ratio between peak energy and cooling energy points toward a moderately-fast cooling regime. In a simple scenario where electrons are accelerated only once, this regime implies weak magnetic fields (< 10-100 G in the fluid comoving frame). In alternative scenarios, these strong constraints on the magnetic field can be relaxed by invoking almost balanced electron cooling and heating rates and/or multiple acceleration in magnetic reconnection islands.