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

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.

8 pages. Updated to include discussions on both magnetars detected by IXPE. To appear in PNAS in 4/2023

References in corpus (8)

• Atmosphere Models of Magnetized Neutron Stars: QED Effects, Radiation Spectra, and Polarization Signals
• Supernova Fallback as Origin of Neutron Star Spins and Spin-kick Alignment
• Polarized x-rays from a magnetar
• Density-Functional-Theory Calculations of Matter in Strong Magnetic Fields: II. Infinite Chains and Condensed Matter
• X-ray spectra and polarization from magnetar candidates
• The Radius of PSR J0740+6620 from NICER and XMM-Newton Data
• Probing magnetar emission mechanisms with spectropolarimetry
• Pair Plasma in Super-QED Magnetic Fields and the Hard X-ray/Optical Emission of Magnetars

Cited by in corpus (9)

• Studying geometry of the ultraluminous X-ray pulsar Swift J0243.6+6124 using X-ray and optical polarimetry
• X-ray Polarisation in Magnetar Atmospheres -- Effects of Mode Conversion
• Prospects for Time-Domain and Multi-Messenger Science with eXTP
• X-ray polarisation signatures in bombarded magnetar atmospheres
• Vacuum birefringence and the polarized X-ray emission from a radio magnetar
• Monte Carlo Simulations of Polarized Radiative Transfer in Neutron Star Atmospheres
• Photon polarization tensor at finite temperature and density in a magnetic field
• Pulsed, Polarized X-ray Emission from Neutron Star Surfaces: the Effects of Vacuum Birefringence in the Magnetosphere
• Resonant Inverse Compton Scattering and Hard X-ray Emission in Magnetar Magnetospheres