Pulsars out for positron count, leaving dark matter in the frame

New modelling discounts a leading theory on antimatter particles hitting Earth. Andrew Masterson reports.

The High-Altitude Water Cherenkov Gamma-Ray Observatory in Mexico.
The High-Altitude Water Cherenkov Gamma-Ray Observatory in Mexico.

A larger-than-predicted number of antimatter particles smashing into the Earth may be the result of the annihilation of dark matter, research suggests.

In a paper in the journal Science, a team led by astronomer A.U. Abeysekara from the University of Utah in the US, raises the idea after presenting evidence that seems to disprove the leading theory to explain their presence.

In 2008, an Italian orbiting probe called PAMELA established that Earth was being hit by a larger number of positrons – the antimatter equivalent of electrons – than standard modelling predicted. The finding was backed up by data from NASA’s Alpha Magnetic Spectrometer-02 detector on board the International Space Station.

As the search began to explain the result, two theories emerged. While some astrophysicists suggested that the decay of dark matter particles might be to blame, others favoured a less exotic idea.

The positrons, astronomers suggested, were being produced by nearby pulsars – collapsed stars that spin around many times a second and throw off powerful jets comprising various types of particles, sending them shooting out into space.

To test this idea, Abeysekara and colleagues used the High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory in Mexico to focus on two candidate pulsars, thought to be theoretically close enough and old enough to be the source of at least some of the extra positrons.

The researchers detected and mapped extended gamma ray emissions coming from the stars, comprising high energy electrons and positrons, measuring their velocity and diffusion pattern. The results showed that the emissions simply did not extend far enough to reach Earth, ruling them out as the birthplace of the antimatter particles.

"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.

Another co-author, Jordan Goodman from the University of Maryland, adds that the finding “strongly disfavours” the idea that pulsars are responsible for the extra positrons – but does not provide direct evidence to support dark matter decay as the explanation.

“Our measurement doesn't decide the question in favour of dark matter, but any new theory that attempts to explain the excess using pulsars will need to match the new data,” he says.

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Andrew Masterson is news editor of Cosmos.
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