Galactic Center’s Gamma Rays Are Not From Dark Matter

All-sky Map of the Milky Way in Gamma-rays by Fermi. NASA

Science often asks more questions that one can answer and often the answers lead to even more questions. In that vein, last week one of the mysteries of our galaxy has just got a little bit more mysterious.

Two papers, published in Physics Review Letters, looked at the emission of powerful gamma rays from the center of the Milky Way. It was expected that a large fraction of these powerful photons is produced by dark matter, but both studies indicated the emissions are from a different astrophysical source altogether – fast-rotating stars called pulsars.

The center of the Milky Way is a complex place. It’s a dense region with star clusters, gas clouds, and a supermassive black hole called Sagittarius A*. As dark matter is thought to only interact gravitationally, astronomers have postulated that it should be found in abundance in dense regions such as the galactic center.

Dark Matter is thought to make up 85 percent of all the matter in the universe, so the researchers looked at dense regions hoping to observe a phenomenon predicted by several theories: dark matter annihilation. If two particles of dark matter interact with each other they would annihilate and convert into pure energy, the gamma ray photons which astronomers detected. If dark matter annihilation was widespread as predicted the emissions should be smooth across the center of our galaxy. Instead the teams' statistical analysis of images captured by the Fermi telescope revealed blotchy emissions.

"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, an assistant professor of physics at Princeton and coauthor of one of the papers, in a statement.

"This is a very complicated region of the sky and there are other astrophysical signals that could be confused with dark matter signals."

Since the researchers didn’t observe the even gamma ray signals they expected of dark matter annihilation, they think the gamma rays must be produced by different sources of high-energy radiation, most likely the fast-rotating stars known as pulsars. These stars pack the mass of the Sun in just a few kilometers in diameter and they spin on their axis hundreds of times per second. This phenomenon, combined with strong magnetic fields, creates a lighthouse effect, meaning these stars appear to pulsate.

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