The 32nd Texas Symposium on Relativistic Astrophysics

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

Not scheduled

20m

Tsung-Dao Lee Institute

Contributed talk in mini symposium

Speaker

Dong Lai (Cornell University; TDLI)

Description

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 ($B> 10^{14}$ G). The detected polarized
X-rays from 4U 0142+61 exhibit a 90 degree linear polarization
swing from low photon energies ($E<4$ keV) to high
energies ($E> 5.5$ 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 $90$ degree swing, consistent with magnetar atmospheric
emission with $B>5\times 10^{14}$ G.