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Very high-energy gamma rays from pulsars, detected by the High-Altitude Water Cherenkov Observatory in Mexico, are consistent with a mysterious antimatter signal.

Henry Romero/Reuters/Newscom

Case weakens for antimatter sign of dark matter

A long debate over a mysterious surplus of antimatter—and whether it’s a sign of dark matter—may be coming to an anticlimactic end. For more than a decade, multiple experiments have found an unexpected excess in the number of high-energy antielectrons, or positrons, in space, and some physicists suggested it could be due to particles of dark matter annihilating one another. Others countered with a more mundane explanation: The positrons come from rapidly rotating neutron stars, or pulsars. Now, a team of theorists has bolstered that more prosaic explanation, showing in detail that pulsars can indeed produce most or all of the excess.

The new study relies on gamma-ray data from the High-Altitude Water Cherenkov Observatory (HAWC) in Mexico. “Before the HAWC observations we didn’t know whether pulsars made up 0.1% of the excess or 100%,” says study leader Dan Hooper, a particle theorist at Fermi National Accelerator Laboratory in Batavia, Illinois. “We now know they make up a very large fraction, and very plausibly all of it.”

Positrons can be created when cosmic rays, charged particles such as protons or helium ions, strike other particles within interstellar space. Conventional astrophysical models show that the higher the positrons’ energy, the rarer they ought to be compared to the number of electrons arriving. But in the last decade, a number of space-based observatories, including the $2 billion Alpha Magnetic Spectrometer (AMS), a giant magnet attached to the International Space Station, have observed a curious rise in the ratio of positrons to electrons between energies of 10 giga-electron volts (GeV) and several hundred GeV.

In principle, those extra positrons could come from dark matter, the unknown stuff that makes up 85% of the matter of the universe. One dark matter candidate is the weakly interacting massive particle (WIMP). Occasionally, two WIMPs could collide, annihilating and creating an electron-positron pair with energies significantly greater than 10 GeV.

In the latest work, Hooper and his colleagues instead consider whether pulsars could do the job. Pulsars have enormous, rotating magnetic fields that accelerate charged particles in their vicinity to immense energies, and when those energies get high enough the particles can generate pairs of electrons and positrons. Hooper says this mechanism is well known to astrophysicists, but no one had checked to see whether specific pulsars could produce enough high-energy positrons to explain the observed anomaly.

To find out, Hooper’s team analyzed gamma rays emitted by the nearby Geminga pulsar and observed by HAWC. They found a halo around the pulsar, extending a few light-years across, that was emitting very high-energy gamma rays. Hooper says that the “only reasonable explanation” for this extended source is a fusillade of high-energy electrons and positrons slamming into photons on their way out from the pulsar, and boosting the photons' energies into the gamma ray part of the spectrum.

The energy of the positrons and electrons making these gamma rays would be far too high—at tens of thousands of GeV rather than a few hundred GeV—to explain the rise seen by the AMS. But Hooper and his colleagues also calculated how many lower energy positrons the pulsar would be expected to generate, based on models. They repeated the exercise for a second nearby pulsar known as B0656+14, and extended their analysis to work out the likely contribution of thousands of other pulsars in the Milky Way. Their conclusion: Pulsars are “very likely” responsible for “much, if not the entirety” of the observed positron excess.

Stéphane Coutu, a cosmic ray physicist at Pennsylvania State University in State College, praises the researchers' new work for “carefully assessing the astrophysical mechanisms of positron production,” work that he describes as “less sexy” than hunting for dark matter. He says the study is somewhat speculative in its modeling of the pulsars, such as how readily the lower energy positrons of interest diffuse through the halo. But, he adds, “The speculations are not far-fetched.”

Hooper acknowledges that there is still enough uncertainty in their models that dark matter could account for roughly half the excess. Just how big the pulsar share is, he says, should become clearer with future observatories with greater sensitivity to lower energy gamma rays, such as the global €400 million Cherenkov Telescope Array, construction of which is due to begin in 2018.