**3. The THESEUS Mission Concept**

The Transient High Energy Sky and Early Universe Surveyor (THESEUS) mission concept (Figure 1), developed in recent years by a large European-led collaboration involving also scientists worldwide, aims to fully exploit the unique and breakthrough potential of GRBs for investigating the Early Universe and substantially advancing multi-messenger astrophysics, while simultaneously vastly increasing the discovery space of most high energy transient phenomena over the entirety of cosmic history and allowing tests of fundamental physics [25]. THESEUS will achieve these ambitious goals through a step change in capabilities for detection and characterisation of GRBs and other transients over a very broad energy band (0.3 keV to 10 MeV) and wide field of view, including on-board near-infrared imaging and spectroscopy, and is designed to be at the forefront of these science fields in the late 2030s.

**Figure 1.** Possible spacecraft design and payload accomodation of THESEUS (Credit: ESA and THESEUS Consortium).

THESEUS will inherently be a mission enabling great synergies with the premier future observatories, providing simultaneous wide sky monitoring, rapid follow-up and real-time alerts (Figure 2). Figure 3 provides a tentative timeline of these observatories with respect to the THESEUS expected operational period. From *ELT* to *ATHENA*, *CTA* to Einstein Telescope, the Vera Rubin Observatory to the Roman Space Telescope, the science returns from combining observations with multiple facilities is a classic case of "the whole being much greater than the sum of the parts" [26]. A broad range of other science programmes will be enabled by THESEUS, including using observations of GRB emission as laboratories of ultra-relativistic matter and, e.g., for testing Lorentz invariance [5], as well as gathering statistics on large populations of other high-energy sources and transients [27]. Thus, THESEUS data will be of interest to a very wide user community, also through its open guest-observer programme.

**Figure 2.** Synergy of THESEUS with next generation very large facilities in the multi-wavelength and multi-messenger domains [26].

**Figure 3.** Planned timelines of the main failicities for Multi-Messenger Astronomy in the next decades with which THESEUS will operate in strong synergy (see Figure 2) [26].
