Decoding the Quantum Riddle of Magnetar X-Rays

Astrophysicist Dong Lai postulates that a quantum electrodynamics (QED) process known as “photon metamorphosis” is the reason behind the unexpected X-ray polarization detected from a magnetar, which is a neutron star with a powerful magnetic field. Lai’s proposition hints that X-ray photons travelling through the magnetar’s magnetized atmosphere might temporarily morph into pairs of “virtual” electrons and positrons, causing variances in the polarizations for low and high-energy X-rays.

A wondrous effect predicted by quantum electrodynamics (QED) might hold the answer to the baffling initial detection of polarized X-rays originating from a magnetar – a neutron star noted for its immense magnetic field strength, suggests a Cornell astrophysicist.

This neutron star, the dense and hot leftover of a giant star, wielding a magnetic field a hundred trillion times stronger than Earth’s, was anticipated to produce unique polarized X-rays, implying that the radiation’s electromagnetic field does not vibrate randomly but rather prefers a specific direction.

However, researchers were taken aback when NASA’s Imaging X-ray Polarimetry Explorer (IXPE) satellite discovered last year that the lower- and higher-energy X-rays demonstrated different polarizations, with their electromagnetic fields arranged perpendicularly to each other.

This occurrence can be logically justified by “photon metamorphosis” – a transformation of X-ray photons that has been theorized but yet to be directly observed, according to Dong Lai, the Benson Jay Simon ’59, MBA ’62, and Mary Ellen Simon, M.A. ’63, Professor of Astrophysics in the College of Arts and Sciences.

“In this observation of radiation from a distant cosmic entity, we discern a magnificent effect that is a testament to the complexity of fundamental physics,” stated Lai. “QED is among the most successful physics theories, but it hasn’t been scrutinized under such intense magnetic field conditions.”

Lai recently penned a study that was featured in the Proceedings of the National Academy of Sciences.

This research is a continuation of calculations Lai and Wynn Ho, Ph.D. ’03, brought forth 20 years ago, merging observations NASA revealed last November of the magnetar 4U 0142+61, situated 13,000 light-years away in the Cassiopeia constellation.

Quantum electrodynamics, the theory that details minuscule interactions between electrons and photons, postulates that as X-ray photons leave the neutron star’s thin atmosphere of hot, magnetized gas, or plasma, they undergo a phase known as vacuum resonance.

In this phase, Lai explained, photons can briefly transform into pairs of “virtual” electrons and positrons that are impacted by the magnetar’s super-strong magnetic field, even in a vacuum, through a process called “vacuum birefringence”. Together with a related process, plasma birefringence, conditions are set for the polarity of high-energy X-rays to pivot 90 degrees relative to low-energy X-rays, as per Lai’s analysis.

“Consider the polarization as two flavors of photons,” he suggested. “A photon abruptly switching from one flavor to another – this is not a common occurrence. But it’s a natural outcome of the physics when you apply the theory under these extreme conditions.”

The IXPE mission did not witness the polarization swing in observations of another magnetar, 1RXS J170849.0-400910, with a stronger magnetic field. Lai noted that this aligns with his calculations, which imply that vacuum resonance and photon metamorphosis would transpire deep within such a neutron star.

Lai’s interpretation of IXPE’s observations of the magnetar 4U 0142+61 has contributed to defining its magnetic field and rotation, and indicated that its atmosphere is probably composed of partially ionized heavy elements.

Further exploration of X-rays from some of the universe’s most extreme entities, including neutron stars and black holes, allows scientists to examine the behavior of matter under conditions impossible to reproduce in laboratories, and enhances our comprehension of the universe’s splendor and diversity.

“IXPE’s observations have created a new avenue for exploring the surface environment of neutron stars,” stated Lai. “This will pave the way for new revelations about these enigmatic objects.”

Reference: “IXPE detection of polarized X-rays from magnetars and photon mode conversion at QED vacuum resonance” by Dong Lai, 18 April 2023, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2216534120

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Frequently Asked Questions (FAQs) about Photon Metamorphosis

More about Photon Metamorphosis

• Quantum Electrodynamics (Wikipedia)
• Magnetar (Wikipedia)
• NASA’s Imaging X-ray Polarimetry Explorer (IXPE)
• Proceedings of the National Academy of Sciences
• Research Paper: “IXPE detection of polarized X-rays from magnetars and photon mode conversion at QED vacuum resonance” by Dong Lai