APJ: Impact of Resonant Compton Scattering on Magnetar X-Ray Polarization with QED Vacuum Resonance

Abstract: We present a general semi-analytical framework for analyzing energy-dependent soft X-ray polarization signature from magnetars, which consistently incorporates both QED vacuum resonance and resonant Compton scattering (RCS) effect. Starting from the polarized radiative transfer equation for RCS, we include mode conversion induced by vacuum resonance as an input; under a first-order approximation in optical depth (valid when τ ≲ 1), we evaluate the influence of magnetic twist (or plasma density), drift velocity of magnetospheric plasma, magnetospheric temperature, and viewing geometry on observed spectrum and polarization. The main results are: (i) magnetic twist (plasma density) and drift velocity are the key parameters controlling RCS re-procession on both the absolute polarization degree and its energy variance in soft X-ray radiation; (ii) in addition to QED, apparent special-relativistic effects (where plasma drift velocity β0 ≳ 0.5) in the scattering process may introduce an extra 90◦ polarization angle (PA) swing in the spectrum; (iii) single RCS has minimal effect on the energy where the initial QED-induced PA swing occurs; (iv) strong RCS (even single scattering) may wash out the PA swing caused by vacuum resonance. This framework offers a new concise paradigm without resorting to complex multidimensional Monte Carlo radiative transfer simulations, and provides a potential pathway toward the analytical treatment of full-surface emission and rotational-phase-resolved modeling of magnetar radiation to support future X-ray polarization missions.

Paper Link: https://arxiv.org/abs/2603.08119