Polarization of accreting X-ray pulsars – II. Hercules X-1

We employ our new model for the polarized emission of accreting X-ray pulsars to describe the emission from the luminous X-ray pulsar Hercules X-1. In contrast with previous works, our model predicts the polarization parameters independently of spectral formation, and considers the structure and dynamics of the accretion column, as well as the additional effects on propagation due to general relativity and quantum electrodynamics. We find that our model can describe the observed pulse fraction and the pulse shape of the main peak, as well as the modulation of the cyclotron line with phase. We pick two geometries, assuming a single accretion column or two columns at the magnetic poles, that can describe current observations of pulse shape and cyclotron modulation with phase. Both models predict a high polarization fraction, between 60 and 80 per cent in the 1–10 keV range, that is phase and energy dependent, and that peaks at the same phase as the intensity. The phase and energy dependence of the polarization fraction and of the polarization angle can help discern between the different geometries. ; © 2020 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). Accepted 2020 October 29. Received 2020 October 29; in original form 2020 May 27. Published: 05 November 2020. We would like to thank Michael Wolff, Peter Becker, and the XMAG collaboration for valuable input, and Sterl Phinney for useful comments. The research was supported by NSERC Canada, Compute Canada, a Burke Fellowship at Caltech, and a Four-Year Fellowship at UBC. Data Availability: The values for the predicted polarization fraction and angle as function of phase and energy displayed in Figs 6 and 7 can be reproduced following the equations introduced in Paper I, and are also ...