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

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 $\sim 20\%$ 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; $F_{3-20\mathrm{keV}}^{FPMA}=2.63\times 10^{-11}\mathrm{erg}{\mathrm{s}}^{-1}\mathrm{c}{\mathrm{m}}^{-2}\mathrm{de}{\mathrm{g}}^{-2}$ and $F_{3-20\mathrm{keV}}^{FPMB}=2.58\times 10^{-11}\mathrm{erg}{\mathrm{s}}^{-1}\mathrm{c}{\mathrm{m}}^{-2}\mathrm{de}{\mathrm{g}}^{-2}$. 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.

23 pages, 15 figures, 4 tables

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