Measuring the Unresolved Cosmic X-ray Background with NuSTAR

The extragalactic cosmic X-ray background (CXB) in the hard band (~3-300keV) encodes the accretion history over all cosmic time of the super massive black holes found in the centers of active galactic nuclei (AGN). Measurements of the flux and detailed spectrum above 7keV from previous missions disagree at the 10-40% level, while the AGN population production at the peak emission (20-30keV) is poorly understood. The X-ray telescope NuSTAR can measure the CXB through its unique design and sensitivity in the band pass of 3-40keV. In order to reliably measure this emission with NuSTAR, certain challenges must be overcome: contamination from local sources, environmental conditions in the satellite’s orbit including activation/fluorescent lines from hard X-rays and soft cosmic rays, and a solar component. This solar component has features of uncertain origin, can dominate the CXB at energies below 7keV, and can vary over time, making it difficult to model. Stacking data when NuSTAR is pointed at both the day and night side of Earth isolates the effective solar spectrum, allowing us to derive an empirical model and keeps it from biasing our future measurement of the CXB.