The Mystery of the Gamma Rays

There’s something strange going on in the center of the Milky Way galaxy.

In 2009, the Fermi Large Area Telescope (principal observing instrument on the Fermi Gamma Ray Space Telescope) discovered an unusually high amount of gamma rays emanating from the center of our galaxy.

It had been speculated that this excessive outpouring of high-energy radiation could be an indication of the annihilation of mysterious “dark matter”—that yet-to-be-found cosmic component that is often invoked to explain the disparities between observed matter and the total gravitational influences at work in the universe.

If true, the detection would be a major breakthrough.  It would constitute a new means for investigating the nature of this fundamental conundrum, this “missing mass” of the universe, and perhaps finally allow scientists to complete their census of all cosmic matter, visible and invisible.

But a new analysis of the gamma ray data has quashed such hopes, at least in the near term, and shown the galactic gamma ray excess to have its roots in somewhat more “mundane” astrophysical phenomena.

Gamma ray map of the Milky Way galaxy, from the Fermi Space Telescope. The insets reveal the point-like, localized sources of the gamma rays, which rules out a dark matter origin. Credit: Christoph Weniger, UvA, UvA/Princeton
The Case of the Millisecond Pulsars

The research was conducted by two different teams, one from Princeton/MIT in the United States, and another from the University of Amsterdam in the Netherlands.  They each used different statistical methods—“non-Poissonian noise” for the Americans, “wavelet transformation” for the Europeans—to filter the Fermi data.

What they found was that the distribution of gamma rays was clumpy and uneven, rather than the smoother, more uniform placement one would expect from continuous dark matter annihilation.

Which indicated, naturally, “unresolved point sources”—in this case, probably millisecond pulsars.  Among the most extreme objects in the universe, millisecond pulsars are neutron stars (the ultra-dense, asteroid-sized remnants of the explosions of supergiant stars) that are rotating so rapidly they complete a turn, or several, every thousandth of a second.  Imagine that—a spherical object with a diameter roughly equal to the length of Manhattan, packing nearly twice the mass of the Sun, completing a full rotation in a fraction of a second!

Naturally, such a bizarre phenomenon produces immense quantities of energy, and this is precisely what millisecond pulsars do, generating more than enough high-energy photons to account for the Fermi telescope’s gamma ray detection.

But the sheer number of gamma rays is so excessive that only a previously unknown and unsuspected population of hundreds or thousands of pulsars could produce the observed signals.

Which, according to Mariangela Lisanti of Princeton University, is intriguing in its own right: “The results of our analysis probably mean that what we are seeing is evidence for a new population of astrophysical sources in the center of the galaxy.  That in itself is something new and surprising.”

So far, the actual source (or sources) of the gamma rays is still unknown; so effectively screened from prying eyes is the galactic center, hidden in its vast dusty cocoon, that telescopes have yet to directly image any such pulsars—or much of anything else, for that matter.  But projected surveys by existing and forthcoming radio telescopes should disclose any hidden pulsars in the core, and resolve the mystery once and for all.

Either way,  it seems that the center of our galaxy is a very strange and mysterious place.

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