Evidence For Unresolved Gamma-Ray Sources In The Inner Galaxy

Two independent studies lead by Princeton University and MIT, have found that the unusual concentration of gamma-rays around the Milky Way’s core likely come from pulsars, and not colliding dark matter particles as was hypothesized.

Pulsars are rapidly spinning neutron stars (the corpses of massive stars) that emit beams of electromagnetic radiation. When observed from Earth, Pulsars emit gamma-rays (the highest energy form of light in the universe) in clumpy intervals. Using observational and statistical analysis, two independent teams found that gamma-ray signals near the Milky Way’s core were uncharacteristic of those expected from colliding dark matter particles, and likely originate from pulsars instead.


Image credit: Christoph Weniger

The two independent groups – the Princeton/MIT group and the Netherlands-based group – used two different techniques, non-Poissonian noise and wavelet transformation, respectively, to independently determine that the gamma ray signals were not due to dark matter annihilation.

“Our analysis suggests that what we are seeing is evidence for a new astrophysical source of gamma rays at the center of the galaxy,” said Mariangela Lisanti, a professor of physics at Princeton, and one of the authors of a study published in the journal Physical Review Letters. “This is a very complicated region of the sky and there are other astrophysical signals that could be confused with dark matter signals,” she added.

Scientists created models of what gamma-rays would look like had they been created by collisions of dark matter particles. Results showed the high energy photons should appear smoothly distributed when viewed with NASA’s Fermi Gamma-ray Space Telescope.

Researchers then analyzed actual images of gamma-rays from the inner Milky Way and found the distribution to be clumpy, rather than smooth, indicating that the gamma-rays were unlikely to be caused by dark matter particle collisions.

According to Lisanti, the clumpy distribution of gamma-rays could be the signature of very old, rapidly rotating stars known as millisecond pulsars. These pulsars release bursts of light in isolated intervals and are more consistent with the clumpy gamma-ray distribution observed in Fermi images.

Currently, that’s the best guess scientists have, but exactly what these sources of gamma-rays are remains unknown.

Researchers suggest it’s possible to try and identify the pulsars by observing the sources using radio telescopes.

And even though their theory goes against it, scientists still leave the door open for a dark matter explanation.

“Either we find hundreds or thousands of millisecond pulsars in the upcoming decade, shedding light on the history of the Milky Way, or we find nothing. In the latter case, a dark matter explanation for the gamma ray excess will become much more obvious,” said team member Christoph Weniger, from the University of Amsterdam.

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