At the highest energies, cosmic rays cannot propagate freely across the Universe but are hindered by interactions with cosmic background photons, so we expect their sources to be relatively nearby, within a few tens of megaparsecs. Galaxies within this range tend to be mostly aligned with a great circle on the celestial sphere known as the Supergalactic Plane. Since cosmic rays are electrically charged, they are deflected by magnetic fields so we cannot directly reconstruct the position of their sources based on their arrival direction as we can do with electromagnetic radiation, neutrinos or gravitational waves, but once enough events are detected we can still hope to find statistical patterns in their distribution. Whereas at large scales a dipole moment has been discovered in Pierre Auger Observatory data, at smaller scales no such pattern has been conclusively identified yet. Tentative indications include an excess around the Centaurus A radio galaxy reported by the Pierre Auger Collaboration, around starburst galaxies reported both in Auger data alone and in a combined dataset also including Telescope Array data, and in two regions of the northern hemisphere reported by the Telescope Array collaboration. All these regions are aligned with the Supergalactic Plane (see Figure 1).
Figure 1: The position of selected nearby galaxies in supergalactic coordinates, compared to the fields of view of the Pierre Auger Observatory and the Telescope Array and to the position of the windows from which the Telescope Array collaboration has reported excesses of events.
Recently, the Pierre Auger Collaboration systematically studied the distribution of ultra-high-energy cosmic rays at an intermediate angular scale on a zone centered along the Supergalactic Plane, using an energy threshold of 20 EeV, lower than in previous searches at such angular scales. The results were published on the Astrophysical Journal. The previously reported excess in the Centaurus region was tentatively confirmed, and found to extend to lower energies than previously studied with no noticeable dependence of its position on the energy threshold used. Conversely, no significant excess was found anywhere else in the region studied with any of the energy thresholds used (see Figure 2), including at the positions where indications had previously been reported by the Telescope Array. Even though these regions are well inside the Telescope Array field of view but at the edge of the Pierre Auger Observatory field of view, the two have accumulated comparable exposures to them, due to the latter's much larger total exposure, due to its larger effective area, age, and zenith angle range. As a result, we would expect any actual excess in the ultra-high-energy cosmic-ray flux in such a region to be detected by both arrays with similar strengths, up to statistical fluctuations, unless the flux is strongly asymmetric even within the region. Future observations with more statistics using AugerPrime, TA×4 and other future observatories will hopefully help elucidate the picture.
Figure 2: Local significances of excesses of Auger events above six energy thresholds from regions along the supergalactic plane
Related Paper:
The distribution of ultra-high-energy cosmic rays along the supergalactic plane measured at the Pierre Auger Observatory
The Pierre Auger Collaboration, ApJ 984 (2025) 123
[arxiv.org/abs/2407.06874] [doi: 10.3847/1538-4357/adbdc5]
