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 Agency’s orbiting XMM–Newton 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 NASA’s 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 XMM–Newton observations is the energy of the light involved, which is connected to the masses of the hypothetical dark matter particles that created it. Fermi’s 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 XMM–Newton 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 neutrinos—heavier cousins of the types produced in many nuclear reactions—or 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 light—the very signal seen by XMM–Newton. 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.
The biggest concern is that it is very hard to rule out other possible x-ray sources. Atoms emit light of specific energies depending on the configuration of their electrons but physicists have not identified all of these energies, especially for x-rays. In other words, it is possible this unidentified signal could be just a new emission line from ordinary atoms or from hot plasma. As CNRS/Université Paris-Sud particle physicist Adam Falkowski wrote, whether you accept the dark matter explanation depends on how you model the plasma emission: “seems that the devil is in the details...(which, frankly speaking, is not reassuring).”
The authors of the latest study think the plasma option is not a likely option, because the Milky Way contains a relatively small amount of it. Another problem is there seems to be a mismatch between the places where the x-rays are emitted and maps of the mass distribution in galaxy clusters that suggest where dark matter is hiding.
A Japanese space agency x-ray observatory named Astro–H scheduled to launch this year will carry instruments with sufficient precision to tell the difference between atomic emission and dark matter signatures. And more data is always better; finding other galaxies and galaxy clusters—especially those farther away—with the same type of x-ray emission would help constrain the possibilities.
Ultimately, astronomers must try to reconcile a big claim—dark matter detection—with a relatively weak astronomical signal, which could have several mundane explanations. “I think people who take weak results and ride them are gonna get in trouble,” says University of Washington particle physicist Leslie Rosenberg of the axion hypothesis. Where things stand, the stretch is pretty big from an anomalous x-ray signal to a definitive detection of the matter making up the lion’s share of all mass in the universe. As with any claim of a striking new discovery, it’s best to hedge one’s bets for now.