Another blow to the dark matter theory

A recent study has tried to identify dark matter at the centre of our galaxy, but it appears neutron stars can account for their observations


The best astrophysical interpretation for the recent gamma-ray studies of the centre of the Milky Way can be best explained by thousands of rapidly spinning neutron stars called ‘millisecond pulsars’. Image credit: NASA Goddard; A. Mellinger, CMU; T. Linden, Univ. of Chicago

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/GRAPPA, Netherlands, and the Laboratoire d’Annecy-le-Vieux de Physique Théorique, France, have now found further evidence that rapidly spinning neutron stars are a much more likely source for this radiation. Their findings are published recently 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 Milky Way.

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.

The entire sky at energies greater than 1 GeV based on five years of data from the LAT instrument on NASA’s Fermi Gamma-ray Space Telescope. Brighter colors indicate brighter gamma-ray sources. Image credit: NASA/DOE/Fermi LAT Collaboration

“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. 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.

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