For more than ten years, astrophysicists have been searching for the source of mysterious gamma-ray emissions at the center of the Milky Way. A new study using machine learning has supported the dark matter hypothesis while casting doubt on the pulsar theory.
Artistic illustration of the center of the Milky Way with diffuse gamma radiation whose nature scientists have still not determined.
A Mysterious Signal
The Galactic Center Excess is a blurred, spherical glow of gamma rays stretching thousands of light-years from the core of the galaxy. It was detected using the Fermi Space Telescope about fifteen years ago.
The signal cannot be explained by known sources of gamma radiation. Scientists are considering two main explanations. One is related to an indistinguishable collection of millisecond pulsars, while the other involves the annihilation of dark matter particles.
Machine Learning Changed the Score
A team of researchers from the University of Vienna and Lawrence Berkeley National Laboratory trained an algorithm on more than one million simulated gamma-ray observations. Unlike previous approaches, this method accounted for the energy of individual photons for the first time.
Previous analyses pointed to relatively bright point sources as the likely explanation for the excess. The new results showed the opposite: such sources would have to be extremely faint.
Too Many Pulsars Would Be Needed
“Our analysis showed that these sources would be so faint that they would be almost impossible to distinguish from the emission expected from annihilating dark matter,” said Nick Rodd, a scientist and one of the study’s authors.
This casts serious doubt on the pulsar hypothesis. If the sources are so faint, then at least 35,000 millisecond pulsars would have to exist in the center of the Milky Way to reproduce the observed signal. Previous estimates of the pulsar population in the galactic center had ranged from several hundred to a few thousand.
Dark Matter Remains in Play
Dark matter makes up about 85% of all matter in the Universe, but it does not interact with light or ordinary matter, which is why it remains invisible. Among the theoretical candidates are so-called weakly interacting massive particles, WIMPs, which have the property of self-annihilation. At the high concentrations expected in the galactic core, such particles would annihilate when they collide, releasing energy in the form of gamma rays.
The new study changes the terms of this debate. If more than 35,000 pulsars are required, and if their emission is practically indistinguishable from a dark-matter signal, then dark matter remains a fully viable explanation for the gamma-ray excess at the center of the galaxy.
Florian List of the University of Vienna summed it up precisely: “Our work does not prove that dark matter is responsible for the signal. However, it shows that it is still too early to rule out that possibility.” The study, authored by Florian List, Eugene Park, Nicholas L. Rodd, Iva Schoen, and Florian Wolf, was published in the peer-reviewed journal Physical Review Letters.
