IceCube search for neutrino emission from extended sources in the galactic plane

The sources of cosmic rays—extremely energetic particles that rain down on Earth—have been a long-standing mystery in the field of astronomy. Cosmic-ray accelerators in the PeV energy range, known as PeVatrons, produce pions when the cosmic rays interact with their surroundings. These pions then decay into gamma rays and nearly massless particles called neutrinos. These high-energy neutrinos are the subject of study for the IceCube Neutrino Observatory at the South Pole. 

The High-Altitude Water Cherenkov (HAWC) Observatory and the Large High Altitude Air Shower Observatory (LHAASO) have detected several sources within the Milky Way that emit photons at more than 100 TeV, also in the same range as high-energy neutrinos. If the accelerated protons crash into each other and produce gamma rays, then by virtue, these same sources should also produce neutrinos. 

Recently, IceCube reported the first evidence of high-energy neutrino emission from the galactic plane. However, definite sources of these galactic neutrinos remain to be identified. 

In a paper recently submitted to The Astrophysical Journal, the IceCube Collaboration presents a search for extended sources of neutrinos within the Milky Way and their correlation with the highest-energy gamma-ray sources. A dedicated search was performed using 10 years of IceCube data, with the most significant location (2.6σ post-trials) coincident with the unidentified TeV gamma-ray source 3HWC J1951+266. Although not significant enough to constitute a source, the researchers put strong constraints on neutrino emission from several candidate cosmic-ray accelerators in the galaxy.

A map showing a portion of the galactic plane with an excess of neutrinos above the background
A local p-value map of a portion of the galactic plane showing the relative excess of neutrinos above background in IceCube data. The positions of the high-energy gamma-ray sources are marked with black crosses. The most significant region can be seen on the right near 3HWC J1951+266. Credit: IceCube Collaboration

The study improved on previous IceCube searches in the galactic plane by 1) using more data and 2) performing a search using a catalog of spatially extended regions. The 10-year dataset consists of track-like events that originate from atmospheric neutrinos and muons produced during the interaction of cosmic rays with the atmosphere. Researchers also analyzed the track-like events to see if they clustered in regions between 0.5° and 2.0˚ in the galaxy (as a reference, the moon has an apparent radius of 0.25˚). 

From the catalog, 20 regions were selected based on their emission of > 50 TeV gamma rays. By performing both the catalog search and the scan across the galactic plane, the researchers could point to or rule out any correlation between neutrinos and gamma rays. 

“Both analyses showed an excess of neutrinos from the same 1.7˚ region in the sky that pointed to a TeV gamma-ray source, 3HWC 1951+266, with a significance of 2.6σ,” says Mehr Un Nisa, a postdoctoral researcher at Michigan State University and study lead. “However, it is not significant enough to constitute evidence for a source.”

With the observed neutrino emission, IceCube places strong constraints on regions suspected of containing PeVatron candidates. The results also bring astrophysicists one step closer to understanding the nature of the highly energetic gamma-ray emitters in the galaxy.

“The identification of a galaxy hot spot is a first step towards deeper studies of galactic neutrino sources,” says Nisa. “The hot spot can now be confirmed or ruled out as a source in future IceCube searches with more data and multiwavelength studies.”

+ info “Search for Extended Sources of Neutrino Emission in the Galactic Plane with IceCube,” IceCube Collaboration: R. Abbasi et al. Submitted to The Astrophysical Journal,