Gamma ray burst studies with THESEUS

Abstract Gamma-ray Bursts (GRBs) are the most powerful transients in Universe, over–shining for a few seconds all other γ -ray sky sources. Their emission is produced within narrowly collimated relativistic jets launched after core–collapse of massive stars or merger compact binaries. THESEUS will open new window use GRBs as cosmological tools by securing statistically significant sample high- z GRBs, well providing large number at low–intermediate redshifts extending current samples to low luminosities. The wide energy band and unprecedented sensitivity Soft X-ray Imager (SXI) X-Gamma rays Imaging Spectrometer (XGIS) instruments provide us route unveil nature prompt emission. For first time, full characterisation spectrum from 0.3 keV 10 MeV with count statistics be possible revealing signatures synchrotron SXI spectra, down keV, constrain local metal absorption and, brightest events, progenitors’ ejecta composition. Investigation internal dissipation mechanisms obtained through systematic study XGIS sub-second variability unexplored so far over such range. follow spectral evolution soft X–ray during early steep decay plateau phase unique ability above these afterglow phases.