Transient High-Energy Sky and Early Universe Surveyor (THESEUS) is aspace telescope mission proposal by theEuropean Space Agency that would studygamma-ray bursts andX-rays for investigating theearly universe.^[1][3] If developed, the mission would investigatestar formation rates andmetallicity evolution, as well as studying the sources and physics ofreionization.

THESEUS is a mission concept that would monitor transient events in the high-energy Universe across the whole sky and over the entirety ofcosmic history. In particular, it expects to make a complete census ofgamma-ray bursts (GRBs) from the Universe's first billion years, to help understand the life cycle of the first stars.^[4] THESEUS would provide real-time triggers and accurate locations of the sources, which could also be followed up by other space- or ground-based telescopes operating at complementary wavelengths.

The concept was selected in May 2018 as a finalist to become the fifth Medium-class mission (M5) of theCosmic Vision programme by theEuropean Space Agency (ESA). The other finalist wasEnVision, a Venus orbiter. The winner,EnVision, was selected in June 2021 for launch in 2031.^[5]

In November 2023, following a new selection process (2022) and a Phase-0 study (2023), THESEUS was selected by ESA for a new 2.5 year Phase-A study as one of the three candidates M7 missions (together with M-Matisse and Plasma Observatory).

The space observatory would study GRBs andX-rays and their association with the explosive death of massive stars,supernova shock break-outs,black holetidal disruption events, andmagnetar flares. This can provide fundamental information on the cosmic star formation rate, the number density and properties of low-mass galaxies, theneutral hydrogen fraction, and the escape fraction ofultravioletphotons from galaxies.^[1]

The conceptual payload of THESEUS includes:^[1]

• Soft X-ray Imager (SXI), sensitive to 0.3-6 keV is a set of 4lobster-eye telescope units, covering a totalfield of view (FOV) of 1 sr with source location accuracy <1-2 arcmin.
• InfraRed Telescope (IRT), sensitive to 0.7-1.8 μm is a 0.7 m NIR telescope with 15x15 arcmin FOV, for fast response, with both imaging and moderate spectroscopic capabilities (R~400). Mass: 112.6 kg.
• X-Gamma ray Imaging Spectrometer (XGIS), sensitive to 2 keV-20 MeV, is a set ofcoded-mask cameras using monolithic X-gamma ray detectors based on bars of silicon diodes coupled withCsI crystalscintillator, granting a 1.5 sr FOV, a source location accuracy of 5 arcmin in 2-30 keV and an unprecedentedly broad energy band. Mass: 37.3 kg.

• Gamma-ray astronomy
• List of proposed space observatories
• X-ray astronomy

• Cosmic Vision
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