IceCube has found a new source of neutrinos - our own Milky Way galaxy, with a significance of 4.5 sigma! This observation was published in a recent paper in Science; a freely available version is available on the arXiv. This study is both technically and scientifically very different from the two observations of neutrinos from active galactic nuclei (AGN) that IceCube previously observed.
Earth is within the Milky Way, which is, from our observation point, largely a plane in space so the source surrounds us. High-energy gamma-rays have been observed coming from the Milky Way The line has a width of a couple of degrees, depending on how you define the width. This is very different from other galaxies, which are point sources (or close to that); the different geometry calls for a different analysis technique. Instead of using muons from muon-neutrinos, this study used `cascades,' which come from electron-neutrinos, and neutral-current interactions of all neutrino flavors. The advantage of using cascades is that the background of atmospheric neutrinos is much lower, so the signal:noise ratio is higher. The disadvantage is that the angular resolution isn't nearly as good. However, IceCube used a machine-learning technique, a convolutional neural network (CNN), to determine cascade directions. A CNN works in a roughly similar manner to our own brains, with neuron-like processing steps that looked at the light deposition in IceCube's sensors. This approach gives a resolution that is about two times better than previous cascade directional studies, lessening the difference with nu_mu's. And, it used many more events. since the Milky Way is not a point source, the angular resolution is less important. The figure below shows how the analysis was done:
The other interesting thing about the neutrinos from the Milky Way is that we are in the galactic plane. This is very different from the previous observations, of neutrinos from NGC1068 and TXS0506, both of which are AGNs, with considerable high-energy activity. We are also observing them from relatively close to their axis of rotation, where high-energy emission is more likely. In contrast, we are in the plane of the Milky Way, and it appears that the neutrinos are coming from many directions.
IceCube found that the neutrino emission was consistent with the pattern of photon emission, assuming that the photons came from pi^0 and neutrinos come from charged pions. However, the measured neutrino flux was considerably higher than one would expect based on pi^0 extrapolations. There could be several reasons for this, including photon absorption en-route to Earth. The neutrinos and pi^0 might be produced in sources, of when high-energy cosmic-rays interact with atoms or dust while moving around the galaxy. However, a very recent new search for emission from a number of known sources of energetic (TeV) gamma-rays did not find evidence for an excess of neutrinos. In short, this is an important observation that raises many interesting new questions.
