2021 Effective Area calibration of the Nuclear Spectroscopic Telescope ARray (NuSTAR)

TL;DR

This paper presents an updated calibration of the NuSTAR X-ray telescope based on 9 years of Crab data, improving detector response, effective area, and flux measurements for more accurate astrophysical observations.

Contribution

The paper introduces a new calibration standard for NuSTAR using extensive Crab data, updating key response functions and effective area models for enhanced measurement accuracy.

Findings

Calibration standard for Crab spectrum established

Flux measurements increased by 5-15% depending on off-axis angle

Improved agreement between detector modules FPMA and FPMB

Abstract

We present here the updated calibration of The Nuclear Spectroscopic Telescope ARray NuSTAR, which was performed using data on the Crab accumulated over the last 9 years in orbit. The basis for this new calibration contains over 250ks of focused Crab (imaged through the optics) and over 500ks of stray-light Crab (not imaged through optics). We measured an epoch averaged Crab spectrum of the stray-light Crab data and define a canonical Crab spectrum of Gamma = 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. The new calibration, released in the CALDB 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 1-4 arcmin, and the measured flux has increased by 5-15%, with 5% below 1 arcmin off-axis angle, 10% between 1-2 arcmin, and 15 above 4arcmin.