Scientific performances of the XGIS instrument on-board THESEUS
The Transient High Energy Sky and Early Universe Surveyor (THESEUS) satellite, currently undergoing a Phase-A study for the M7 ESA mission selection, will mainly rely on the XGIS (X and Gamma Imaging Spectrometer) instrument to discover and precisely localise gamma-ray bursts and other high-energy transients. The XGIS instrument has been designed to provide accurate sky images in a wide field of view (>2 steradians) in the 2-150 keV band, coupled to coverage of nearly half of the sky with good timing and spectroscopic capabilities up to several MeV. This will be achieved thanks to the combination of two identical cameras, based on the coded mask imaging technique, oriented to provide partially overlapping fields of view. We present the capabilities of the XGIS, focussing in particular on the sensitivity and source location accuracy. We discuss the expected scientific advances in the GRB field, estimated through detailed Monte-Carlo simulations which take into account the design of the XGIS imaging system and the properties of the GRB population at high redshifts.