In-flight PSF calibration of the NuSTAR hard X-ray optics

TL;DR

This paper details the calibration of NuSTAR's hard X-ray optics PSF using in-orbit observations and ray-trace models, improving accuracy and confirming stability over 300 days.

Contribution

It introduces empirical corrections to the ray-trace model for better PSF calibration of NuSTAR's optics in orbit.

Findings

Empirical corrections significantly improve the ray-trace model fit.

Uncertainty in the PSF's enclosed energy fraction is less than 3%.

The PSF remained stable over approximately 300 days.

Abstract

We present results of the point spread function (PSF) calibration of the hard X-ray optics of the Nuclear Spectroscopic Telescope Array (NuSTAR). Immediately post-launch, NuSTAR has observed bright point sources such as Cyg X-1, Vela X-1, and Her X-1 for the PSF calibration. We use the point source observations taken at several off-axis angles together with a ray-trace model to characterize the in-orbit angular response, and find that the ray-trace model alone does not fit the observed event distributions and applying empirical corrections to the ray-trace model improves the fit significantly. We describe the corrections applied to the ray-trace model and show that the uncertainties in the enclosed energy fraction (EEF) of the new PSF model is < 3% for extraction apertures of R > 60" with no significant energy dependence. We also show that the PSF of the NuSTAR optics has been stable over a period of ~300 days during its in-orbit operation.