Polarization Evolution in A Strongly Magnetized Vacuum: QED Effect and Polarized X-ray Emission from Magnetized Neutron Stars

Abstract

X-ray photons emitted from the surface or atmosphere of a magnetized neutron star is highly polarized. However, the observed polarization may be modified due to photon propagation through the star's magnetosphere. For photon frequencies much larger than the typical radio frequency, vacuum birefringence due to strong-field quantum electrodynamics dominates over the plasma effect. We study the evolution of photon polarization in the magnetized QED vacuum of a neutron star magnetosphere, paying particular attention to the propagation effect across the quasi-tangential (QT) point, where the photon momentum is nearly aligned with the magnetic field. In agreement with previous studies, we find that in most regions of the magnetosphere, the photon polarization modes are decoupled due to vacuum birefringence, and therefore a large net linear polarization can be expected when the radiation escapes the magnetosphere. However, we show that X-ray polarization may change significantly when the photon passes through the QT region. When averaging over a finite emission area, the net effect of QT propagation is to reduce the degree of linear polarization; the reduction factor depends on the photon energy, magnetic field strength, geometry, rotation phase and the emission area, and can be more than a factor of two. We derive the general conditions under which the QT propagation effect is important, and provide an easy-to-use prescription to account for the QT effect for most practical calculations of X-ray polarization signals from magnetic neutron stars. For a neutron star with a dipole magnetic field, the QT effect can be important for emission from the polar cap for certain magnetic field and energy ranges, and is negligible for emission from the entire stellar surface.

29 pages, 17 figures, accepted by MNRAS

References in corpus (5)

• Physics of Strongly Magnetized Neutron Stars
• Corona of Magnetars
• Atmosphere Models of Magnetized Neutron Stars: QED Effects, Radiation Spectra, and Polarization Signals
• Isolated neutron stars: Magnetic fields, distances, and spectra
• Wave Modes in the Magnetospheres of Pulsars and Magnetars

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• Radiative properties of magnetic neutron stars with metallic surfaces and thin atmospheres
• IXPE Detection of Polarized X-rays from Magnetars and Photon Mode Conversion at QED Vacuum Resonance
• Physics in Very Strong Magnetic Fields: Introduction and Overview
• Nonlinear QED effects in X-ray emission of pulsars
• X-ray Polarimetry of the accreting pulsar 1A~0535+262 in the supercritical state with PolarLight
• Polarized X-rays from Magnetized Neutron Stars
• Criticality and dominance of axion physics in highly magnetized vacuum