Scientists must now decide whether the anomalous signal is truly exotic or has a more mundane provenance

When astronomers recently discovered x-rays with no obvious origin, it sparked an exciting hypothesis. Credit: Chandra X-Ray Observatory

Many major discoveries in astronomy began with an unexplained signal: pulsars, quasars and the cosmic microwave background are just three out of many examples. When astronomers recently discovered x-rays with no obvious origin, it sparked an exciting hypothesis. Maybe this is a sign of dark matter, the invisible substance making up about 85 percent of all the matter in the universe. If so, it hints that the identity of the particles is different than the prevailing models predict.

The anomalous x-rays, spotted by the European Space Agencys orbiting XMMNewton telescope, originate from two different sources: the Andromeda Galaxy and the Perseus cluster of galaxies. The challenge is to determine what created those x-rays, as described in a study published last month in Physical Review Letters. (See also an earlier study published in The Astrophysical Journal.) The signal is real but weak and astronomers must now determine whether it is extraordinary or has a mundane explanation. If that can be done, they can set about the work of identifying what kind of dark matter might be responsible.

If the [x-ray emission] line is conclusively shown to be due to dark matter, the implications are of course profound, wrote University of California, Irvine, astrophysicist Kevork Abazajian in a commentary published December 15, 2014, in Physical Review Letters.

If this observation sounds familiar, it is because researchers using NASAs Fermi Gamma-Ray Space Telescope detected anomalous gamma rays near the Milky Way center, which some think could be from dark matter particles colliding and annihilating. The difference between the Fermi and the XMMNewton observations is the energy of the light involved, which is connected to the masses of the hypothetical dark matter particles that created it. Fermis gamma rays are more than a million times more energetic than the x-rays, so the particles that created the former would be more massive than a proton.

The x-rays, on the other hand, would have to originate from particles significantly lighter than an electron. (For those keeping track at home, the x-rays have an energy of about 3.5 kilo-electron volts, corresponding to less than one one-hundredth of the electron mass.) If the XMMNewton detection is a sign of dark matter, however, it would not be due to weakly interacting massive particles (WIMPs), which are researchers most popular candidates for what constitutes dark matter.

Other potential dark matter particles could include sterile neutrinosheavier cousins of the types produced in many nuclear reactionsor more exotic possibilities such as axions, originally predicted to solve an unrelated problem in particle physics. Both of these particles remain hypothetical, but if they exist, they would be much less massive than electrons.

If the culprits are sterile neutrinos, they would possess masses slightly larger than the energy of the x-ray photons. They decay into the well-known standard neutrinos, with the rest of their mass converted into x-ray lightthe very signal seen by XMMNewton. This idea, however, presents a few problems: there are no equivalent x-rays in the Milky Way and other experiments hunting for sterile neutrinos have turned up empty.

By contrast, axions are stable but they transform into photons in the presence of strong magnetic fields. Because galaxies and galaxy clusters generate such intense magnetism, they are prime axion makers. The particles (technically axionlike particles) required to make the anomalous x-rays would be of higher mass than the typical axion, but within constraints allowed by some theories.

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Weird X-Rays Spur Speculation about Dark Matter Detection