IXPE Detection of Polarized X-rays from Magnetars and Photon Mode Conversion at QED Vacuum Resonance

TL;DR

This paper explains the observed polarization swing in X-ray emissions from magnetars as a result of photon mode conversion at vacuum resonance, influenced by strong magnetic fields and plasma effects, providing insights into magnetar atmospheres and magnetic field strengths.

Contribution

It introduces a model of photon polarization mode conversion at vacuum resonance to explain X-ray polarization observations of magnetars, linking polarization features to magnetic field strength and atmospheric composition.

Findings

Polarization swing explained by vacuum resonance mode conversion.

Atmospheric composition likely includes partially ionized heavy elements.

Magnetic field estimates are consistent with spin-down measurements.

Abstract

The recent observations of the anomalous X-ray pulsars 4U 0142+61 and 1RXS J170849.0-400910 by the Imaging X-ray Polarimetry Explorer (IXPE) opened up a new avenue to study magnetars, neutron stars endowed with superstrong magnetic fields ($B\gtrsim 10^{14}$~G). The detected polarized X-rays from 4U 0142+61 exhibit a 90$^\circ$ linear polarization swing from low photon energies ($E\lesssim 4$~keV) to high energies ($E\gtrsim 5.5$~keV). We show that this swing can be explained by photon polarization mode conversion at the vacuum resonance in the magnetar atmosphere; the resonance arises from the combined effects of plasma-induced birefringence and QED-induced vacuum birefringence in strong magnetic fields. This explanation suggests that the atmosphere of 4U 0142 be composed of partially ionized heavy elements, and the surface magnetic field be comparable or less than $10^{14}$~G, consistent with the dipole field inferred from the measured spindown. It also implies that the spin axis of 4U 0142+61 is aligned with its velocity direction. The polarized X-rays from 1RXS J170849.0-400910 do not show such $90^\circ$ swing, consistent with magnetar atmospheric emission with $B\gtrsim 5\times 10^{14}$~G.