Search for ultra-high energy neutrons from Galactic sources with the Pierre Auger Observatory

TL;DR

This study searches for ultra-high-energy neutrons from Galactic sources using Pierre Auger Observatory data, aiming to identify sources of cosmic rays by detecting neutron fluxes, but finds no significant excess and sets upper limits.

Contribution

It introduces a method to search for neutron fluxes from over 1000 Galactic sources using extensive cosmic-ray data, providing new upper limits on neutron emissions.

Findings

No significant neutron flux detected from any candidate source.

Established upper limits on neutron fluxes for over 1000 sources.

Demonstrated the feasibility of using cosmic-ray data to search for Galactic neutrons.

Abstract

Deflections in the propagation of charged ultra-high-energy cosmic rays (UHECRs) caused by magnetic fields make the identification of their sources challenging. On the other hand, the arrival directions at Earth of neutrons point directly to their origin. The emission of UHECRs from a source is expected to be accompanied by the production of neutrons in its vicinity through interactions with ambient matter and radiation. Since free neutrons travel a mean distance $d/\text{kpc}=9.2(E/\text{EeV})$ before decaying, a neutron flux in the EeV range could be detected on Earth from sources of UHECRs in our Galaxy. Using cosmic-ray data from the Phase\,I of the Surface Detector of the Pierre Auger Observatory, we search for neutron fluxes from Galactic candidate sources. We select more than 1000 objects of astrophysical interest, stacking them into target sets. The targets all have declinations within the exposure of the Observatory, ranging from $-90^\circ$ up to $+45^\circ$ for energies above 1 EeV (and up to $+20^\circ$ for energies down to 0.1 EeV). Given that a neutron air shower is indistinguishable from a proton one, there is a significant background due to cosmic rays. A neutron flux from the direction of a candidate source would be identified by a celestial density of events that significantly exceeds the expected density of cosmic rays for that direction. No significant excess is found at any tested target direction, and an upper limit on the neutron flux is calculated for each candidate source.