Exploring a Magnetospheric Origin for Fast Radio Bursts

Astronomers are starting to close in on the origins of fast radio bursts — powerful, fleeting flashes of radio waves seen at extragalactic distances. Highly magnetized neutron stars called magnetars might be responsible for many of these far-flung events, but how exactly do these extreme objects generate fast radio bursts?

A Cosmic Conundrum

The mystery of where fast radio bursts come from — some of them, anyway! — seemed to be solved when the first radio burst within the Milky Way was found to come from a magnetar — an ultra-dense stellar remnant with a magnetic field roughly 100 billion to 10 trillion times stronger than a typical refrigerator magnet. But as is so often the case in astrophysics, solving one puzzle prompted many more: how do magnetars generate these strong, brief bursts of radio waves, and do they arise from near the magnetar’s surface or from material surrounding it?

In today’s article, Andrei Beloborodov (Columbia University and Max Planck Institute for Astrophysics, Germany) explored whether a fast radio burst generated close to the surface of a magnetar could escape the confines of its magnetosphere — the region of space where charged particles bend to the will of the magnetar’s intense magnetic field.

Magnetar Models

Magnetospheres have great importance within our solar system — Earth’s magnetosphere shields us from energetic particles generated by the Sun, and Jupiter’s magnetosphere would dominate the sky if our eyes were adapted to see radio wavelengths. The magnetosphere of a magnetar, though, is far stranger than these nearby examples; the magnetized neutron star at the center generates a plasma of electrons and their positively charged counterparts, positrons, that fills the magnetosphere and might prevent a radio burst generated near a magnetar’s surface from escaping into space.

Beloborodov used plasma physics equations to understand how a radio burst might interact with the charged particles and magnetic fields within a magnetar’s magnetosphere. As the radio burst travels outward, it compresses the magnetosphere, transferring its momentum to the magnetic fields and the plasma. The oscillating electrons and positrons emit gamma rays that can collide to produce even more electrons and positrons — creating a cascade of particles and gamma rays that scatter the radio wave and sap its energy. For the radio wave, there’s no escape; Beloborodov found that it’s extremely unlikely that a fast radio burst could escape the magnetosphere’s grasp if it were generated within 100,000 km of the magnetar’s surface.

More to Learn

While Beloborodov’s findings rule out the possibility of fast radio bursts arising from within the inner magnetosphere, there are other ways that magnetars can power these events. Another possibility is that fast radio bursts form much farther from the magnetar where magnetospheric flares collide with the wind flowing out from the magnetar.

Luckily, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) is poised to add new observations to its existing catalog of hundreds of fast radio bursts, helping us to understand the sources of these mysterious events and the complex physics behind them.

Citation

“Can a Strong Radio Burst Escape the Magnetosphere of a Magnetar?,” Andrei M. Beloborodov 2021 ApJL 922 L7. doi:10.3847/2041-8213/ac2fa0