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

TL;DR

This work reports a targeted search for ultra-high-energy neutrons from Galactic sources using Phase I data from the Pierre Auger Observatory. It employs a per-event celestial density approach, accounting for angular uncertainties, and contrasts observed densities with isotropic expectations derived from scrambled backgrounds. An extensive catalog of 12 source classes (1092 targets for the high-exposure SD-1500 and 70 for SD-750 within 1 kpc) is analyzed, with both unweighted and weighted stacking to enhance sensitivity. No significant neutron flux is detected; upper limits are set across energy bins, improving previous limits by about a factor of 2 and providing the strongest direct constraints on hadronic acceleration in Galactic objects above 100 PeV. The results inform models of UHECR production and motivate future analyses, including time-dependent searches and the Phase II AugerPrime upgrades.

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.

Figures (1)

• Figure 1: Spectral energy distributions (SEDs) for the Crab pulsar (top) and the Galactic center (bottom). For the Crab pulsar, the electromagnetic SED is from Pheno_Crab with data available in Pheno_Crab_Data. For the Galactic center, the majority of data is obtained through the tool SED Builder (https://tools.ssdc.asi.it/SED) with data from GC_GammaGC_IRGC_PlanckGC_RadioGC_VLAGC_VizierJ with the manual addition of VHE-$\gamma$ data from MAGIC GC_Magic, Veritas GC_Veritas, and H.E.S.S. GC_HESS.