Positrons—the antimatter counterpart of electrons—are created in high-energy cosmic processes. When normal matter meets its antimatter counterpart, they annihilate, or vanish, and produce a distinctive 511 keV gamma-ray signal. Scientists use this signal to detect where these annihilations occur.
Now, 20 years’ worth of this kind of data has revealed that positron annihilations might be happening in unexpected places and at far greater rates than previously thought. The new study, published in Astronomy & Astrophysics, describes how astronomers are interpreting a new positron annihilation map and whether the results represent true annihilations or just imaging artifacts.
Tracking down positron annihilation hotspots
The team involved in the new study examined a sky map made from more than 20 years of data from the spectrometer aboard the International Gamma-Ray Astrophysics Laboratory (INTEGRAL/SPI). The map highlighted areas where the 511 keV gamma-ray signal was detected. They masked bright Milky Way regions and compared remaining hotspots with maps of fast-moving gas clouds and a catalog of nearby galaxies.
Earlier INTEGRAL work had already established strong positron-annihilation emission signals from the Milky Way’s bulge and disk. A 2025 reconstruction of the long INTEGRAL dataset also found persistent off-plane hotspots even after several checks.
“High-exposure regions off the galactic plane are found in the regions of Andromeda, Ursa Major, Hydra, Sextans, and Virgo. In addition to these regions, two more hotspots were found in which hardly any nearby galaxy is located. These regions appear to coincide with high-velocity clouds, and in particular, the Magellanic Stream,” the study authors wrote.
Extragalactic sources hint at higher Milky Way positron production
The team assessed the reconstructed 511 keV signal detections around high-velocity clouds, large accumulations of nearby galaxies and the Magellanic Stream—a vast ribbon of gas that trails behind the Large and Small Magellanic Clouds as they orbit the Milky Way.
They found that the two brightest hotspots lined up with the Magellanic Stream and another giant gas structure outside the Milky Way, called Complex C. The researchers say that this alignment may mean positrons are flowing out of the Milky Way and annihilating in surrounding gas. This is unexpected because previous analyses indicated that positrons should be moving too slowly to reach the outer parts of the galaxy before annihilating. Four additional weaker hotspots also pointed toward concentrations of nearby galaxies, including the Andromeda, Ursa Major, Hydra and Virgo regions.
When the team estimated the expected contribution of positrons originating from the Milky Way galaxy based on its findings, the results were 2–3 times higher than previous estimates. The researchers say it is possible that extragalactic positron-annihilation signals from nearby galaxy groups, such as around Andromeda and Virgo, might explain the additional signal. They also note that positron annihilation from across the universe may add only about 1% to the cosmic gamma-ray background, but potentially more than 10% above a few MeV under some assumptions.
The potential for artifacts
There is still considerable uncertainty surrounding the study’s results, with the possibility that much of the additional signal is due to imaging artifacts. These signals are faint, and image reconstruction can create false structures. In addition, INTEGRAL’s sky coverage is uneven, which can make some regions more vulnerable to artifacts or harder to detect. But the team says it thinks this is unlikely.
They write, “While there is still a chance that some high-latitude 511 keV emission features are imaging artifacts, it appears at odds if all hotspots are mere background fluctuations.”
A more sensitive, more uniformly surveyed MeV gamma-ray sky may confirm or reject the hotspots in the future. NASA’s upcoming COSI mission, expected to launch in 2027, could test for individual extragalactic 511 keV sources, providing additional evidence.
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Publication details
Thomas Siegert et al, Possible extragalactic positron annihilation signal, Astronomy & Astrophysics (2026). DOI: 10.1051/0004-6361/202659502
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Extragalactic positron-annihilation hotspots might mean Milky Way produces far more positrons than thought (2026, July 27)
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