Performance and Results of the Regolith X-ray Imaging Spectrometer (REXIS)

The Regolith X-ray Imaging Spectrometer (REXIS) is a student-built instrument onboard NASA’s OSIRIS-REx spacecraft. REXIS was designed to measure the relative elemental abundances of iron, magnesium, silicon, sulfur, and oxygen on the surface of asteroid (101955) Bennu through solar-induced x-ray fluorescence. REXIS incorporates novel technologies for a planetary science x-ray experiment: CCD detectors with sensitivity below 0.5keV and a coded-aperture mask to enable imaging analysis of the elemental distribution across the surface of the asteroid.

In July and November 2019, REXIS collected approximately 617 hours of data with the asteroid in the instrument field of view. During these data-taking periods, REXIS encountered several data quality issues which complicated the analysis, but serendipitously observed the astrophysical x-ray transient MAXI J0637-430 from around the limb of the asteroid, demonstrating the sensitivity of the REXIS CCDs. In this talk, we describe the performance of the REXIS instrument, its calibration and response modeling, and the results of our observational campaign and possible implications for the nature of asteroid Bennu.