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

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. (Results from the REgolith X-ray Imaging Spectrometer (REXIS) at Bennu, 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.

Appendices

Appendix A: Angular Resolution and Point Source Localization in Coded-Aperture Imaging

The angular resolution ($\Delta \theta$) and the localization error ($\delta \theta$) of a point source in coded-aperture imaging are given as

$$\begin{aligned} \Delta \theta = & \tan ^{-1}\left ( \frac{\sqrt{\Delta m^{2}+ \Delta d^{2}}}{s}\right ), \\ \delta \theta \sim & \frac{\Delta \theta }{\sigma }, \end{aligned}$$

where $\Delta m$ is the mask pixel size, $\Delta d$ the detector pixel size, $s$ the separation from the mask and detector, and $\sigma$ the signal-to-noise ratio (SNR) of the point source.

The upper limit in the localization error is set by the minimum SNR (≳5) needed to confidently claim a detection. Coded aperture imagers with a random mask pattern such as REXIS also have a upper limit in the SNR they can achieve due to the coding noise arising from the random nature of the mask pattern.

The upper limit in the SNR from a detector plane image is the square root of the number of mask pixels that the detector plane image can capture. For REXIS, the upper limit in SNR ranges from ∼ 18 to ∼ 25, depending the number of nodes used in the detector plane image. This in turn sets the lower limit in the localization error. Even without the coding noise, there is a fundamental systematic error in localization, which originates from the finite detector pixel size (25″) and the pointing jitter ($\sim 10''$). Thus, the localization error of REXIS would be limited to ≳ 30″, unless algorithms for the subpixel randomization and active jitter compensation are employed.

Appendix B: Modeling the CCD Response of the Mn-K_α Line

The response $f(E)$ of the 5.9 keV Mn-K_α line in Fig. 7 can be described by a Gaussian function and an exponential low-energy tail component:

$$\begin{aligned} f(E) = A_{1} \exp \left (- \frac{(E - E_{0})^{2}}{\Delta _{1}^{2}/\log 16 }\right ) & \\ + A_{2} \bar{\theta }(E-E_{0}) e^{C_{0} (E - E_{0})} \left [ 1 - \exp \left (-\frac{(E - E_{0})^{2} }{ \Delta _{2}^{2}/ \log 16} \right ) \right ] &, \end{aligned}$$

where $A_{1}$ and $A_{2}$ are the normalization constants, $E_{0}$ the line energy, $\Delta _{1}$ and $\Delta _{2}$ the FWHM of the Gaussian and tail components, $C_{0}$ the exponential decay parameter. $\bar{\theta }(x)$ is a reverse step function, where

$$\begin{aligned} \bar{\theta }(x) = \textstyle\begin{cases} 1,& \text{if } x \leq 0 \\ 0, & \text{otherwise.} \end{cases}\displaystyle \end{aligned}$$

Appendix C: Pointing Coordinates of the Crab Nebula and Sco X-1 Observations

Table 6 lists the boresight coordinates of the Crab Nebula observations during the Crab Calibration Operation and the Sco X-1 observations during Mask Calibration Operation, respectively. See Fig. 10.