The High Energy X-ray Probe (HEX-P): Unveiling Accretion Around Neutron Stars

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. HEX-P will characterize X-ray binary systems over an unprecedented range of fluxes, energies, and time-scales, allowing us to answer questions about binary evolution, neutron star physical properties, and feedback into their environment. We present spectral simulations of accreting neutron stars to demonstrate the power of the HEX-P observatory. We show that HEX-P will (1) detect cyclotron line features above 80 keV, directly measuring B-fields stronger than ever before; (2) provide constraints on neutron star radii with higher accuracy using X-ray reflection modeling; (3) yield unique information about the accretion geometry and accretion state in these systems for the whole range of mass accretion rates from ULX-like luminosities to the onset of the propeller effect or quiescence; and (4) characterize the diverse environments these systems live in, including stellar winds and dust shrouds, intervening Be-star disks or warped accretion disks, as well as outflows. More information on HEX-P, including the full team list, is available at hexp.org.