Tracking Down Diffuse Gamma Rays

The disk of the Milky Way glows with continuous emission of high-energy gamma-ray photons. Where does this diffuse emission come from? A new study suggests that we may be missing the complete picture.

Smashing Cosmic Rays

Roughly 80% of the gamma-ray photons detected by the Fermi LAT gamma-ray detector come from diffuse emission — emission produced in the plane of our galaxy that isn’t associated with specific sources. Scientists have identified speeding cosmic rays as the primary culprit: high-energy protons and atomic nuclei whiz through space at nearly the speed of light, slamming into the interstellar medium and producing byproducts of gamma rays, neutrinos, and more.

Photograph of a detector array spread over a large surface of a plateau surrounded by mountains.

Observations from this cosmic-ray observatory in Tibet reveal the diffuse gamma-ray emission in our galaxy’s disk. [Institute of High Energy Physics of the Chinese Academy of Sciences]

But does this picture tell the whole story? In a recent study, Nanjing University scientists Ruo-Yu Liu and Xiang-Yu Wang point out a possible concern with this model: if cosmic-ray collisions produce the galaxy’s diffuse gamma-ray emission … where are all the neutrinos?

A Conflict from Missing Neutrinos

By modeling the interaction of galactic cosmic rays with the interstellar medium in the Milky Way, Liu and Wang illustrate the problem: in order to reproduce the spectrum of diffuse gamma-ray emission recently observed by detectors on the Tibetan Plateau, the cosmic-ray collisions would also produce a large number of neutrinos — so many, in fact, that they should be observable by detectors on Earth, like the IceCube neutrino observatory. The problem? Based on IceCube’s most recently released results, these predicted neutrinos aren’t there!

Since the neutrino and diffuse gamma-ray observations conflict, Liu and Wang argue, then the model must be missing something. The source of the seemingly diffuse gamma-ray emission from the galactic disk cannot only be cosmic-ray collisions with the interstellar medium. Instead, there must be a contribution from some additional source that produces high-energy gamma rays without also creating lots of neutrinos.

What is that source? The authors have found a potential culprit.

Another Player

Image of a complicated star-forming nebula with bright regions of gamma-ray emission

A grayscale infrared map of the Cygnus cocoon is overlaid here with colored gamma-ray data showing the excesses of high-energy photons. [IFJ PAN / HAWC]

Of the highest-energy diffuse gamma-rays detected by the Tibet observatory, 40% come from a single region: the center of the Cygnus cocoon, a superbubble surrounding a site of massive star formation. Could this area be producing the extra gamma rays observed in the diffuse emission?

Liu and Wang describe several potential sources of gamma rays in the cocoon — like the massive star cluster Cygnus OB2, the supernova remnant γ Cygni, and a pulsar wind nebula — and demonstrate that, by adding contributions from these sources, they can successfully reproduce the diffuse gamma-ray emission we observe while not exceeding the upper limits on neutrino production set by the IceCube observations.

More exploration of this picture is still needed, but the authors’ work shows that we still plenty more to learn about the sources that produce high-energy particles in our galaxy!


“Origin of Galactic Sub-PeV Diffuse Gamma-Ray Emission: Constraints from High-energy Neutrino Observations,” Ruo-Yu Liu and Xiang-Yu Wang 2021 ApJL 914 L7. doi:10.3847/2041-8213/ac02c5