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

Abstract

The recent observation of the anomalous X-ray pulsar 4U 0142+61 by the Imaging X-ray Polarimetry Explorer (IXPE) opened up a new avenue to study magnetars, neutron stars endowed with superstrong magnetic fields (B 10 14 G). The detected polarized X-rays exhibit an intriguing 90 • linear polarization swing from low photon energies (E 4 keV) to high photon energies (E 5.5 keV). We show that this swing can be naturally 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, and the mode conversion is analogous to the MSW effect for neutrino oscillations. This explanation suggests that the atmosphere of 4U 0142 be composed of partially ionized heavy elements (such as Fe), 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.