Is the Fermi gamma-ray excess evidence for dark matter or due to milli-second pulsars?

Fermi Satellite, 3d model.
Image Source: NASA

The Large Area Telescope on board the Fermi spacecraft looks out for the most extreme events in the cosmos. Launched 2008, it scans the whole sky for gamma rays in the very high energy range, from 20 MeV to about 300 GeV; a full scan takes about 3 hours. One of Fermi’s most interesting findings is an unexpectedly large amount of gamma-rays, ordinary light but at enormous energy, stemming from the center of our galaxy.

The Fermi gamma-ray excess has proved difficult to explain with standard astrophysical processes. The spectral distribution of the observed gamma-rays over energies bulges at about 2 GeV and it is hard to come up with a mechanism that produces particles at such high energies that prefer this particular spectral feature. The other puzzle is that whatever the sources of the gamma-rays, they seem homogeneously distributed in the galactic center, out to distances of more than ten degrees, which is about 5,000 light years – a huge range to span.

The most exciting proposal to solve the riddle is that the gamma-rays are produced by dark matter annihilation. Annihilation spectra often bulge at energies that depend on both the mass of the particles and their velocity. And the dark matter distribution is known to be denser towards centers of galaxies, so one would indeed expect more emission from there. While dark matter isn’t entirely homogeneous but has substructures, the changes in its density are small, which would result in an overall smooth emission. All of this fits very well with the observations.

For this reason, many particle physicists have taken their dark matter models to see whether they can fit the Fermi data, and it is possible indeed without too much difficulty. If the Fermi gamma-ray excess was due to dark matter annihilation, it would speak for heavy dark matter particles with masses of about 30 to 100 GeV, which might also show up at the LHC, though nothing has been seen so far.

Astrophysicists meanwhile haven’t been lazy and have tried to come up with other explanations for the gamma-ray excess. One of the earliest and still most compelling proposals is a population of millisecond pulsars. Such objects are thought to be created in some binary systems, where two stars orbit around a common center. When a neutron star succeeds in accreting mass from a companion star, it spins up enormously and starts to emit large amounts of particles including gamma rays reaching up to highest energies. This emission goes into one particular direction due to the rapid rotation of the system, and since we only observe it when it points at our telescopes the source seems to turn on and off in regular intervals: A pulsar has been created.

Much remains to be understood about millisecond pulsars, including details of their formation and exact radiation characteristics, but several thousands of them have been observed so far and their properties are observationally well documented. Most of the millisecond pulsars on record are the ones in our galactic neighborhood. From this we know that they tend to occur in globular clusters where particularly many stars are close together. And the observations also tell us the millisecond pulsar spectrum peaks at about 2 GeV, which makes them ideal candidates to explain the Fermi gamma-ray excess. The only problem is that no such pulsars have been seen in the center of the galaxy where the excess seems to originate, at least so far.

There are plausible reasons for this lack of observation. Millisecond pulsars tend to be detected in the radio range, at low energies. Only after astronomers have an idea of where exactly to look, they can aim precisely with telescopes to confirm the pulsation in the higher energy range. But such detections are difficult if not impossible in the center of the galaxy because observations are shrouded by electron gas. So it is quite plausible that the millisecond pulsars are in the center, they just haven’t been seen. Indeed, model-based estimates indicate that millisecond pulsars should also be present in the galactic center, as laid out for example in this recent paper. It would seem odd indeed if they weren’t there. On the other hand it has been argued that if millisecond pulsars were the source of the gamma-ray excess, then Fermi should also have been able to pinpoint a few of the pulsars in the galactic center already, which has not been the case. So now what?

The relevant distinction between the both scenarios to explain the gamma ray excess, dark matter annihilation or millisecond pulsars, is whether the emission comes from point sources or whether the sources are indeed homogeneously distributed. This isn’t an easy question to answer because Fermi basically counts single photons and their distribution is noisy and inevitably sometimes peaks here or there just by coincidence. Making estimates based on such measurements is difficult and requires sophisticated analysis.

In two recent papers now, researchers have taken a closer look at the existing Fermi data to see whether it gives an indication for point-like sources that have so far remained below the detection threshold beyond which they would be identified as stellar objects. For this they have to take the distribution of the measured signals, extract peaks ordered by magnitude, and test this measured distribution against a random distribution.

Strong support for the millisecond pulsar origin of the Galactic center GeV excess
Richard Bartels, Suraj Krishnamurthy, Christoph Weniger
arXiv:1506.05104 [astro-ph.HE]

Evidence for Unresolved Gamma-Ray Point Sources in the Inner Galaxy
Samuel K. Lee, Mariangela Lisanti, Benjamin R. Safdi, Tracy R. Slatyer, Wei Xue
arXiv:1506.05124 [astro-ph.HE]

The difference between these papers is the method they used to identify the point sources. The first paper by Bartels et al uses a wavelet analysis on the data, that is somewhat like a Fourier transform with a local profile, to pick up potential sources with low statistical significance. The Lee et al paper tries to generate a pattern close to the observed one by using various mixtures of noise and point sources of particular spectra. In both papers the researchers find that the data indicates the origin of the gamma-rays is point sources and not entirely smoothly distributed. In the first paper, the authors moreover extract the necessary density of pulsars in the galactic center to explain the observations, and demonstrate that it is possible the pulsars give rise to the observed excess and might so far have stayed just below the detection threshold for point sources.

Taken together, it looks like the evidence has now shifted in favor of millisecond pulsars. As Christopher Weniger from the University of Amsterdam put it “[The pulsars] are there, we know they are there, and they have the right spectrum. We first have to rule out that this isn’t what we see.” Rather than ruling out astrophysical sources as origin of the gamma-ray excess however, the researchers are now well on the way to confirm it’s the pulsars that cause the signal.

Finding definite evidence that the Fermi gamma-ray excess is due to millisecond pulsars is difficult but not impossible. What is needed is more statistics that will allow resolving the point sources better, and more time will bring more statistics. The puzzle isn’t solved yet, but chances are good it will be solved within the next years. What constitutes dark matter however at least for now remains a mystery.