If positrons don’t come from pulsars, are they evidence of dark matter?

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  Last updated November 29, 2017 at 2:36 pm


A flood of positrons hitting the Earth has scientists baffled for years, and now a new paper calls into question the leading hypothesis of where they come from.

The HAWC experiment near Puebla, Mexico, uses more than 300 tanks of water to detect the signatures of gamma rays.

The antimatter particles, measured by an orbiting probe called PAMELA, are hitting Earth in far greater numbers than modelling suggests should be the case. Until now, scientists have suspected they are been jettisoned from pulsars, the spinning remnants of exploded stars.

But a study from the High-Altitude Water Cherenkov Observatory (HAWC) published in Science casts doubt on this.

At the same time, the researchers from the University of Utah offer an alternative, and more exotic, suggestion – that the particle may come from the breakdown of dark matter.

The team, led by astronomer A.U. Abeysekara, tested how positrons travel through space by measuring gamma rays from two “nearby” pulsars — Geminga and Monogem.

Two pulsars, Geminga and Monogem, are seen in this image in gamma rays, high-energy radiation produced when positrons and electrons collide with of particles of light. Credit: A.U. Abeysekara et al

These collapsed stars spin at massive speeds shooting out a range of particles and the gamma rays are produced when positrons and electrons slam into lower energy photons, producing a glow.

But the intensity of this gamma ray glow shows that the positrons dissipated rapidly after leaving the pulsars – much too rapidly to make it to Earth, and certainly not in the numbers being measured by PAMELA.

“Our analysis does not support previous claims that the two nearby pulsars are responsible for the excess of positrons detected by two space-born telescopes,” says report co-author Petra Huentemeyer of the Michigan Technical University.

While the pulsar theory may have taken a hit, it does not necessarily mean the dark matter explanation is the right one, but, as another co-author, Jordan Goodman from the University of Maryland, says “any new theory that attempts to explain the excess using pulsars will need to match the new data”.

Not everyone agrees with the findings.

Astrophysicist Dan Hooper of Fermilab in Illinois told ScienceNews that he still thinks pulsars are the best explanation and that the gamma ray measurements may not be telling the full story.

“I have every confidence that those particles are now reaching the solar system,” he was quoted as saying.


About the Author

Bill Condie
Bill is Head of Publishing at the Royal Institution of Australia. Previously he was Publisher of the popular science magazine, Cosmos, based in Melbourne, Australia. Bill has been a journalist for more than 30 years and his work has been published in Cosmos magazine, The Guardian, The Observer, The Times, The Daily Telegraph and the London Evening Standard.