For the past decade or so scientists have noticed Earth is being bombarded with far more antimatter than expected. Now they are closing in on this strange bombardment’s source, tentatively linking it with the enigmatic dark matter thought to make up roughly five sixths of all matter in the universe.
Every particle of normal matter has an antimatter counterpart, equal in mass but opposite in charge. For instance, the antiparticle of the negatively charged electron is the positively charged positron, a particle that makes up the bulk of antimatter striking the top of Earth’s atmosphere from outer space. When a particle meets its antiparticle, they annihilate each other, typically releasing energetic photons called gamma rays as a result.
In 2008 researchers using the space-based PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) detector found the number of high-energy protons hitting Earth to be roughly three times greater than predicted by standard models. These results were later confirmed by the Fermi Gamma-ray Space Telescope as well as the Alpha Magnetic Spectrometer (AMS-02), which is mounted on the International Space Station. Ever since, astrophysicists have been debating where all these excess positrons might come from.
One promising explanation: the excess could come from nearby pulsars, rapidly whirling remnants of expired massive stars that violently expel positrons and a menagerie of other high-energy particles as they spin. A more exotic possibility is that this antimatter is given off by decaying particles of dark matter, the theoretical invisible substance astronomers have so far “detected” only via its apparent gravitational effects on normal matter.
To see if pulsars might solve this antimatter puzzle, an international team of researchers analyzed data from the High-Altitude Water Cherenkov (HAWC) gamma-ray observatory, located on the flanks of a volcano about four hours east of Mexico City. When gamma rays from annihilated positrons smash into Earth’s atmosphere, they rip apart atoms, generating showers of particles that hurtle downward at nearly the speed of light. These showers produce minuscule flashes of light when they hit the water held in 300 corrugated steel tanks at HAWC. Observers can detect and record the flashes, using them to deduce the energies and cosmic origins of the gamma rays that triggered the particle cascades.
The scientists focused on gamma rays originating from the vicinity of two nearby pulsars that could blast Earth with positrons—Geminga and PSR B0656+14, located some 815 and 950 light-years from Earth, respectively. “HAWC scans about one third of the sky overhead, giving us the first wide-angle view of high-energy light from the sky,” says study co-author Jordan Goodman, a particle astrophysicist at the University of Maryland, College Park. “Before HAWC there were observatories that were highly sensitive to high-energy gamma rays, but they had relatively limited fields of view. With HAWC, we can see how gamma rays are diffusing from these pulsars across wide regions of sky.” The researchers detailed their findings in the November 17 Science.
Goodman and his colleagues used HAWC to map how these pulsars’ positron-generated gamma-ray emissions radiated across patches of sky, each about 65 light-years across. They found each pulsar is surrounded by a kind of murky “fog,” from which relatively few positrons can escape to strike Earth. Positrons help create this gamma-ray fog when they slam into the cosmic microwave background radiation, the photons of light left over from the big bang that formed the universe. By mapping the extent of the fog around each pulsar, the HAWC team could estimate how fast and how far most of the positrons can travel. Their results suggest these pulsars “cannot be the source of the excess positrons measured by AMS-02 and preceding experiments,” says John Wefel, a particle astrophysicist emeritus at Louisiana State University who did not take part in the study.
Even so, the positron excess remains “one of the most exciting mysteries in astrophysics,” says Justin Vandenbroucke, a particle astrophysicist at the University of Wisconsin–Madison, also unaffiliated with the work. It could be that the positrons come from other conventional astrophysical sources such as high-energy particles produced by black holes siphoning gas from nearby companion stars. Or they do come from pulsars, but somehow take more complicated paths through space to reach Earth than those assumed by the HAWC team. More certainty may come as HAWC and the upcoming Cherenkov Telescope Array gather data from a larger numbers of nearby pulsars.
For now, though, when it comes to all the potential suspects in the antimatter mystery, “the only one left standing seems to be dark matter,” Goodman says. “We’re not saying what AMS and PAMELA have detected is dark matter—we’re just saying that we’ve given an alibi to the other major suspects, the pulsars.”