The distribution of ultra-high-energy cosmic rays along the supergalactic plane measured at the Pierre Auger Observatory

TL;DR

The study probes intermediate-scale anisotropies of ultra-high-energy cosmic rays (UHECRs) along the local supergalactic plane using data from the Pierre Auger Observatory with $E \ge 20\,\mathrm{EeV}$ up to 2022. It adopts a windowed, fixed-angular-scale search across six energy thresholds and employs a binomial-likelihood framework to compare flux inside circular windows to the exterior, deriving both local significances and 99% CL upper limits. The strongest signal arises in the Centaurus region with a post-trial significance of $3.1\sigma$, and the excess extends down to $20\,\mathrm{EeV}$ with a harder spectrum ($\gamma\approx 2.6$) than the background; no statistically significant excess is found in regions reported by the Telescope Array. The results reinforce a link between the UHECR distribution and local large-scale structure while highlighting tension with TA-reported regions and underscoring the need for future upgrades (e.g., AugerPrime, TAx4) and composition-sensitive analyses to decipher the origin and mass with energy.

Abstract

Ultra-high-energy cosmic rays are known to be mainly of extragalactic origin, and their propagation is limited by energy losses, so their arrival directions are expected to correlate with the large-scale structure of the local Universe. In this work, we investigate the possible presence of intermediate-scale excesses in the flux of the most energetic cosmic rays from the direction of the supergalactic plane region using events with energies above 20 EeV recorded with the surface detector array of the Pierre Auger Observatory up to 31 December 2022, with a total exposure of 135,000 km^2 sr yr. The strongest indication for an excess that we find, with a post-trial significance of 3.1σ, is in the Centaurus region, as in our previous reports, and it extends down to lower energies than previously studied. We do not find any strong hints of excesses from any other region of the supergalactic plane at the same angular scale. In particular, our results do not confirm the reports by the Telescope Array collaboration of excesses from two regions in the Northern Hemisphere at the edge of the field of view of the Pierre Auger Observatory. With a comparable integrated exposure over these regions, our results there are in good agreement with the expectations from an isotropic distribution.

Figures (8)

• Figure 1: Local Li--Ma significance $Z_\text{LM}$ of excesses over the isotropic expectation as a function of the window center position. The $Z_\text{LM}$ in windows whose center lies outside the FoV of the Observatory was not computed (shown as the gray disk wrapping around the left and right edges of each panel; see also \ref{['fig:regions']}). In each panel, the energy threshold used is written in the upper right corner. The solid circle is the window position with the highest $Z_\text{LM}$ in the whole strip; the dashed one is that with the highest $Z_\text{LM}$ excluding those overlapping with the solid one. Labels indicate the position of Council of Giants galaxies McCall for reference only; they are not taken into account in the analysis in any way.
• Figure 2: The maximum-likelihood value of the ratio $\Phi_\text{in}/\Phi_\text{out}$, i.e., $N_\text{in}/N_\text{bg}$, as a function of the window center position
• Figure 3: The windows in which the TA collaboration reported excesses of events, as of their latest update TA_ICRC2023, compared to the FoV of the Pierre Auger Observatory and of the Telescope Array
• Figure 4: The directional exposure of the Pierre Auger Observatory and the Telescope Array as a function of declination, compared to the declinations of the windows shown in \ref{['fig:regions']} (horizontal bars; note that the bar lengths denote the sizes of the windows, not the uncertainties in their position).
• Figure 5: Binomial probability that $N_\text{in}$ events would be observed in our dataset in the first (a) of the windows reported by TA and shown in \ref{['fig:regions']}. The thin blue histogram assumes that the value of the flux ratio $\Phi_\text{in}/\Phi_\text{out}$ is exactly the one reported by TA, whereas the thick one is the marginal distribution of $\Phi_\text{in}/\Phi_\text{out}$ over TA statistical uncertainties. The complete figure set (4 images) is available in the online journal.