The High-Energy X-ray Probe (HEX-P): Constraining Black Hole Growth History With Population Spin Measurements

HEX-P is a probe-class mission concept that will combine high spatial resolution X-ray imaging (<10 arcsec FWHM) and broad spectral coverage (0.1-150 keV) with an effective area far superior to current facilities (including XMM-Newton and NuSTAR), to enable revolutionary new insights into a variety of important astrophysical problems. Among these, constraining the primary growth channel of supermassive black holes (SMBH) remains one the most actively debated questions in the context of cosmological structure formation. The nature of their growth history dictates the expected SMBH spin distribution, and thus SMBH spin measurements offer a rare observational window into this question. With its broad bandpass and high throughput, HEX-P is an excellent probe for reflection spectroscopy and reverberation mapping, promising to deliver spin measurements for large samples of SMBHs at redshift z=0 and beyond. Here, we investigate the capability of HEX-P to distinguish between different primary SMBH growth channels, based on the distinct mass vs spin distributions predicted for these channels by state-of-the-art cosmological simulations. More information on HEX-P, including the full team list, is available at https://hexp.org.