Effective area calibration of the Nuclear Spectroscopic Telescope Array

Abstract

We present here the updated calibration of the Nuclear Spectroscopic Telescope Array, which was performed using data on the Crab accumulated over the last nine years in orbit. The basis for this new calibration contains over 250 ks of focused Crab observations (imaged through the optics) and over 500 ks of stray-light (SL) Crab observations (not imaged through optics). We measured an epoch averaged spectrum of the SL Crab data and define a canonical Crab spectrum of Γ = 2.103 ± 0.001 and N = 9.69 ± 0.02 keV⁻¹ cm⁻² s⁻¹ at 1 keV, which we use as our calibration standard. This calibration released in the Calibration Data Base update 20211020 provides significant updates to: (1) the detector absorption component, (2) the detector response function, and (3) the effective area vignetting function. The calibration improves agreement between FPMA and FPMB across detectors with a standard deviation of 1.7% for repeat observations between off-axis angles of 1' to 4'. As a consequence of the measured SL observations, the absolute flux of the instrument has increased by 5% to 15%, with 5% below 1' off-axis angle, 10% between 1 and 2', and 15% above 4'.