The Unresolved Cosmic X-ray Background as Measured by NuSTAR

The extragalactic cosmic X-ray background emission (CXB) in the “hard” band (~3-300 keV) is dominated by AGN, which contains information about the accretion history over all cosmic time. While previous X-ray background missions agree on the overall shape of the CXB, the absolute calibration between each of these missions can vary from ~10-40%, due to a lack of an absolute X-ray calibration source. Furthermore, a complete census of the AGN populations that contribute to the peak emission in this range (20-30keV) can be difficult to measure due to the hydrogen column density that can surround and obscure some of the AGN. NuSTAR, with a reliable sensitivity from 3-40 keV, is well suited for making these measurements, due to an absence of shielding around the mast, which allows stray light to leak onto the detectors. This stray light process, which collects ~10 times the number of photons than the focused portion, is almost entirely from the CXB. By understanding how these photons are modulated by the geometry of the spacecraft, we can accurately and precisely measure the normalization of the CXB by eliminating the dependence on the calibration of the mirrors. The difficulties in making such a measurement lie with eliminating all other contributing sources such as instrumental, solar, and other x-ray sources that contaminate our measurements. Using ~50 Ms of observational and ~15 Ms of earth occulted data from NuSTAR’s archive, we present a preliminary measurement of the CXB through the methods described.We acknowledge support from the NuSTAR mission and from NASA Astrophysics Data Analysis Program grant 16-ADAP16-0118.