  The REgolith X-ray Imaging Spectrometer (REXIS) instrument on board NASA's
OSIRIS-REx mission to the asteroid Bennu is a Class-D student collaboration
experiment designed to detect fluoresced X-rays from the asteroid's surface to
measure elemental abundances. In July and November 2019 REXIS collected ~615
hours of integrated exposure time of Bennu's sun-illuminated surface from
terminator orbits. As reported in Hoak et al. (2021), the REXIS data do not
contain a clear signal of X-ray fluorescence from the asteroid, in part due to
the low incident solar X-ray flux during periods of observation. To support the
evaluation of the upper limits on the detectable X-ray signal that may provide
insights for the properties of Bennu's regolith, we present an overview of the
REXIS instrument, its operation, and details of its in-flight calibration on
astrophysical X-ray sources. This calibration includes the serendipitous
detection of the transient X-ray binary MAXI J0637-430 during Bennu
observations, demonstrating the operational success of REXIS at the asteroid.
We convey some lessons learned for future X-ray spectroscopy imaging
investigations of asteroid surfaces.
