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The XGIS (X and Gamma Imaging Spectrometer) is one of the three instruments onboard the THESEUS mission (ESA M5, currently in Phase-A). Thanks to its wide field of view and good imaging capabilities, it will efficiently detect and localize gamma-ray bursts and other transients in the 2-150 keV sky, and also provide spectroscopy up to 10 MeV. Its current design has been optimized by means of scientific simulations based on a Monte Carlo model of the instrument coupled to a state-of-the-art description of the populations of long and short GRBs extending to high redshifts. We describe the optimization process that led to the current design of the XGIS, based on two identical units with partially overlapping fields of view, and discuss the expected performance of the instrument., Comment: Proceedings of the SPIE 2020, paper 11444-276

Space Telescopes and Instrumentation 2020: Ultraviolet to Gamma Ray; Virtual, Online; United States; 14 December 2020 through 18 December 2020; Code 166330.--Proceedings of SPIE - The International Society for Optical Engineering Volume 11444, 2020, Article number 114442K.--Full list of authors: Labanti, C.; Amati, L.; Frontera, F.; Mereghetti, S.; Gasent-Blesa, J. L.; Tenzer, C.; Orleanski, P.; Kuvvetli, I.; Campana, R.; Fuschino, F.; Terenzi, L.; Virgilli, E.; Morgante, G.; Orlandini, M.; Butler, R. C.; Stephen, J. B.; Auricchio, N.; De Rosa, A.; Da Ronco, V.; Evangelisti, F. Melchiorri, M.; Squerzanti, S.; Fiorini, M.; Bertuccio, G.; Mele, F.; Gandola, M.; Malcovati, P.; Grassi, M.; Bellutti, P.; Borghi, G.; Ficorella, F.; Picciotto, A.; Zanini, V.; Zorzi, N.; Demenev, E.; Rashevskaya, I.; Rachevski, A.; Zampa, G.; Vacchi, A.; Zampa, N.; Baldazzi, G.; La Rosa, G.; Sottile, G.; Volpe, A.; Winkler, M.; Reglero, V.; Connell, P. H.; Pinazo-Herrero, B.; Navarro-González, J.; Rodríguez-Martínez, P.; Castro-Tirado, A. J.; Santangelo, A.; Hedderman, P.; Lorenzi, P.; Sarra, P.; Pedersen, S. M.; Tcherniak, D.; Guidorzi, C.; Rosati, P.; Trois, A.; Piazzolla, R., THESEUS (Transient High Energy Sky and Early Universe Surveyor) is one of the three missions selected by ESA as fifth medium class mission (M5) candidates in its Cosmic Vision science program, currently under assessment in a phase A study with a planned launch date in 2032. THESEUS is designed to carry on-board two wide and deep sky monitoring instruments for X/gamma-ray transients detection: a wide-field soft X-ray monitor with imaging capability (Soft X-ray Imager, SXI, 0.3 - 5 keV), a hard X-ray, partially-imaging spectroscopic instrument (X and Gamma Imaging Spectrometer, XGIS, 2 keV - 10 MeV), and an optical/near-IR telescope with both imaging and spectroscopic capability (InfraRed Telescope, IRT, 0.7 - 1.8 µm). The spacecraft will be capable of performing fast repointing of the IRT to the error region provided by the monitors, thus allowing it to detect and localize the transient sources down to a few arcsec accuracy, for immediate identification and redshift determination. The prime goal of the XGIS will be to detect transient sources, with monitoring timescales down to milliseconds, both independently of, or following up, SXI detections, and identify the sources performing localisation at, The Phase A study of the THESEUS/XGIS instrument is supported by ASI-INAF Agreement n. 2018-29-HH.0, OHB Italia/ - INAF-OASBo Agreement n.2331/2020/01, by the European Space Agency ESA through the M5/NPMC Programme and by the AHEAD2020 project funded by UE through H2020-INFRAIA-2018-2020. By the Spanish Ministerio de Ciencia e Innovación, PID2019-109269RB-C41. By Polish National Science Center, Project 2019/35/B/ST9/03944 and Foundation for Polish Science, Project POIR.04.04.00-00-5C65/17-00.