Sky-high observatory sheds light on origin of excess anti-matter

UNIVERSITY PARK, Pa. — The High-Altitude Water Cherenkov (HAWC) Observatory in Mexico, built and operated by an international team that includes Penn State scientists, has captured the first wide-angle view of very-high-energy light emanating from two rapidly spinning stars. The fresh perspective on these stellar neighbors — which are both close to Earth in cosmic terms — casts serious doubt on one possible origin for a mysterious excess of particles near Earth whose origin has tantalized scientists in recent years.

“Scientists have debated the cause of an unexpectedly high number of positrons — the anti-matter cousins of electrons — near the Earth since a space-borne detector measured the anomaly in 2008,” said Miguel Mostafá, professor of physics and of astronomy and astrophysics and principal investigator of the HAWC group at Penn State. “Some have speculated that the extra positrons have an exotic source, perhaps originating from as-yet undetected processes involving dark matter — the invisible but pervasive substance seen only through its gravitational pull. Others have suggested something more pedestrian: The extra particles might originate from nearby collapsed stars, called pulsars, that spin around several times a second and throw off electrons, positrons and other matter with violent force.”

Now, using new data from the HAWC Gamma-Ray Observatory, an international team of researchers has made the first detailed measurements of two pulsars, Geminga and its unnamed sibling, that had been identified as possible sources of the mysterious excess of positrons. By catching and counting particles of light streaming forth from these nearby stars, the HAWC collaboration has shown that the two pulsars are very unlikely to be the origin of the positron excess despite being the right age and the right distance from Earth to contribute. Positrons from these sources simply have not traveled far enough to reach the Earth in great numbers. The results appear in the Nov. 17 issue of the journal Science.

“After excluding two of the main source candidates, we are closer to understanding the origin of the positron excess,” said Francisco Salesa Greus, lead author of the new paper. A postdoctoral researcher in the HAWC group at Penn State at the time of the research, Salesa Greus is now at the Polish Academy of Sciences Institute of Nuclear Physics in Krakow, Poland.

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 continuously scans about one-third of the sky overhead, providing researchers with a wide field-of-view that was critical to accurately observing these nearby pulsars.

“Relatively close galactic objects, like these two pulsars, appear so extended that one can only see them with an instrument with a wide field-of-view,” said Mostafá. “Using data from the HAWC Observatory, we provide a unique and complementary view of these very extended sources that is not possible with the more pointed gamma-ray telescopes.”

The HAWC Observatory consists of 300 massive water tanks that monitor for cascades of particles initiated by high-energy packets of light called gamma rays — many of which have more than a million times the energy of a dental X-ray. When gamma rays smash into the Earth’s 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.

“Thanks to its wide field-of-view, HAWC provides unique measurements on the very-high-energy gamma ray profiles caused by the particle diffusion around nearby pulsars, which allows us to determine how fast the particles diffuse more directly than previous measurements,” said Hao Zhou, a scientist at Los Alamos National Lab in New Mexico.

Observing many gamma rays from the same region of the sky allows HAWC to build sharp images of individual gamma-ray sources. The highest energy photons — light particles — originate in the graveyards of big stars, such as the spinning pulsar remnants of supernovae. This light is not created by the star directly, but rather when electrons accelerated to extremely high energies by the spinning star smash into lower-energy photons left over from the early universe.

By identifying the size of this stellar debris field, measured by the patch of sky that glows bright in gamma rays, researchers infer how quickly matter moves relative to a local astrophysical body — in this case, the nearby pulsar. This, in turn, enables researchers to estimate how many positrons could have reached Earth from the source.

Using the most complete catalog of HAWC data to date, scientists have absolved the nearby pulsar Geminga and its unnamed sibling as sources of the positron excess at Earth. Even though the two pulsars are old enough and close enough to account for the excess, matter is not drifting away from the pulsars fast enough to have reached Earth.

“The gamma rays HAWC measures demonstrate that there are high energy positrons escaping from these sources,” said Rubén López-Coto, a scientist at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. “But according to our measurement, they could not be significantly contributing to the extra positrons seen at the Earth.” 

This work was supported by the US National Science Foundation (NSF); the U.S. Department of Energy (DOE) Office of High-Energy Physics; the Laboratory Directed Research and Development program of Los Alamos National Laboratory (LANL); and the Mexican National Council of Science and Technology (CONACyT).

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Last Updated November 16, 2017