Gamma-Ray Bursts (GRBs) are intense bursts of high-energy photons which, in just a few seconds, outshine all other $\gamma$-ray emitters in the sky. Due to their extreme luminosity, GRBs are not only important as high-energy astrophysical phenomena but also serve as valuable probe models of the far, high-redshift Universe. The importance of these events has pushed the High-Energy Astrophysics community to propose new mission concepts over the past decade, prompting dedicated research and development efforts to achieve the required technological readiness levels. The X and Gamma-Ray Imager and Spectrometer (XGIS) is one of the two GRB monitors onboard the proposed, upcoming THESEUS space mission. Building on strong heritage from previous studies, ongoing developments and optimizations are focused on enhancing the instrument’s capabilities and increasing its technological maturity. This work presents the current status of the XGIS instrument and the latest technological advancements achieved in preparation for its deployment on THESEUS.

This paper presents the procedures employed for experimental functional and performance characterization of a 2 × 2 pixel prototype detection system tailored specifically for the X and Gamma-ray Imaging Spectrometer (XGIS) instrument onboard the THESEUS mission. The XGIS system comprises of two coded masked wide field cameras integrated with monolithic SDDs (Silicon Drift Detectors) and CsI:Tl (Thallium doped-Cesium Iodide) scintillators, contributing to its broad X and $\gamma$-ray detection range. Given the space instrumentation complexity, thorough requirement qualification and testing procedures are essential. This work focuses on working principle, the testing setup utilized, and observed performance for the small scale four-pixel XGIS prototype. Furthermore, the alignment of light output performance of the four-pixel SDD and scintillator prototype detection system with the XGIS instrument requirements is emphasized.

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.

Polarimetry of the transient sky is an emerging field in high-energy astrophysics. The Soft-X and gamma-ray WIde-field Polarimeter (SWIPE), a new concept mission, aims to integrate photoelectric and Compton polarimeters. At low energies (0.2–30 keV), SWIPE employs photoelectric absorption in gas mixtures, optimizing both quantum efficiency and polarization sensitivity. For hard X-rays and gamma-rays (20–200 keV), Compton scattering techniques are usually preferred. The SWIPE Compton polarimeter consists of an array of plastic and inorganic scintillator bars coupled to Silicon Photomultipliers or Silicon Drift Detectors at both bar ends. Typically, Compton polarimeters reconstruct events considering only the azimuthal scattering angle, simplifying the analysis but limiting performance for off-axis sources. Instead, the peculiar readout configuration of SWIPE, applied on both sides of the bars, enables the measurement of the Depth of Interaction, by comparing the intensity of the two acquired signals for each detected event. This allows us to estimate the polar scattering angle improving sensitivity and reducing systematic errors in polarization measurements. This work presents the SWIPE Compton polarimeter concept and the ongoing development of its prototype.

The X and Gamma Imager and Spectrometer (XGIS) on board THESEUS is a finely pixelized and modular instrument designed for broadband high-energy transient detection. XGIS consists of two cameras, each composed of 10 supermodules, with each supermodule further divided into 10 modules and each module made with 64 independent readout pixels based on Silicon Drift Detectors coupled with 5 × 5 × 30 mm³ CsI scintillator bars. An algorithm to quickly read out the signals from the 64 pixels and send them in chronological order through the module and supermodule logic up to the camera logic is under development. Furthermore, a challenge for space-based high-energy instruments is distinguishing X-/gamma-ray photons while effectively rejecting background photons and particles, including electrons, protons, and heavier cosmic rays. Unlike traditional systems that rely on anticoincidence systems, XGIS aims to achieve background rejection through an innovative readout logic that analyzes the spatial and temporal properties of energy deposits in the detector. By leveraging the finely pixelized structure, the readout system can differentiate single-photon events from charged-particle tracks based on energy deposition patterns and event topology.