  We are entering a new era for high energy astrophysics with the use of new
technology to increase our ability to both survey and monitor the sky. The Soft
X-ray Imager (SXI) instrument on the THESEUS mission will revolutionize
transient astronomy by using wide-field focusing optics to increase the
sensitivity to fast transients by several orders of magnitude. The THESEUS
mission is under Phase A study by ESA for its M5 opportunity. THESEUS will
carry two large area monitors utilizing Lobster-eye (the SXI instrument) and
coded-mask (the XGIS instrument) technologies, and an optical-IR telescope to
provide source redshifts using multi-band imaging and spectroscopy. The SXI
will operate in the soft (0.3-5 keV) X-ray band, and consists of two identical
modules, each comprising 64 Micro Pore Optics and 8 large-format CMOS
detectors. It will image a total field of view of 0.5 steradian instantaneously
while providing arcminute localization accuracy. During the mission, the SXI
will find many hundreds of transients per year, facilitating an exploration of
the earliest phase of star formation and comes at a time when multi-messenger
astronomy has begun to provide a new window on the universe. THESEUS will also
provide key targets for other observing facilities, such as Athena and 30m
class ground-based telescopes.
