Milky Way’s gamma ray emissions may not be caused by dark matter

NASA Goddard; A. Mellinger/CMU; T. Linden/U. Chicago

Milky Way’s gamma ray emissions may not be caused by dark matter

There’s more gamma ray radiation coming from our galaxy’s inner realm than known objects in that region can generate. Among other sources of such radiation, scientists have proposed that interactions between bits of dark matter (which make up a large fraction of the universe’s mass but haven’t yet been directly detected) in a halo around the galactic center may be creating the surplus gamma rays. But two new analyses to be published in a forthcoming issue of Physical Review Letters hint that the excess gamma-ray radiation (depicted in the image above in varying colors, red being the most intense, and superimposed on a visible-light image of the center of the Milky Way) could stem from large numbers of fast-spinning neutron stars called pulsars. One reason: The pattern of gamma-ray photons streaming from the galactic center is clumpy rather than smooth, a strong sign that individual sources of gamma rays (rather than a diffuse cloud of particles that only occasionally interact) may be to blame. In the new studies, the two teams—one from the University of Amsterdam in the Netherlands, and the other made up of scientists from Massachusetts Institute of Technology and Princeton University—used slightly different statistical methods to estimate how many pulsars would be needed to produce the pattern of gamma-ray radiation that’s been observed by an Earth-orbiting gamma-ray telescope. Nicely, they came up with approximately the same number. In the sphere of space that lies within about 5000 light-years of the center of the galaxy, a little more than 60 such pulsars of varying size and brightness could generate about half the excess gamma rays seen emanating from that region, and about 200 could account for all of the excess radiation at those wavelengths. The pulsars can’t be seen individually, the researchers say, because they’re incredibly small, whereas the width of an image pixel at the distance of the galactic center is about 50 light-years across. Future observations should spot hundreds of the elusive pulsars, one researcher suggests. If not, theorists may need to revert to the dark-matter explanation—or come up with another idea entirely—for the excess gamma rays. 

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