Last year a team of astronomers detected an unexplained gamma-ray signal from the core of the Milky Way and scientists got pretty excited about it, as it could have been the first evidence of dark matter annihilation.
A different team has looked at the signal again, trying to model all the possible explanations put forward originally, and they are quite confident that the signal is actually pulsars rather than dark matter. Their results will be published in the Astrophysical Journal.
Pulsars were a strong candidate from the very beginning but the signal didn’t appear to be very pulsar-like. It appeared rather extended and smooth, which instead matched the expected profile for dark matter annihilation. Pulsars also tend to have a very short lifespan (about 10 million years) and the stars at the center of the Milky Way tend to be much older.
But pulsars weren’t dismissed. This signal only appeared in the Milky Way and not in other galaxies, which rather favor a less universal “dark matter” solution. Follow-up observations suggested that the signal was actually speckled and not as smooth as originally thought.
"Considering that about 70 percent of all point sources in the Milky Way are pulsars, they were the most likely candidates but we used one of their physical properties to come to our conclusion," Mattia Di Mauro from the Kavli Institute for Particle Astrophysics and Cosmology, said in a statement.
"Pulsars have very distinct spectra – that is, their emissions vary in a specific way with the energy of the gamma rays they emit. Using the shape of these spectra, we were able to model the glow of the galactic center correctly with a population of about 1,000 pulsars and without introducing processes that involve dark matter particles."
For such a large population of pulsars to persist for such a long time, they require companions from which they can steal gas and undergo a “rejuvenation” process. Pulsars are the collapsed cores of stars, the end product of smaller supernovae. They are incredibly dense and spin hundreds of times per second. Robbing a companion object of its gas allows them to keep this spin up. By spinning so rapidly, and possessing an intense magnetic field, they produce periodic pulsation that can be detected by telescopes.
This study is good news for astronomers that want to know more about the central regions of the Milky Way but will disappoint dark matter hunters. Dark matter, whose existence is still in question, should accumulate in the core of galaxies and produce gamma rays but maybe there’s less there than expected.
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