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X-Ray Observations from the Milky Way's Core Offer Yet Another Dark Matter Tease

But not much more.
Image: ESO

Physicists from Leiden University in the Netherlands have isolated a new x-ray signal indicative of distant dark matter. Their conclusion, which is described in a paper accepted for publication in the Physical Review Letters, is based on data collected by the European Space Agency's XMM-Newton orbiting observatory and suggests a pool of decaying dark matter particles concentrated within the heart of the Milky Way.


Dark matter being dark, we can't easily go look for it directly, though many physicists are certainly trying. We know it exists mainly because of its gravitational effects on galaxies and its apparent role in the evolution of large-scale cosmic structures. The Leiden team is hunting for another, somewhat more direct effect of dark matter, which is its decay products, particularly in the form of x-rays.

"The signal passes a very nontrivial consistency check."

Physicists have mostly been interested in a theorized form of dark matter known as weakly interacting massive particles (WIMP). These are indeed huge particles that only interact with other matter via gravity and the weak force, which is the fundamental force governing the radioactive decays of particle nuclei. Their allure is a precise match between the number of dark matter particles that should have crystallized out of the quickly cooling post-Big Bang universe, the number that would have survived until now, and the amount of dark matter required to maintain the universe as we see it today. This is known as the "WIMP miracle," and from it physicists have been able to assign a predicted mass to WIMPs, a target range that can be probed in direct detection experiments.

That range hasn't proved fruitful, however. The Leiden researchers are instead looking for particles just a millionth the size of the theorized WIMPs. Dark matter particles of this size would be prone to decay, and it's these decay products they're interested in.

The same group of physicists reported in 2014 peaks in the x-ray spectra being emitted by the nearby Andromeda Galaxy and Perseus galaxy cluster. Right around 3.5 thousand electron-volts there was a little something extra. Around the same time, physicists from the Harvard-Smithsonian Center for Astrophysics reported a similar spike in data collected from the combined spectra of 73 galaxy clusters.

And so here it is again, right in the heart of the Milky Way. "We can't prove that dark matter is coming from the center of the Milky Way because we don't know the physics around the galactic center well enough," Alexey Boyarsky, the Leiden group's lead researcher, tells Science. "But the signal passes a very nontrivial consistency check."

Frustratingly, there isn't all that much more that can be done aside from measure the signal better. As Boyarsky says, we know precious little about what goes on in the center of a galaxy in the first place. A new x-ray observatory aboard a satellite called ASTRO-H will go into orbit next year, and should offer a better resolution to the data and, hopefully, the ability to determine if the signal has to do with regular old atoms or the still-elusive dark matter.