The High Energy X-ray Probe: Diagnosing compact objects and their accretion flows with spectral-timing techniques

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. The X-ray signal from an accreting compact object has encoded within it the properties of the object itself and the extreme gravitational field that surrounds it. However, the X-ray emitting region is far too small to directly image. Spectral-timing techniques circumvent this problem by using the observed rapid variability of the X-ray spectrum to indirectly map the inner regions of the accretion flow. We will present simulations of HEX-P observations of accreting black holes to demonstrate the dramatic improvement in spectral-timing diagnostics that can be made over current instrumentation. We will demonstrate the ability of HEX-P to measure black hole mass and spin using X-ray reverberation mapping – a technique that exploits the light-crossing delay between X-rays that reach us directly from the inner accretion flow and those that take a longer path by first reflecting off the accretion disk. We will demonstrate the constraints that the same technique can achieve on the distance to accreting black holes, thus providing an independent estimate of the Hubble constant by measuring the distance to nearby active galactic nuclei. Finally, we will show the insights into the mechanism driving the quasi periodic oscillations (QPOs) observed in the flux of black hole X-ray binaries that can be achieved by determining how the X-ray spectrum evolves with QPO phase. More information on HEX-P, including the full team list, is available at https://hexp.org.