*3.1. Scientific Goals and Requirements*

The scientific goals for the exploration of the early Universe require the detection, identification, and characterization of several tens of long GRBs occurring in the first billion years of the Universe (*z* > 6) within the 4 years of nominal mission lifetime of THESEUS [28]. This would be a giant leap with respect to what has been obtained in the last 20 years (8 GRBs at *z* > 6), using past and current GRB dedicated experiments like *Swift/BAT*, *Fermi/GBM*, *Konus-WIND* combined with intensive follow-up programs from the ground with small robotic and large telescopes (e.g., VLT). This breakthrough performance can be achieved by overcoming the current limitations through an extension of the GRB monitoring passband to the soft X-rays with an increase of at least one order of magnitude in minimum detectable flux with respect to previously-flown wide-field X-ray monitors. As well, a substantial improvement of the efficiency of counterpart detection, spectroscopy and redshift measurement will be enabled through prompt on-board near-infrared (NIR) follow-up observations (Table 1).

At the same time, the goals for multi-messenger astrophysics and time domain astronomy require:(i) a substantial advance in the detection and localization, over a large (>2 sr) FoV of short GRBs as electromagnetic counterparts of GW signals coming from BNS, and possibly NS-BH mergers; (ii) monitoring the high-energy sky with an unprecedented combination of sensitivity, location accuracy and field of view in the soft X-rays; iii) imaging up to the hard X-rays and spectroscopy/timing of the soft gamma-rays [27,29,30].

**Table 1.** Key science performance requirements of THESEUS<sup>1</sup> . The sensitivity requirements assume a power-law spectrum with a photon index of 1.8 and an absorbing column density of <sup>5</sup> <sup>×</sup> <sup>10</sup><sup>20</sup> cm−<sup>2</sup> .

