Weak gamma-ray emission around the core of the Milky Way still has no reliable explanation. One of the leading theories links it to a population of rapidly rotating neutron stars. A new analysis of the data using machine-learning methods has shown that there would have to be an order of magnitude more such objects than previously estimated. Therefore, the annihilation of dark matter particles cannot yet be ruled out as a possible explanation.
Radiation Without a Source
The excess gamma-ray emission in the central part of the Galaxy was detected in 2009 using the orbiting Fermi telescope. The excess has an approximately spherical shape and extends for thousands of light-years around the core.
Deep observations have not revealed any specific object capable of producing it. The signal became known as the Galactic Center Excess.
Theories of Its Origin
Several explanations have been proposed. Some studies pointed to millisecond pulsars, neutron stars with rotation periods measured in thousandths of a second.
Others linked the emission to the dense stellar population of the central region or to the supermassive black hole Sirius A*, as Universe Today writes. None of these explanations has received decisive confirmation because the region is too bright and crowded with sources.
A third explanation relies on dark matter. The hypothesis works if each dark matter particle is also its own antiparticle. When two such particles collide, they annihilate and produce gamma-ray photons. The most likely candidate is considered to be weakly interacting massive particles (WIMPs), whose existence remains purely theoretical.
The Energy of Each Photon
Previous statistical analyses relied primarily on how the radiation is distributed across the sky. The energy of each detected photon was largely left out, even though it is precisely this information that distinguishes the contribution of point-like objects from a diffuse glow.
An international team from the University of Vienna and Lawrence Berkeley National Laboratory trained a neural network on more than one million simulated gamma-ray observations, according to a press release from the University of Vienna. For the first time, this approach combined spatial and spectral information in a single analysis.
How Many Pulsars Are Needed
The picture changed significantly once photon energies were included in the calculations. Earlier models described relatively bright unresolved point sources, whereas the new analysis indicates that such sources would have to be much fainter.
According to study co-author Nick Rodd, the sources would have to be so dim that they would be almost indistinguishable from the emission expected from dark matter annihilation. For this explanation to work, there would need to be at least 35,000 pulsars in the center of the Galaxy. Previous estimates ranged from several hundred to several thousand.
Over the entire history of radio astronomy, around three thousand pulsars have been detected throughout the Galaxy, and only a few hundred of them are millisecond pulsars. The new calculations suggest that the central region alone would have to contain a population an order of magnitude larger than the entire known catalog.
What the New Estimate Changes
The authors do not claim that the mystery has been solved. Their conclusion is that one of the strongest arguments against the dark matter hypothesis has lost some of its weight.
The results were published in the peer-reviewed journal Physical Review Letters. First author Florian List calls the origin of the excess one of the longest-running debates in astrophysics and adds that it is still too early to dismiss the dark matter explanation.
