Multi-messenger constraints on transient accelerators of ultra-high energy cosmic rays

TL;DR

The paper investigates the origin of ultra-high-energy cosmic rays by testing two competing source-distribution scenarios—production rates tracing the cosmic star-formation rate or the stellar-mass density—using Auger spectrum and composition data. It introduces a multi-messenger framework centered on galaxy clusters, combining a large nearby galaxy catalog with propagation simulations and magnetic-deflection modeling to predict UHECR arrival patterns and secondary neutrino/gamma-ray fluxes. By comparing these predictions with current limits, the study constrains source energetics, maximum energies, and burst rates, highlighting the role of cluster environments in shaping transport and observables. The results favor long gamma-ray bursts as the most plausible stellar-scale transients, with possible contributions from binary neutron star mergers, and outline future facility prospects (CTA, GRAND, POEMMA, GCOS) to further test these scenarios.

Abstract

The origin of ultra-high-energy cosmic rays (UHECRs) remains an open questions in astrophysics. We explore two primary scenarios for the distribution of UHECR sources, assuming that their production rate follows either the cosmic star-formation-rate or stellar-mass density. By jointly fitting the UHECR energy spectrum and mass composition measured by the Pierre Auger Observatory above the ankle (10^{18.7} eV), we derive constraints on the acceleration mechanisms, source energetics, and elemental abundances at escape. Using these constraints, we generate sky maps above 40 EeV based on a catalog of over 400,000 galaxies out to 350 Mpc, providing a near-infrared flux-limited sample that maps the two stellar-activity tracers across the full sky. A crucial factor in understanding UHECR propagation is the influence of large-scale cosmic structures, particularly galaxy clusters, the largest gravitationally bound systems in the Universe, which are filled with magnetized diffuse plasma. Intermittent sources hosted in galaxies within such structures, coupled with cosmic magnetic fields, shape the observed UHECR arrival directions and provide insights into the burst rate of the sources. We show that these environments can significantly impact UHECR transport, making them particularly opaque to heavy nuclei. Additionally, we compute the expected secondary neutrino and photon fluxes from UHECR interactions in these environments and compare them with current experimental limits, constraining the maximum energy that particles can achieve. Finally, we assess the compatibility of these constraints with astrophysical candidates, identifying long gamma-ray bursts as the most promising sources.

Figures (5)

• Figure 1: Left panel: Magnetic field profiles adopted for Virgo, Perseus and Coma. Right panel: Gas density profiles adopted for Virgo, Perseus and Coma. In both scenarios, the assumptions underlying these expectations are described in detail in Condorelli.
• Figure 2: Cumulative stellar mass (in solar masses) and star formation density as a function of the comoving radius for Virgo cluster. $R_{500}$ is the radius within which the cluster density is 500 times the critical density of the universe at the cluster redshift, and it is a crucial parameter to describe clusters environment.
• Figure 3: Relevant timescale in galaxy clusters. Inverse compton (IC) on CMB and different EBL models are shown with dashed lines; pair production timescales are shown with solid line, while synchrotron timescales are shown in dotted lines for different magnetic field values.
• Figure 4: Expected neutrino flux (left) and photon flux (right) at Earth for SMD and SFR scenarios. For the neutrino flux, only corrections due to the adiabatic expansion of the Universe are accounted for, in comparison with Auger limit for point-like sources Auger2019PointNeutrino. In contrast, the photon flux includes attenuation due to propagation through the intra-cluster medium as well as interactions in the extragalactic space. The black points are the upper limit on the photon flux from Perseus Cluster placed by MAGIC. Aleksic2012_AA541A99.
• Figure 5: UHECR burst rate as a function of kinetic energy of the outflow. The allowed regions are based on an efficiency of conversion of kinetic energy to particles, $\eta$, between $1\,\%$ and $10\,\%$. Rectangular markers indicate the statistical uncertainty associated with the number of X-ray bursts observed for each type of source. The vertical error bars show the range associated with the beaming correction factor, while the horizontal error bars illustrate the range of kinetic energy per burst in each population. The color bar shows the threshold luminosity above which each source density is measured. The low luminosities excluded by the Hillas-Lovelace-Waxman-Blandford criterion are shown as a green hatched region of the color bar. Horizontal shaded bands mark the corresponding range of local merger rates inferred from gravitational-wave observations.