Covering the celestial sphere at ultra-high energies

TL;DR

This paper maps the distribution of ultra-high energy cosmic rays across the entire sky at energies beyond key spectral features, providing new constraints on anisotropy and source distribution using combined data from major observatories.

Contribution

It presents the first full-sky maps of UHECR flux at energies above 10 EeV and 50 EeV, enabling comprehensive anisotropy analysis without assumptions on higher multipoles.

Findings

Constraints on low-order multipole moments established

No significant large-scale anisotropy detected

Full-sky coverage improves source distribution understanding

Abstract

Despite deflections by Galactic and extragalactic magnetic fields, the distribution of ultra-high energy cosmic rays (UHECRs) over the celestial sphere remains a most promising observable for the identification of their sources. Thanks to a large number of detected events over the past years, a large-scale anisotropy at energies above 8 EeV has been identified, and there are also indications from the Telescope Array and Pierre Auger Collaborations of deviations from isotropy at intermediate angular scales (about 20 degrees) at the highest energies. In this contribution, we map the flux of UHECRs over the full sky at energies beyond each of two major features in the UHECR spectrum -- the ankle and the flux suppression --, and we derive limits for anisotropy on different angular scales in the two energy regimes. In particular, full-sky coverage enables constraints on low-order multipole moments without assumptions about the strength of higher-order multipoles. Following previous efforts from the two Collaborations, we build full-sky maps accounting for the relative exposure of the arrays and differences in the energy normalizations. The procedure relies on cross-calibrating the UHECR fluxes reconstructed in the declination band around the celestial equator covered by both observatories. We present full-sky maps at energies above 10 EeV and 50 EeV, using the largest datasets shared across UHECR collaborations to date. We report on anisotropy searches exploiting full-sky coverage and discuss possible constraints on the distribution of UHECR sources.