XGIS instrument onboard THESEUS: design and development overview

The X and Gamma-ray Imager and Spectrometer (XGIS) is the high-energy monitor proposed for THESEUS (Transient High-Energy Sky and Early Universe Surveyor), a candidate mission within ESA M7 program aimed at exploring the transient high-energy sky for investigating the Early Universe and advancing in Multi-Messenger Astrophysics. XGIS is designed to detect and localize transient X-ray and gamma-ray sources across a broad energy range (2 keV–10 MeV), enabling high-resolution timing and spectroscopy. The instrument consists of two identical cameras, employing coded mask techniques to image the sky in the range 2–150 keV, while acting as a half-sky monitor in the 150 keV–10 MeV energy range. Its innovative detection system is based on Silicon Drift Detectors optically coupled to CsI(Tl) scintillator bars. This configuration, combined with a low-noise distributed electronics system (ORION ASICs), enables spectroscopy over an unprecedentedly wide energy band within a single, modular and compact device. The design of XGIS emphasizes modularity and robustness, with each camera integrating 100 detector modules that require extensive development, testing and qualification. The ongoing development of XGIS includes performance testing of the silicon sensors and the ORION readout chain, alongside optimization of the mechanical assembly. In this review I will outline the XGIS architecture and summarize the technological advancements made in the current M7 Phase A.