AGN Synthesis of the Cosmic X-ray Background in the Era of NuSTAR

The origin of the cosmic X-ray background (CXB) has been a central problem in high-energy astrophysics since its discovery by Riccardo Giacconi and colleagues in rocket flights in the 1960’s. The CXB is now known to be comprised mainly of emission from individual active galactic nuclei (AGN), the majority of which must be heavily obscured in order to reproduce the observed peak of the background at ~30 keV. With the launch of NuSTAR, we can now directly probe the emission from individual AGN at these peak energies, and begin to perform direct measurements of the synthesis of the CXB. I will present the results of a stacking analysis of AGN in the NuSTAR extragalactic surveys, and show that known AGN detected by NuSTAR or Chandra account for the majority of the CXB at energies up to 24 keV. I will compare these results to the predictions of recent CXB synthesis models (one based on a complete treatments of the X-ray AGN population, another utilizing semi-numerical prescription of galaxy and black hole formation). These CXB synthesis models show excellent agreement with the NuSTAR observations, providing an important step toward a complete understanding of the origin of the CXB. This work was supported in part by NASA through award numbers NNX15AU32H and NNX15AU32H, and the National Science Foundation through award number 1554584.