Gamma Rays Point to Pulsars, Not Dark Matter

Gamma rays are the most powerful form of electromagnetic radiation in the universe.  With wavelengths as small at atoms, they usually result from the most powerful interactions known, such as the collision of two particles, or the release of energy from the accretion disk of a black hole.  But there is another potential source of gamma rays that has not yet been confirmed: Dark Matter.

Gamma Ray signals from the Galaxy shown with two different statistical techniques. Credit: Image courtesy of Christoph Weniger, UvA. © UvA/Princeton

The leading candidate for dark matter is the theorized Weakly Interacting Massive Particle (WIMP), though it is not as wimpy as its namesake suggests, making up 5 times as much mass as the visible matter in the universe.

WIMPs are thought to collided every so often, and when they do it’s expected that they will produce a gamma ray flash, one that should be visible from Earth.  For an Earthbound observer, we should see a smattering of gamma rays across the entire galactic halo where we expect to find the dark matter.  So what did we see?

Two independent teams of astrophysicists have looked at data from NASA’s Fermi Gamma Ray Space Telescope, and found that the gamma ray emission is concentrated near the galactic center in a few bright sources, as opposed to the random faint sources expected for WIMP interactions.

This could mean two things:

  1. The gamma rays are coming from some other source
  2. WIMPs don’t release gamma rays upon collision

The potential other source could be millisecond pulsars, neutron stars the size of cities that are spinning once every millisecond.  This rapid spin creates incredibly powerful magnetic fields and released gamma radiation.  This would explain the observed brighter concentrated sources.

Although it complicates the search for dark matter, every step is a step in the right direction for scientists.  According to MIT’s Douglas Finkbeiner “Our job as astrophysicists is to characterize what we see in the universe, not get some predetermined, wished-for outcome. Of course it would be great to find dark matter, but just figuring out what is going on and making new discoveries is very exciting.”

Christoph Weniger from the University of Amsterdam, lead author of one of the independent studies says that the finding is a win-win situation. “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.”

Whatever dark matter turns out to be, it won’t be some simplistic thing.  If it was, it wouldn’t be so elusive and difficult to detect.  We are moving in the right direction, and I hope to see a full understanding of dark matter in my lifetime.


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