Unveiling the puzzling phenomenon of magnetar X-rays

Magnetar magnetic star illustration

Astrophysicist Dong Lai theorizes that a quantum electrodynamics (QED) effect called “photon metamorphosis” explains the unexpected observations of X-ray polarization from a magnetar, a neutron star with an intense magnetic field. Lai’s theory suggests that X-ray photons passing through the magnetized atmosphere of the magnetar can temporarily transform into “virtual” pairs of electrons and positrons, leading to different polarizations for low- and high-energy X-rays.

A “magnificent effect” predicted by quantum electrodynamics (QED) may provide an explanation for the baffling initial observations of polarized X-rays radiating from a magnetar – a type of neutron star characterized by an immensely powerful magnetic field, according to a Cornell astrophysicist.

The extremely dense and hot remnant of a massive star, equipped with a magnetic field that exceeds that of the Earth by 100,000 billion times, was predicted to produce sharply polarized X-rays. This means that the electromagnetic field of the radiation does not vibrate randomly but has a preferred direction.

But scientists were surprised when NasaLast year, the Imaging X-ray Polarimetry Explorer (IXPE) satellite detected that low- and high-energy X-rays were polarized differently, with electromagnetic fields oriented at right angles to each other.

The phenomenon is naturally explained byphoton metamorphosis” – a transformation of X-ray photons that has been theorized but never directly observed, said Dong Lai, Ph.D. ’94, the Benson Jay Simon ’59, MBA ’62, and Mary Ellen Simon, MA ’63, professor of astrophysics at the College of Arts and Sciences.

“In this observation of radiation from a distant celestial object, we see a beautiful effect that is a manifestation of complex and fundamental physics,” Lai said. “QED is one of the most successful physical theories, but it had not been tested under such strong magnetic field conditions.”

Lai is the author of a recent study published in Proceedings of the National Academy of Sciences.

The research is based on calculations by Lai and Wynn Ho, Ph.D. ’03, published 20 years agoincorporating NASA observations reported last November magnetar 4U 0142+61, located 13,000 light-years away in the constellation Cassiopeia.

Quantum electrodynamics, which describes the microscopic interactions between electrons and photons, predicts that when X-ray photons exit the thin atmosphere of hot, magnetized gas of the neutron star, or plasmathey go through a phase called vacuum resonance.

There, Lai said, the photons, which have no charge, can temporarily convert into “virtual” pairs of electrons and positrons that are influenced by the magnetar’s super-strong magnetic field even in a vacuum, a process called “vacuum birefringence.” Combined with a related process, plasma birefringence, conditions are created for the polarity of high-energy X-rays to oscillate 90 degrees relative to low-energy X-rays, according to Lai’s analysis.

“You can think of polarization as two flavors of photons,” he said. “A photon suddenly jumping from one flavor to another – you don’t usually see that sort of thing. But it’s a natural consequence of physics if you apply the theory in these extreme conditions.

The IXPE mission did not see the polarization wobble in observations of another magnetar, called 1RXS J170849.0-400910, with an even stronger magnetic field. Lai said this matches his calculations, which suggest that vacuum resonance and photon metamorphosis would occur very deep inside such a neutron star.

Lai said his interpretation of IXPE observations of magnetar 4U 0142+61 helped limit its magnetic field and spin, and suggested its atmosphere was likely composed of partially ionized heavy elements.

Ongoing X-ray studies of some of the most extreme objects in the universe, including neutron stars and black holes, he said, allow scientists to probe the behavior of matter in conditions that cannot be replicated in the laboratory, and add to our understanding of the beauty and diversity of the universe.

“The IXPE observations have opened a new window to study the surface environment of neutron stars,” Lai said. “This will lead to new insights into these enigmatic objects.”

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

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