Astronomers have detected a mysterious signal in X-ray data from a study of galaxy clusters, and they think the X-rays could have been produced by the decay of sterile neutrinos, a type of particle proposed as a candidate for dark matter.

Dark matter constitutes 85 percent of the matter in the universe, though it doesn’t emit or absorb light like protons, neutrons, and electrons -- the “normal” matter that make up the planets and stars. That’s why scientists use indirect methods to search for invisible dark matter clues; its gravitational influence on the movements and appearance of objects like galaxies can be detected.

The mysterious X-ray signal was revealed in a new study of the Perseus galaxy cluster using NASA's Chandra X-ray Observatory and 73 clusters with ESA's XMM-Newton. Perseus is one of the most massive objects in the universe, containing thousands of galaxies in a cloud of superheated hydrogen gas. At over 10 million degrees Celsius, that gas is hot enough to emit X-rays. The image of Perseus above combines 17 days’ worth of data taken over a decade.

The new findings consist of an unidentified X-ray emission line: a spike in intensity at a very specific wavelength of X-ray light. In this case, it was centered on about 3.56 kiloelectron volts (keV). The emission line could be a signature from the decay of these hypothetical sterile neutrinos, which are predicted to interact with normal matter only via gravity.

"If this strange signal had been caused by a known element present in the gas, it should have left other signals in the X-ray light at other well-known wavelengths, but none of these were recorded," Esra Bulbul of the Harvard-Smithsonian Center for Astrophysics says in an ESA news release.

She adds in a NASA release: “We know that the dark matter explanation is a long shot, but the pay-off would be huge if we're right.” However uncertainties abound: The detection of the emission line pushed the sensitivity capabilities of both observatories, and it’s possible that normal matter in the cluster could have produced the emission line.

The work is described in The Astrophysical Journal this month, although Bulbul’s team posted a preprint on arXiv, the publicly accessible database. A week after it appeared on the online forum, a team led by Alexey Boyarsky of Leiden University in the Netherlands placed a paper there reporting evidence for an emission line at the same energy in XMM-Newton observations of the galaxy M31 and the outskirts of the Perseus cluster -- strengthening the evidence that the emission line is real.

Their next step is to combine data from Chandra with JAXA’s Suzaku to see if they can find the same X-ray signal. Here’s a different view of Perseus, combining data from Chandra in the inner regions of the cluster and XMM data in the outer regions.

Images: X-ray: NASA/CXC/SAO/E.Bulbul, et al. (top)