A Mysterious X-ray Signal

X-ray observation of the Perseus galaxy cluster. (Credit: NASA/CXC/SAO/E.Bulbul, et al.). Astronomers at NASA’s Chandra X-ray Observatory have detected a mysterious X-ray signal coming from Perseus galaxy cluster. Although the signal still requires more investigation, there’s a possibility that it represents a concrete discovery of a form of dark matter.

Dark matter is still something of a mystery in the realm of astronomy and physics. Astronomers hypothesize that dark matter comprises roughly 85% of the matter in the universe, and doesn’t interact with light (which is why it would be difficult to detect). Its existence is presumed because the visible matter that can be seen doesn’t explain the gravitational behavior that astronomers have been able to observe. 

The astronomers at Chandra were observing signals coming from the Perseus galaxy cluster, which is one of the largest objects observed in the universe. The cluster consists of thousands of galaxies that are surrounded by a superheated cloud of gas. At the center of the cluster is the galaxy NGC 1275, which is about 237 million light years from Earth.

The lead researcher on this project, Dr. Elsa Bulbul, first noted this X-ray signal when she was going through about 15 years worth of X-ray observations of the Perseus cluster.

“After spending a year reducing, carefully examining, and stacking the XMM-Newton X-ray observations of 73 galaxy clusters, I noticed an unexpected emission line at about 3.56 kiloelectron volts (keV), a specific energy in the X-ray range,” she wrote in a blog post. “I remember being so puzzled when I first saw the line. Its wavelength did not correspond to any of the known atomic transitions.” 

Further study of the emission lines provided several possibilities for the source of the signal. But one particularly intriguing interpretation of the is that it might have been caused by the decay of sterile neutrinos, a hypothesized form of dark matter.

“My team and I came up with a few astrophysical processes, which could explain this line. We went through these processes one by one carefully,” wrote Bulbul. “However none of them could produce an emission line at 3.56 keV. Since 80% of galaxy clusters consists of dark matter, one possibility we considered was the decay signature of sterile neutrinos.”

If they exist, sterile neutrinos are thought to be a heavier form of neutrinos that do not interact with other forms of matter in any way except gravitationally. When they decay, they’re expected to produce the neutrinos that we are familiar with, plus a burst of radiation – which, if the astronomers are right, is what they’re seeing in that X-ray signal. 

There’s still a lot of work to do before concluding that these signals indicate the decay of a form of dark matter, however. As the researchers note in their paper, which was published in the Astrophysical Journal, there are still a lot of uncertainties that have to be resolved.

“Since this line is weak, it is important to confirm it with other satellites,” wrote Bulbul.

To that end, she’ll continue to explore this signal using Japan’s Suzaku satellite, which has instruments for looking at this type of X-ray emission. Additionally, in 2015, Japan is launching its Astro-H mission, which has instruments for detecting spectra at higher resoltuions.

With these instruments. Bulbul wrote, “we hope we will be able to unambiguously distinguish an astrophysical line from a dark matter signal and tell us what this new X-ray emission truly is.”

By: forbes.com