Astronomers from the Australian National University, the University of New South Wales, the University of Canterbury and the University of Tokyo have found an alternative explanation for an anomalous gamma-ray signal coming from the center of the Milky Way, which was long claimed as a signature of dark matter.
An artist’s impression of a millisecond pulsar. Image credit: NASA’s Goddard Space Flight Center.
Gamma-rays are a form of electromagnetic radiation with the shortest wavelength and highest energy.
The Galactic Center excess is an unexpected concentration of gamma-rays emerging from the center of our Milky Way Galaxy.
The signal was first detected in 2009 by the Large Area Telescope, the primary instrument on NASA’s Fermi Gamma-ray Space Telescope.
Its origin has been debated, with proposed sources prominently including self-annihilating dark matter and an undetected population of millisecond pulsars.
“Our work does not throw any doubt on the existence of the signal, but offers another potential source,” said Dr. Roland Crocker, an astronomer in the Research School of Astronomy and Astrophysics at the Australian National University.
“It is based on millisecond pulsars — neutron stars that spin really quickly — around 100 times a second.”
“Scientists have previously detected gamma-ray emissions from individual millisecond pulsars in the neighborhood of the Solar System, so we know these objects emit gamma-rays.”
“Our model demonstrates that the integrated emission from a whole population of such stars, around 100,000 in number, would produce a signal entirely compatible with the Galactic Center excess.”
The team’s discovery may mean astronomers have to re-think where they look for clues about dark matter.
“The nature of dark matter is entirely unknown, so any potential clues garner a lot of excitement,” Dr. Crocker said.
“But our results point to another important source of gamma-ray production.”
“For instance, the gamma-ray signal from Andromeda, the next closest large galaxy to our own may be mostly due to millisecond pulsars.”
The team’s paper was published in the journal Nature Astronomy.
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A. Gautam et al. Millisecond pulsars from accretion-induced collapse as the origin of the Galactic Centre gamma-ray excess signal. Nat Astron, published online April 28, 2022; doi: 10.1038/s41550-022-01658-3
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