X-rays from highly magnetic neutron stars reveal strange ``photon metamorphosis'' phenomenon

What is it about?

-- Background: Magnetars are neutron stars (NSs) whose energy outputs are dominated by the dissipation of super-strong magnetic fields, with field strength of $10^{14}$ Gauss or more (that is 100 trillion times more than Earth's surface magnetic field). The surface of a magnetar is hot (a few million degree Kelvin) and emits X-rays. Because of the strong magnetic field, these X-rays are expected to be highly polarized, i.e., the electric field of the X-ray radiation has a preferred direction rather than being random. Recently the NASA/ASI Imaging X-ray Polarimetry Explorer (IXPE) reported the detection of linearly polarized x-ray emission from a magnetar called AXP 4U 0142+61. This is the first time that polarized x-rays have been detected from any astrophysical point sources. Interestingly, the observation shows that there is a substantial variation of the polarization signal with the photon energy. In particular, the polarization (electric field) direction changes (or swings) by 90 degrees from low energy (2-4~keV) to high energy (5.5-8~keV). This is very puzzling. --- What is the paper about? We show that this 90-degree swing can be explained by photon polarization conversion (``photon metamorphosis'') at the so-called vacuum resonance in the magnetar's thin atmosphere, which consists of hot, magnetized plasma. The resonance arises from the combined effects of plasma-induced birefringence and QED-induced vacuum birefringence in strong magnetic fields. (``Birefringence'' is a general term that refers to the optical property of a material where the light travel speed depends on the polarization and propagation direction; many mineral crystals are birefringent.) -- In a bit more detail: Quantum electrodynamics (QED) governs the microscopic interactions between electrons and photons, and is one of the most successful physics theory ever developed. Although a photon (which has no charge) does not directly interact with an external magnetic field (e.g. that produced by a magnetar), it can temporarily convert into a pairs of electron and positron, and therefore becomes affected by the magnetic field, even in vacuum. This is the ``vacuum birefringence''. When a photon propagates in the atmosphere plasma, it also experience ``plasma birefringence'' since the photon affects the electrons in the plasma and the electron's motion is influenced by the external magnetic field. It turns out that the ``vacuum birefringence'' and ``plasma birefringence'' operate in an orthogonal way, such that at the vacuum resonance, the effects cancel each other. As a result, when a linearly polarized photon travels across the resonance, its direction of linear polarization can change by 90 degree. This photon polarization conversion/metamorphosis lies at the heart of the observed X-ray polarization swing from AXP 4U 0142+61. Very recently, IXPE observed polarized X-rays from from another magnetar, AXP 1RXS J170849.0-400910. This magnetar does not show the $90^\circ$ polarization swing. In this case, because the magnetar magnetic field is stronger (larger than $5\times 10^{14}$~G), the polarization conversion at vacuum resonance takes place very deep in the neutron star atmosphere, and therefore the emitted X-rays do not exhibit the 90-degree swing.

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Photo by Bryan Goff on Unsplash

Photo by Bryan Goff on Unsplash

Why is it important?

The photon polarization conversion at the vacuum resonance provides a natural explanation of the 90-degree polarization swing observed in AXP 4U 0142+61. It demonstrates a beautiful effect QED (``vacuum birefringence'') which has never been directly observed. It also puts useful constraints on the neutron star. The calculation reported in the PNAS paper suggests that the atmosphere of AXP 4U 0142 is composed of partially ionized heavy elements, and the surface magnetic field be comparable or less than $10^{14}$~G, consistent with other indirect constraint. It also implies that the spin axis of AXP 4U 0142+61 is aligned with its velocity direction -- This has interesting implications for the formation of the neutron star. More generally, the photon polarization conversion has many analogies in other areas of sciences. For example, it is is analogous to the Mikheyev-Smirnov-Wolfenstein neutrino oscillation that takes place in the Sun, the Landau-Zener transition in atomic physics, and electromagnetic wave propagation in inhomogeneous media and metamaterials.

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