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Astronomers Detect Mysterious Signal 240 Million Light-Years From Earth

1316 Astronomers Detect Mysterious Signal 240 Million Light-Years From Earth
Chandra’s latest view of the Perseus Cluster, where red, green, and blue show low, medium, and high-energy X-rays respectively / X-ray: NASA/CXC/SAO/E.Bulbul, et al.

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. 

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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.

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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)


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  • neutrinos,

  • x-ray,

  • galaxy clusters

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