Ultra-high-energy cosmic rays (UHECRs) are the most energetic particles ever detected, with energies exceeding 10^18 eV. Their origin remains a mystery, as their paths are deflected by magnetic fields in space, making it difficult to trace them back to their sources. Scientists use models of the Galactic magnetic field (GMF) to account for these deflections and try to pinpoint the sources of UHECRs. A recent study used a new suite of GMF models called UF23, which provides a more accurate representation of the Milky Way's magnetic field. The study found that the dipole amplitude of UHECRs, which is a measure of the anisotropy in their arrival directions, is significantly smaller than predicted by previous models. This discrepancy is attributed to the demagnification effect of the GMF, where the magnetic field deflects UHECRs coming from certain directions so strongly that they never reach Earth.

The study also highlighted the importance of considering the inhomogeneous distribution of UHECRs arriving at the Milky Way. The flux of UHECRs is enhanced in the direction of the Virgo cluster, which is a massive cluster of galaxies.

The combination of the demagnification effect and the inhomogeneous flux distribution significantly affects the predicted dipole amplitude and direction of UHECRs. This finding has important implications for the search for UHECR sources, as it suggests that some popular source candidates, such as M87, may be hidden from our view due to demagnification.

Publication: T. Bister et al., "The large-scale anisotropy and flux (de-) magnification of ultra-high-energy cosmic rays in the Galactic magnetic field", arXiv:2408.00614 

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: BBC