Calibration and Performance of the REgolith X-Ray Imaging Spectrometer (REXIS) Aboard NASA's OSIRIS-REx Mission to Bennu

TL;DR

The paper discusses the calibration, operation, and lessons learned from the REXIS instrument on NASA's OSIRIS-REx mission, which aimed to detect asteroid surface X-ray fluorescence but faced challenges due to low solar X-ray flux.

Contribution

It provides an overview of REXIS calibration, in-flight performance, and insights for future asteroid X-ray spectroscopy missions.

Findings

REXIS successfully operated and detected a transient X-ray binary.

No clear X-ray fluorescence signal from Bennu was observed.

Lessons learned for future asteroid surface X-ray studies.

Abstract

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.