Synopsis: Wind Gusts Could Explain Gamma-Ray Flares

A mysterious gamma-ray emission from the Crab Nebula—a giant interstellar cloud of gas—may be due to fluctuations in the wind blown out from the pulsar in the nebula’s center.

The Crab Nebula was created by a stellar explosion, observed on Earth in the year 1054. But the fireworks aren’t over yet, as recent observations have revealed ongoing flares of gamma rays from this expanding cloud of gas. A new theoretical model suggests that these sudden spikes in emission may be due to fluctuations in the wind of plasma emanating from a pulsar that forms the beating heart of the Nebula.

Gamma-ray emission from the Crab Nebula was long thought to be stable, so astronomers were stunned in 2011 by the discovery of flares in which the gamma-ray brightness increased by a factor of 30 or more. Researchers have had difficulty explaining the flares’ rapid variability, as well as the high energy of their gamma rays (over 100 MeV).

To resolve these issues, John Kirk and Gwenael Giacinti from the Max Planck Institute for Nuclear Physics, Germany, considered the role of the pulsar wind. This wind—generated by the rotating magnetic fields around the pulsar—is known to be responsible for much of the Nebula’s high-energy emission: When the wind’s electrons and positrons slam into the gas and dust surrounding the pulsar, they emit synchrotron radiation. Kirk and Giacinti propose that flares occur when the wind in a certain direction experiences a temporary drop in electrons and positrons. The resulting low-density pocket speeds up, driven by electromagnetic forces trying to maintain a constant current flow in the wind. The accelerated electrons and positrons slam harder into Nebula material, thus producing gamma rays with higher-than-usual energy. If this idea is correct, then the resulting flare emission should be polarized, which is something future telescopes might be able to test.

This research is published in Physical Review Letters.

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.

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