spaceSpace and Physics

Controversial X-Ray Signal Might Be Dark Matter, New Study Claims


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockDec 20 2017, 14:53 UTC

Composite X-ray, Radio, and optical images of the Perseus cluster. X-ray: NASA/CXO/Oxford University/J. Conlon et al. Radio: NRAO/AUI/NSF/Univ. of Montreal/Gendron-Marsolais et al. Optical: NASA/ESA/IoA/A. Fabian et al.; DS

Dark matter is a theoretical invisible component of the universe used to explain many different phenomena, but so far it has eluded our detection. Now researchers have come up with an explanation for how a controversial X-ray emission might be the dark matter detection we've been hoping for.

Researchers from the University of Oxford have published a study on this widely debated signal, which is an emission line in the X-ray spectrum at the specific wavelength of 0.354 nanometers, or for the X-ray astronomers, 3.5 keV, a measure of the energy of the photons. The study, available from Physical Review D, tries to solve the controversy and also explain what dark matter should be like if it was indeed the source of the signal.


The whole affair started in 2014 when two independent groups witnessed the specific X-ray signal in question. The emission was first observed in 73 galaxy clusters using NASA’s Chandra and ESA’s XMM-Newton observatories and subsequently confirmed in Andromeda and in the outskirts of the Perseus cluster’s central galaxy.

Follow-up studies were a bit hit and miss. Some observations detected the emission and some didn’t. Then, last year, the Japanese X-ray satellite Hitomi failed to detect the 3.5 keV line in the Perseus cluster. Hitomi was aptly designed to look at specific emission lines, so the lack of detection seemed to suggest that this “dark matter” signal was not there.

"One might think that when Hitomi didn't see the 3.5 keV line that we would have just thrown in the towel for this line of investigation," said co-author Francesca Day in a statement. "On the contrary, this is where, like in any good story, an interesting plot twist occurred."


The team realized that Hitomi’s field of view encompassed the whole central galaxy, not just the outskirts. So they checked with past observations of the central region from Chandra and XMM-Newton and discovered a deficit in the 3.5 keV emission. Once the center and outskirts were reconstructed together, they showed a picture consistent with Hitomi’s observations.

The team points the finger at dark matter to explain the distribution of the emission. They suggest that dark matter particles absorb X-rays from the center of galaxies and then re-emit them in random directions. So we don’t see the signal when looking directly at it, but we do detect it in the outskirts.

"This is not a simple picture to paint, but it's possible that we've found a way to both explain the unusual X-ray signals coming from Perseus and uncover a hint about what dark matter actually is," added co-author Nicholas Jennings.


While this is intriguing, more observations are necessary to understand if this signal is caused by dark matter or another phenomenon entirely. It is too early to tell.

"We expect that this result will either be hugely important or a total dud," said Joseph Conlon of Oxford University, who led the new study. "I don't think there is a halfway point when you are looking for answers to one of the biggest questions in science."

How the X-ray signal might be produced by dark matter. NASA/CXC/M. Weiss

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