Enlarge / Fermi’s view of the galaxy’s core.

Not so long ago, I attended a general physics conference where a lot of people were very excited about the Fermi Large Area Telescope. Fermi observed high-energy radiation from the galactic center and found an excess that was hard to explain. Might this be a long-awaited sign of dark matter? Early calculations seemed promising.

Now, it seems that the signal might due to pulsars and not dark matter.

What did Fermi see?

The Fermi telescope watches the sky for gamma rays. These are photons with energies in the range of 10 million electron Volts (eV) up to about 300 billion eV. For comparison sake, light in the visible range is less than 10eV, while a standard X-Ray machine in a hospital has photons with an energy of about 2,000eV. So gamma rays are photons that pack a serious punch.

Creating gamma ray photons obviously takes a lot of energy. We’re talking nuclear fusion and fission or extremely high magnetic fields. Indeed, you only really find magnetic fields strong enough to power gamma rays around black holes, pulsars, and magnetars.

But when Fermi turned its eye toward the galactic center, it needed to be fit with gamma ray sunglasses. The emission of gamma rays with energies around 50GeV was quite a bit brighter than expected. To make it even more confusing, there were no obvious sources for the gamma rays.

Wait, isn’t space kind of empty?

It’s important to realize that the center of the galaxy is hot and relatively dense. This makes viewing the center a bit like looking into a fog. We can see the light, but we can’t always see where it comes from. To be more precise, for high-energy photons, the galactic center is hot and dense, while for lower-energy photons, the galactic center is much more transparent.

So seeing a diffuse gamma ray glow was not that surprising since we expect things to look smeared out at these energies. But we can measure the total energy, and we have a pretty good idea of what that should be since we know what’s in the galactic center from other telescope studies.

The problem with the Fermi data is that the two sets of observations simply didn’t seem to add up. The high density of the galactic center is also why dark matter was such a hopeful candidate. Dark matter only interacts rarely, but the Milky Way’s core is where dark matter is most dense and has the highest chance of revealing itself through some sort of light-emitting process.

Even more exciting, the bright gamma-ray glow fit quite well to a dark matter being annihilated into a pair of matter/antimatter quarks. These would then collide with each other and vanish in a puff of gamma ray smoke. And just to get the juices really flowing: the spatial distribution of the gamma ray glow was pretty close to the expected dark matter distribution.

We were all pretty excited.

Reality bites

The extra gamma rays had one other intriguing feature: the energy range (or spectrum) over which they are observed is very similar to that emitted by millisecond pulsars.

To check if this link had any significance, a group of researchers took what they know about the shape of galactic centers and used that to model the gamma ray spectrum. As they state in the paper, the center of the galaxy has a “boxy/peanut-shaped bulge/bar” within it. This had not been taken into account in previous calculations.

The size and mass of the peanut at the center of the galaxy is relatively well measured, although the distribution of stars (e.g., the star types and their exact locations) within the shape is less well known. The number of millisecond pulsars is even less well known. So the researchers used a model they had recently developed to calculate the expected gamma ray spectrum and spatial distribution from the center of the galaxy with different distributions of stars.

They showed that they could fit the Fermi telescope observations using millisecond pulsars. Their model also produces mass densities that fit with observations of the center of the galaxy. Furthermore, the fit is better than that obtained using dark matter. This combination of results is what gives the researchers some confidence that their explanation is more likely than dark matter annihilation.

A critical point is that it takes a lot of millisecond pulsars: about ten times more per unit of mass than is observed in the galactic disk. However, the researchers offer no comment on whether this increase in millisecond pulsars in the center of the galaxy is something we might have expected.

The main point is that there is a confluence of evidence that points to pulsars rather than dark matter as the most likely explanation. The next critical datapoint will be the density of millisecond pulsars, and that will have to await the commissioning the square kilometer array or meerKAT radio telescopes.

Nature Astronomy, 2018, DOI: 10.1038/s41550-018-0531-z, (About DOIs).




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