For almost ten years, astronomers have been studying a mysterious diffuse radiation coming from the centre of our Galaxy. Originally, it was thought that this radiation could originate from the elusive dark matter particles that many researchers are hoping to find. However, physicists from the University of Amsterdam and the Laboratoire d’Annecy-le-Vieux de Physique Théorique have now found further evidence that rapidly spinning neutron stars are a much more likely source for this radiation. Their findings are published today in Nature Astronomy.
Observations of the gamma-ray radiation from the Galactic centre region with the Fermi Large Area Telescope have revealed a mysterious diffuse and extended emission. Discovered almost 10 years ago, this emission generated a lot of excitement in the particle physics community, since it had all the characteristics of a long-sought-after signal from the self-annihilation of dark matter particles in the inner Galaxy. Finding such a signal would confirm that dark matter, a substance that so far has only been observed through its gravitational effects on other objects, is made out of new fundamental particles. Moreover, it would help to determine the mass and other properties of these elusive dark matter particles. However, recent studies show that arguably the best astrophysical interpretation of the excess emission is a new population in the Galactic bulge of thousands of rapidly spinning neutron stars called millisecond pulsars, which have escaped observations at other frequencies up to now.
‘Understanding in detail the morphology [the location and shape] and spectrum [the combined frequencies] of the excess emission is of vital importance for discriminating between the dark matter and astrophysical interpretations of the Galactic Centre excess radiation’, says Christoph Weniger, one of the researchers that conducted the study. A new study by researchers at the University of Amsterdam and the Laboratoire d’Annecy-le-Vieux de Physique Théorique, a research unit of the French Centre National de la Recherche Scientifique, found strong evidence that the emission actually seems to come from regions where there is also a large amount of stellar mass in the Galactic bulge (the ‘boxy bulge’) and centre (the ‘nuclear bulge’). Furthermore, the researchers found that the light-to-mass ratio in the Galactic bulge and centre are mutually consistent, so that the gamma ray GeV emission is a surprisingly accurate tracer of stellar mass in the inner Galaxy – see figure 2. This study was based on a new analysis tool, SkyFACT (Sky Factorization with Adaptive Constrained Templates), developed by the researchers themselves, which combines physical modelling with image analysis.
The findings support the millisecond pulsar interpretation of the excess emission, since neither a dark matter signal nor other astrophysical interpretations are expected to show such a correlation. ‘The results will help guide upcoming radio searches for this hidden population of millisecond pulsars in the Galactic bulge with MeerKAT and the future Square Kilometre Array’, said Francesca Calore, another of the paper’s authors. ‘This makes these upcoming searches even more promising.’
R. Bartels, E. Storm, C. Weniger and F. Calore, The Fermi-LAT GeV excess traces stellar mass in the Galactic bulge, Nature Astronomy, 6 Aug 2018.
Richard Bartels, Suraj Krishnamurthy and Christoph Weniger: 'Strong support for the millisecond pulsar origin of the Galactic center GeV excess’ in: Physical Review Letters, (February 4, 2016). http://arxiv.org/abs/1506.05104
Samuel K. Lee, Mariangela Lisanti, Benjamin R. Safdi, Tracy R. Slatyer and Wei Xue: 'Evidence for Unresolved Gamma-Ray Point Sources in the Inner Galaxy' in: Physical Review Letters, February 4, 2016, http://arxiv.org/abs/1506.05124