Neutrinos are tiny, nearly massless particles that travel cosmological distances unhindered, acting as messengers that carry information about their sources. Since the recent detection of high-energy neutrino emission from the Milky Way, the IceCube Neutrino Observatory at the South Pole is working to pinpoint the exact nature of the Galactic emission contributing to the astrophysical neutrino signal. These neutrinos can be produced by individual sources, such as pulsars and supernova remnants, or by cosmic rays interacting with Galactic dust and matter. In order to narrow down the possibilities, more research is needed to better understand the neutrinos emanating from our own galaxy.
In a study recently published in The Astrophysical Journal, a team of researchers used an approach that simulated Galactic neutrino sources in order to better understand the results presented by the IceCube Collaboration. The study constrains the number of individual neutrino emission regions, encompassing both discrete astrophysical sources and individual points of diffuse emission.
Because the IceCube signal follows the shape of the Galaxy in two dimensions, the team can infer the approximate shape in three dimensions. They wrote a simulation package dubbed SNuGGY (for “simulation of neutrino and gamma-ray Galactic yield”) that simulates Galactic point-like neutrino sources in two steps. First, the sources were mapped and positioned using a 3D spatial probability density function, from which flux values were assigned to the simulated sources. Additionally, the researchers ensured that the total neutrino flux from the simulated sources matched the total Galactic neutrino flux measured by IceCube.
“Once we had SNuGGY to simulate our galaxy, we used the sensitivity and discovery potential curves of IceCube datasets to see if our simulated neutrino sources would be detected and resolved by IceCube,” said Abhishek Desai, former John Bachall postdoctoral fellow at the Wisconsin IceCube Particle Astrophysics Center (WIPAC) at the University of Wisconsin–Madison (UW–Madison), who led the study. “By comparing our results to what was reported by IceCube, it allows us to report the minimum number of point-like neutrino sources in our galaxy that would be needed to explain the total flux observed by IceCube.”
In addition to a lower limit on the number of emission points, the team set an upper limit on the luminosity of each source.
Desai, now a NASA Postdoctoral Program (NPP) fellow at the NASA Goddard Space Flight Center, was part of a team that included UW–Madison associate professor Justin Vandenbroucke, Clemson University research fellow Samalka Anandagoda, UW–Madison physics PhD student Jessie Thwaites, and UW–Madison postbaccalaureate researcher MJ Romfoe.
“SNuGGY is designed to have a much broader use with the possibility of adding more functionality over time, including the use of observational data for improved simulations. It can also simulate gamma-ray sources in the Galaxy and can be used for other neutrino-related simulation studies,” says Anandagoda, who worked on the development of the SNuGGY code. The SNuGGY package is made available to the public.
Desai and the team found that at least eight sources are required to account for the Galactic neutrino flux at a set luminosity or intrinsic brightness. This includes ruling out a single bright source at the center of the Galaxy that is responsible for the total neutrino emission.
“The exact nature of Galactic neutrino sources is still a mystery, but this simulation uses what we currently know to better understand the population of Galactic neutrino emitters,” says Thwaites, who presented results from this simulation at the International Cosmic Ray Conference in 2023. “Our simulation shows that future studies should look for populations of sources in the Galaxy that individually have lower luminosities, rather than looking for only a single bright source.”
The results also demonstrate the detection and resolving power of IceCube datasets using both cascade and track-like event morphologies for individual point sources, motivating future neutrino studies using IceCube data.
“The Galactic neutrinos that IceCube discovered raise exciting questions about their origins,” says Vandenbroucke. “With our new simulation software, we have shown that there must be more than a few spots in the Galaxy where they originate. Future work with IceCube and IceCube-Gen2 can unveil exactly what is emitting these neutrinos.”
+info “Constraints on the Origins of the Galactic Neutrino Flux Detected by IceCube,” Abhishek Desai et al, The Astrophysical Journal, 966 (23), (2024). DOI: 10.3847/1538-4357/ad2a5e