An international research collaboration between the University of Vienna and Lawrence Berkeley National Laboratory in the United States has used machine learning to re-examine one of the most hotly debated signals in astrophysics. The so-called Galactic Center Excess (GCE), a faint, roughly spherical glow of gamma rays at the center of the Milky Way, has fascinated physicists for more than a decade. The new results suggest that an explanation in terms of dark matter cannot currently be ruled out. The results have now been published in the journal Physical Review Letters.

The Galactic Center Excess (GCE) is a roughly spherical glow of gamma rays extending over thousands of light years around the center of the Milky Way. Several explanations have been proposed for this unusual signal: theoretical predictions are consistent with self-annihilating dark matter. Another possibility is a large population of rapidly rotating neutron stars known as millisecond pulsars. The origin of the signal at the center of our galaxy therefore remains unresolved.
“Interpreting the signal is particularly difficult because the Galactic Center is an exceptionally bright and crowded region of the gamma-ray sky,” explains Florian List, study author and researcher at the University of Vienna.

Including Photon Energies for the First Time Brings a Decisive Change

The pulsar hypothesis has been supported by previous statistical studies. However, earlier analyses did not include a crucial piece of information: the energy of each individual detected photon. In the new study, the research group developed a machine-learning method trained on more than a million simulated gamma-ray observations. The aim was to evaluate spatial and spectral information simultaneously for the first time.

Including this energy information changes the picture substantially. Whereas earlier analyses pointed to comparatively bright, unresolved light sources (point sources), the new results show that these point sources would have to be extremely faint. “Our new analysis shows that the sources would have to be so faint that they would be almost indistinguishable from the emission expected from annihilating dark matter”, says Nick Rodd, study author and scientist at the Lawrence Berkeley National Laboratory.

For the pulsar hypothesis, this would imply that there must be at least 35,000 such sources in the center of the Milky Way — significantly more than the few hundred to few thousand sources assumed in some previous studies.

Dark matter remains plausible in the debate about the center of the Milky Way

“The origin of the Galactic Center Excess is one of the longest-running debates in astrophysics,” says Florian List. “Our work does not show that dark matter is responsible for the signal. However, it suggests that it is still too early to rule out this possibility.”

The new results weaken one of the strongest arguments so far against the dark-matter hypothesis. Although the study does not provide direct evidence for dark matter, the hypothesis that the Galactic Center Excess is due to dark matter remains a plausible explanation in the debate.

Summary:

• The Galactic Center Excess (GCE) is a roughly spherical glow of gamma rays at the center of the Milky Way.
• One possible origin of this glow is a population of rapidly rotating neutron stars, known as millisecond pulsars. The new results show that dark matter also remains a plausible explanation.
• In the new study, the research group developed a machine-learning method that incorporated photon energies for the first time.
• The study does not show that dark matter is responsible for the signal. However, it suggests that it is still too early to rule out this possibility.

About the University of Vienna:

At the University of Vienna, curiosity has been the core principle of academic life for more than 650 years. For over 650 years the University of Vienna has stood for education, research and innovation. Today, it is ranked among the top 100 and thus the top four per cent of all universities worldwide and is globally connected. With degree programmes covering over 180 disciplines, and more than 10,000 employees we are one of the largest academic institutions in Europe. Here, people from a broad spectrum of disciplines come together to carry out research at the highest level and develop solutions for current and future challenges. Its students and graduates develop reflected and sustainable solutions to complex challenges using innovative spirit and curiosity.