HAWC gamma-ray observatory sheds light on origin of excess anti-matter

HAWC gamma-ray observatory sheds light on origin of excess anti-matter

A mountaintop observatory in Mexico, built and operated by an international team of scientists, has captured the first wide-angle view of gamma rays emanating from two rapidly spinning stars. The High-Altitude Water Cherenkov Gamma-Ray Observatory (HAWC) provided the fresh perspective on high-energy light streaming from these stellar neighbors, casting serious doubt on one possible explanation for a mysterious excess of anti-matter particles near Earth.

Two Michigan State University physicists, who are founding members of the HAWC Collaboration – Jim Linnemann and Kirsten Tollefson – contributed to the study, which is featured in the current issue of Science.

MSU Physics and Astronomy faculty members Kirsten Tollefson (left) and Jim Linnemann are founding members of the HAWC collaboration and co-authors of a recent paper in Science that sheds light on the origin of excess anti-matter particles near Earth.
[Photo by Harley Seeley]

In 2008, astronomers observed an unexpectedly high number of positrons – the anti-matter cousins of electrons – in orbit a few hundred miles above the Earth's atmosphere. Ever since, scientists have debated the cause of the anomaly, split over two competing theories of its origin. Some suggested a simple explanation: The extra particles might come from nearby collapsed stars called pulsars, which spin around several times a second and throw off electrons, positrons and other matter with violent force. Others speculated that the extra positrons might come from processes involving dark matter – the invisible but pervasive substance seen so far only through its gravitational pull.

Using new data from the HAWC observatory, researchers made the first detailed measurements of two pulsars previously identified as possible sources of the positron excess. By catching and counting particles of light streaming from these nearby stellar engines, HAWC collaboration researchers found that the two pulsars are unlikely to be the origin of the positron excess. Despite being the right age and the right distance from Earth, the pulsars are surrounded by an extended murky cloud that prevents most positrons from escaping.

“This new measurement is tantalizing because it strongly disfavors the idea that these extra positrons are coming to Earth from two nearby pulsars, at least when you assume a relatively simple model for how positrons diffuse away from these spinning stars,” said Jordan Goodman, professor of physics at the University of Maryland and the lead investigator and U.S. spokesperson for the HAWC collaboration. “Our measurement doesn't decide the question in favor of dark matter, but any new theory that attempts to explain the excess using pulsars will need to account for what we’ve found.”

Jim Linnemann, professor of high energy physics in the MSU Department of Physics and Astronomy, led the reviewing and editing of the result inside the HAWC collaboration. Linnemann praised the HAWC team's interdisciplinary effort.

“This exciting result combines the chief scientific interests of our group at MSU by interpreting a result in high energy galactic astronomy in the context of searches for dark matter,” he said. “We look forward to extending these results to other pulsars, and to verifying the diffusion model by measuring the energy spectra more precisely.”

As with an ordinary camera, collecting lots of light allows HAWC to build sharp images of individual gamma-ray sources. The most energetic gamma rays originate in the graveyards of big stars, around stellar remains like the spinning pulsar remnants of supernovae. But that light doesn’t come from the stars themselves. Instead, it's created when the spinning pulsar accelerates particles to extremely high energies, causing them to smash into lower-energy photons left over from the early universe.

The size of the debris field around powerful pulsars, measured by the patch of sky that glows bright in gamma rays, tells researchers how quickly matter moves relative to the spinning stars. This enables researchers to estimate how quickly positrons are moving and how many positrons could have reached Earth from a given source.

Using a recently published HAWC catalog of the high-energy sky, scientists have absolved the nearby pulsar Geminga and its sister – the pulsar PSR B0656+14 – as sources of the positron excess. Even though the two are old enough and close enough to account for the excess, matter isn’t drifting away from the pulsars fast enough to have reached the Earth.

This measurement wouldn't have been possible without HAWC's wide view. It continuously scans about one-third of the sky overhead, which provided researchers with a broad view of the space around the pulsars. Other observatories watching for high-energy gamma rays with a much narrower field of view missed the extended nature of the pulsars.

The HAWC Observatory sits at an elevation of 13,500 feet, flanking the Sierra Negra volcano inside Pico de Orizaba National Park in the Mexican state of Puebla. It consists of more than 300 massive water tanks that sit waiting for cascades of particles initiated by high-energy packets of light called gamma rays – many of which have more than 10 million times the energy of a dental X-ray.

[Image of TeV photon emissions around the Geminga and PSR B0656+14 pulsars, with an overlaid lunar image for size comparison]

The emission around the pulsars, smoothed by averaging over 1 degree, showing the extent of the TeV photon emission region. The emission regions are large compared with HAWC's angular resolution, which is typically the size of the moon or smaller (moon image overlay shown for size comparison).
[Photon emission image courtesy of the HAWC Collaboration]
[Moon image credit: Gregory H. Revera]

When these gamma rays smash into the upper atmosphere, they blast apart atoms in the air, producing a shower of particles that moves at nearly the speed of light toward the ground. When this shower reaches HAWC's tanks, it produces coordinated flashes of blue light in the water, allowing researchers to reconstruct the energy and cosmic origin of the gamma ray that kicked off the cascade.

It's possible that a new insight about the astro­physics of these pulsars and their local environments could account for the posi­tron excess at Earth, but it would require a more complicated theory of positron diffusion than physicists in the collaboration think is likely. On the other hand, dark matter may be the right explanation, but more evidence will ultimately be needed to decide.

For more information, see the article in the 17 Nov 2017 issue of Science Magazine.

[Banner graphic courtesy HAWC Collaboration]