The High Energy X-ray Probe (HEX-P): X-ray Spectro-Timing of Extragalactic Nuclear Transients

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. We will present simulations of HEX-P observations of a variety of extragalactic nuclear transient phenomena including tidal disruption events, AGN outbursts and quasi-periodic eruptions. We will show that HEX-P will (1) probe the evolution of soft X-ray continuum emission from newly formed accretion disks around supermassive black holes; (2) track the emergence of hard, non-thermal X-ray coronae, reflection features from the disk, and ultrafast outflows; (3) enable broadband X-ray timing studies to search for quasi-periodic oscillations; (4) provide independent and complementary probes of the spin of an otherwise dormant central massive black hole; (5) facilitate an unprecedented view of highly relativistic jets as they are launched; and (6) enable time-resolved spectroscopy of quasi-periodic eruptions, which are potential multi-messenger targets. More information on HEX-P, including the full team list, is available at https://hexp.org.