Strange X-rays point to possible ‘dark’ matter

An orbiting telescope has just spotted a strange X-ray signal. It’s raising hopes that the source may be dark matter. That’s the unknown — and unseen — substance that scientists believe constitutes most of the matter in the cosmos.

NASA’s Chandra X-ray Observatory is an Earth-orbiting telescope. It looks for X-rays, a type of radiating energy, coming from deep space. Chandra picked up an excess of X-rays possessing a particular energy. They made a bump on a plot of the radiation. That bump, or “line,” corresponds to the X-rays’ energy. And it’s unusual.

Astronomers have seen such a line before, but not often. Several other telescopes have claimed to see it. Others looked and didn’t see it. But finding it again, with a different instrument, ups the odds that the signal is real. It also helps rule out that it’s due to some mistake or other cause.

“This is a very exciting thing,” says Nico Cappelluti. He’s an astrophysicist at Yale University in New Haven, Conn. He also coauthored a report of the new finding at arXiv.org on January 29.

What also makes these X-rays special, he says, is where they were seen.

The new analysis comes from data taken when the telescope was observing deep space. It was not pointing at a particular cluster of galaxies. So if the signal indicated dark matter, the authors say, it would be due to particles in the region surrounding the Milky Way. That area is known as our galaxy’s halo.

Dark matter particles may not exist, although physicists strongly suspect that they do. Those scientists have several different ideas for what these particles might be. If the particles are a type known as a sterile neutrino, they should “decay,” giving off energy. That energy could come in the form of X-rays at the energy of the line — or about 3,500 electron volts.

Cappelluti and his colleagues compared the strength of the X-rays from the Milky Way’s halo to X-rays that other telescopes saw coming from the center of our galaxy. Those measurements were consistent with the expected variation in concentrations of dark matter in different parts the galaxy.

Dark matter isn’t the only possible explanation. The physics of how atoms behave in space might also explain the bonus X-rays at this energy level. As such, “There’s definitely a lot of debate,” says Shunsaku Horiuchi. An astroparticle physicist, he works at Virginia Tech in Blacksburg and was not involved with the new work. The line certainly “looks like it’s real,” he admits. Still, he is not convinced yet as to what caused it.

Although there’s a small chance that the result could be some fluke in the data, the analysis rules out certain other possibilities. Scientists had proposed that the line could be the result of sulfur ions grabbing an electron from hydrogen atoms in space. That, however, couldn’t explain the new data, Cappelluti and colleagues conclude. Likewise, a quirk of the telescope itself couldn’t explain the line, they have determined.

Indeed, Horiuchi acknowledges: “It’s kind of getting other people excited.”