Characterization of the Background Components of NuSTAR to Measure the Cosmic X-Ray Background in the 3-30 keV Energies

By characterizing the contribution of stray light to a large ~8 Ms and ~18 Ms dataset from the NuSTAR X-ray Observatory collected over 2012-2017, we report a measurement the cosmic X-ray background in the 3-30 keV energy range. These data represent 20% sky coverage while avoiding Galactic Ridge X-ray Emission and are less weighted by deep, contiguous survey fields than previous measurements with NuSTAR. Images in narrow energy bands are then stacked in detector space and spatially fit with a model representing the stray light pattern expected from the cosmic X-ray background and the instrumental background. 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 from HEAO-1, with 10% lower flux than most subsequent measurements. We discuss these results in light of previous measurements of the cosmic X-ray background and the impact of systematic uncertainties on our spectra.