A new study of the central region of the Perseus galaxy cluster, shown in this image, using NASA's Chandra X-ray Observatory and 73 other clusters with ESA's XMM-Newton has revealed a mysterious X-ray signal in the data. This signal is represented in the circled data points in the inset, which is a plot of X-ray intensity as a function of X-ray energy. The signal is also seen in over 70 other galaxy clusters using XMM-Newton. This unidentified X-ray emission line - that is, a spike of intensity at a very specific energy, in this case centered on about 3.56 kiloelectron volts (keV) - requires further investigation to confirm both the signal's existence and nature as described in the latest Chandra press release.

One intriguing possible explanation of this X-ray emission line is that it is produced by the decay of sterile neutrinos, a type of particle that has been proposed as a candidate for dark matter. While holding exciting potential, these results must be confirmed with additional data to rule out other explanations and determine whether it is plausible that dark matter has been observed.

There is uncertainty in these results, in part, because the detection of this emission line is pushing the capabilities of both Chandra and XMM-Newton in terms of sensitivity. Also, there may be explanations other than sterile neutrinos if this X-ray emission line is deemed to be real. For example, there are ways that normal matter in the cluster could have produced the line, although the team's analysis suggested that all of these would involve unlikely changes to our understanding of physical conditions in the galaxy cluster or the details of the atomic physics of extremely hot gases.

This image is Chandra's latest view of hot gas in the central region of the Perseus Cluster, where red, green, and blue show low, medium, and high-energy X-rays respectively. It combines data equivalent to more than 17 days worth of observing time taken over a decade with Chandra. The Perseus Cluster is one of the most massive objects in the Universe, and contains thousands of galaxies immersed in an enormous cloud of superheated gas. In Chandra's X-ray image, enormous bright loops, ripples, and jet-like streaks throughout the cluster can be seen. The dark blue filaments in the center are likely due to a galaxy that has been torn apart and is falling into NGC 1275 (a.k.a. Perseus A), the giant galaxy that lies at the center of the cluster. A different view of Perseus, shown below, combines data from Chandra in the inner regions of the cluster and XMM data in regions further out.

Chandra+ XMM full field

A paper describing the detection of this mysterious emission line was published in the July 1st issue of The Astrophysical Journal and a preprint is available online. The first author is Esra Bulbul of Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, MA, and NASA's Goddard Space Flight Center (GSFC) in Greenbelt, MD. The co-authors are Maxim Markevitch from GSFC, Adam Foster and Randall Smith from CfA, Michael Loewenstein from University of Maryland in College Park MD and GSFC, and Scott W. Randall from CfA.

Only a week after Bulbul et al. placed their paper on the arXiv, a different group, led by Alexey Boyarsky of Leiden University in the Netherlands, placed a paper on the arXiv 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. This strengthens the evidence that the emission line is real and not an instrumental artifact.