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

TL;DR

This study presents a detailed physical model of the X-ray spectrum of Hercules X-1, revealing insights into the accretion flow and plasma conditions near the neutron star, advancing understanding beyond phenomenological descriptions.

Contribution

We develop and apply a radiation-dominated radiative shock model based on Becker & Wolff's analytic work, enabling physical parameter estimation of the accretion flow in Hercules X-1.

Findings

Good fit to 4-78 keV spectrum during main-on phase

Estimated accretion rate and magnetic polar cap radius

Derived plasma temperature and scattering opacity parameters

Abstract

We report new spectral modeling of the accreting X-ray pulsar Hercules X- 1. Our radiation-dominated radiative shock model is an implementation of the analytic work of Becker & Wolff on Comptonized accretion flows onto magnetic neutron stars. We obtain a good fit to the spin-phase averaged 4 to 78 keV X-ray spectrum observed by the Nuclear Spectroscopic Telescope Array 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 phenomenological models that characterized the shape of the X-ray spectrum but could not determine the physical parameters of the accretion flow. We describe the spectral fitting details and discuss the interpretation of the accretion flow physical parameters.