NICER Calibration Overview

NICER is an X-ray observatory on the International Space Station, launched in 2017. It consists of 52 independently-operating single-pixel X-ray telescopes. Each of them is calibrated independently to achieve accurate spectral and timing performance. Here we present an overview of the NICER calibration status. The calibrated NICER energy range is 0.25-10 keV. Over that range the energy scale is calibrated to approximately 5 eV. The NICER effective area curve is calibrated to the Crab nebula and pulsar, and detector residuals are less than 1.5%. In comparison to NuSTAR’s Crab flux measurement, NICER agrees at the ~5% level. Off-axis sources are properly accounted for by NICER’s software. The array-average resolution at 6.4 keV is 143 eV (FWHM). Both resolution and low energy threshold depend on optical loading, which is also accounted for. As of HEASoft 6.31 (November 2022), three background models (SCORPEON, 3C50, and Space Weather) are included in NICER’s standard software and calibration products.