The NuSTAR X-ray Spectrum of Hercules X-1: A Radiation-Dominated Radiative Shock

We report on new spectral modeling of an observation of the accreting X-ray pulsar Her X-1 by the Nuclear Spectroscopic Telescope Array (NuSTAR). We utilize a radiation-dominated radiative shock model that is an implementation of the analytic work of Becker & Wolff (2007) on Comptonized accretion flows onto magnetic neutron stars within the XSPEC analysis environment. We obtain a good fit to the Her X-1 spin-phase averaged 4 to 78 keV X-ray spectrum observed by NuSTAR during a main-on phase of the Her X-1 35-day accretion disk precession period. This model allows us to estimate the accretion rate, the Comptonizing temperature of the radiating plasma, the radius of the magnetic polar cap, and the average scattering opacity parameters in the accretion column. This is in contrast to previous spectral models that characterized the shape of the X-ray spectrum but could not determine the physical parameters of the accretion flow. We describe the details of our spectral fitting model and we discuss the interpretation of the resulting accretion flow physical parameters.This research is supported by the NASA Astrophysics Data Analysis Program.