Measuring the Cosmic X-ray Background in 3-20keV with Straylight from NuSTAR

TL;DR

This study measures the cosmic X-ray background in the 3-20 keV range using NuSTAR data, carefully modeling stray light contributions to obtain results consistent with historic measurements and highlighting the importance of systematic uncertainty analysis.

Contribution

The paper introduces a novel method for characterizing stray light in NuSTAR data to accurately measure the cosmic X-ray background in the 3-20 keV range.

Findings

Measured CXB flux values are consistent with HEAO 1 results.

Results are approximately 10% lower than most recent measurements.

Validated the fitting method using Crab observations.

Abstract

By characterizing the contribution of stray light to large datasets from the NuSTAR X-ray observatory collected over 2012--2017, we report a measurement of the cosmic X-ray background in the 3--20 keV energy range. These data represent $\sim20\%$ sky coverage while avoiding Galactic Ridge X-ray emission and are less weighted by deep, survey fields than previous measurements with NuSTAR. Images in narrow energy bands are stacked in detector space and spatially fit with a model representing the stray light and uniform pattern expected from the cosmic X-ray background and the instrumental background, respectively. We establish baseline flux values from Earth-occulted data and validate the fitting method on stray light observations of the Crab, which further serve to calibrate the resulting spectra. We present independent spectra of the cosmic X-ray background with the FPMA and FPMB detector arrays, which are in excellent agreement with the canonical characterization by HEAO 1 and are $10\%$ lower than most subsequent measurements; $F_{\rm{3-20~keV}}^{FPMA} = 2.63 \times 10^{-11}~\rm{erg~s^{-1}~cm^{-2}~deg^{-2}}$ and $F_{\rm{3-20~keV}}^{FPMB} = 2.58 \times 10^{-11}~\rm{erg~s^{-1}~cm^{-2}~deg^{-2}}$. We discuss these results in light of previous measurements of the cosmic X-ray background and consider the impact of systematic uncertainties on our spectra.