Excess antielectrons aren’t from nearby dead stars, study says

New observations of the whirling cores of dead stars have deepened the mystery behind a glut of antimatter particles raining down on Earth from space.

The particles are antielectrons, also known as positrons, and could be a sign of dark matter — the exotic and unidentified culprit that makes up the bulk of the universe’s mass. But more mundane explanations are also plausible: Positrons might be spewed from nearby pulsars, the spinning remnants of exploded stars, for example. But researchers with the High-Altitude Water Cherenkov Observatory, or HAWC, now have called the pulsar hypothesis into question in a paper published in the Nov. 17 Science.

Although the new observations don’t directly support the dark matter explanation, “if you have a few alternatives and cast doubt on one of them, then the other becomes more likely,” says HAWC scientist Jordan Goodman of the University of Maryland in College Park.

Earth is constantly bathed in cosmic rays, particles from space that include protons, atomic nuclei, electrons and positrons. Several experiments designed to detect the showers of spacefaring particles have found more high-energy positrons than expected (SN: 5/4/13, p. 14), and astrophysicists have debated the excess positrons’ source ever since. Dark matter particles annihilating one another could theoretically produce pairs of electrons and positrons, but so can other sources, such as pulsars.
It was uncertain, though, whether pulsars’ positrons would make it to Earth in numbers significant enough to explain the excess. HAWC researchers tested how positrons travel through space by measuring gamma rays, or high-energy light, from two nearby pulsars — Geminga and Monogem — around 900 light-years away. Those gamma rays are produced when energetic positrons and electrons slam into low-energy light particles, producing higher-energy radiation.
The size and intensity of the resulting gamma-ray glow indicated that the positrons slowly dissipated away from their pulsar birthplaces, getting bogged down by magnetic fields that permeate the galaxy and twist up the particles’ trajectories. That sluggish departure suggests the particles wouldn’t have made it all the way to Earth, the researchers conclude, and therefore couldn’t explain the excess.

Astrophysicist Dan Hooper of Fermilab in Batavia, Ill., disagrees. He still thinks pulsars are the best explanation for the rogue antimatter. The gamma ray measurements are just one method for studying how cosmic ray particles propagate through space. Other methods indicate that the pulsars’ positrons should be able to make the trek across the galaxy swiftly enough to get to Earth, he says. “I have every confidence that those particles are now reaching the solar system.”

Ruling out pulsars still wouldn’t point the finger at dark matter. “I think they’ve made a good case that these pulsars are not the source,” says astrophysicist Gregory Tarlé of the University of Michigan in Ann Arbor. Instead, Tarlé thinks that scientists can explain the excess positrons by better understanding what happens as cosmic ray particles travel through space. Protons interacting with the interstellar medium — particles that permeate the spaces between stars — could produce positrons that would explain the observations, without invoking either dark matter or pulsars.

The conflict leaves physicists with their work cut out for them. “In order to prove that it’s dark matter, you have to prove that it’s not something ordinary,” says HAWC researcher Brenda Dingus of Los Alamos National Laboratory in New Mexico. Although the new result disfavors the most obvious ordinary candidates, Dingus says, other possibilities are still in the running. “We need to look harder.”