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Simulating the propagation of cosmic rays heavier than iron in SimProp

Armando di Matteo

TL;DR

This work addresses the possibility that ultra-high-energy cosmic rays include nuclei heavier than iron and demonstrates how to extend SimProp v2r4 to simulate such nuclei using TALYS-based photodisintegration cross sections. It outlines minimal command-line and code modifications, constructs cross sections for a wide range of $A$, and provides example scenarios showing that heavy nuclei can reach Earth largely intact, altering interpretations of source distances and composition. The study highlights limitations such as beta-decay treatment and neglect of intergalactic magnetic fields, and points to the upcoming SimProp Sirente release and other codes (e.g., CRPropa 3.3) for broader heavy-nuclei modeling. Overall, the results suggest heavy nuclei can travel longer distances than previously assumed, with important implications for source identification and cosmological evolution of UHECRs that warrant further investigation.

Abstract

Ultra-high-energy cosmic rays (UHECRs) have long been assumed to entirely consist of iron and/or lighter atomic nuclei, and this assumption has been hard-coded in a great deal of software for UHECR simulations and data analysis. However, in the last few years several authors have started questioning this assumption and entertaining the possibility that UHECRs might at least partly consist of nuclei of elements heavier than iron, especially at the highest energies. Thoroughly testing this hypothesis will require upgrading software so that it can handle such nuclei. In this contribution I will describe the minimal modifications required for the last publicly released version of SimProp, a code for Monte Carlo simulations of the intergalactic propagation of UHECRs, to be able to treat heavier nuclei, and discuss the applicability of approximations first introduced for lighter nuclei.

Simulating the propagation of cosmic rays heavier than iron in SimProp

TL;DR

This work addresses the possibility that ultra-high-energy cosmic rays include nuclei heavier than iron and demonstrates how to extend SimProp v2r4 to simulate such nuclei using TALYS-based photodisintegration cross sections. It outlines minimal command-line and code modifications, constructs cross sections for a wide range of , and provides example scenarios showing that heavy nuclei can reach Earth largely intact, altering interpretations of source distances and composition. The study highlights limitations such as beta-decay treatment and neglect of intergalactic magnetic fields, and points to the upcoming SimProp Sirente release and other codes (e.g., CRPropa 3.3) for broader heavy-nuclei modeling. Overall, the results suggest heavy nuclei can travel longer distances than previously assumed, with important implications for source identification and cosmological evolution of UHECRs that warrant further investigation.

Abstract

Ultra-high-energy cosmic rays (UHECRs) have long been assumed to entirely consist of iron and/or lighter atomic nuclei, and this assumption has been hard-coded in a great deal of software for UHECR simulations and data analysis. However, in the last few years several authors have started questioning this assumption and entertaining the possibility that UHECRs might at least partly consist of nuclei of elements heavier than iron, especially at the highest energies. Thoroughly testing this hypothesis will require upgrading software so that it can handle such nuclei. In this contribution I will describe the minimal modifications required for the last publicly released version of SimProp, a code for Monte Carlo simulations of the intergalactic propagation of UHECRs, to be able to treat heavier nuclei, and discuss the applicability of approximations first introduced for lighter nuclei.

Paper Structure

This paper contains 8 sections, 4 figures.

Table of Contents

  1. Introduction
  2. Command-line options and modifications to the source code
  3. The cross sections
  4. Example scenarios
  5. Mass composition at Earth
  6. Source distances
  7. Limitations
  8. Conclusions and future directions

Figures (4)

  • Figure 1: An example input file used with TALYS 2.0
  • Figure 2: Energy loss lengths for various nuclei
  • Figure 3: Predicted spectra at Earth from the injection of the three elements heavier than iron (left) and the three "conventional" elements (right) with no injection cutoff, grouped by atomic numbers at Earth
  • Figure 4: Predicted spectra at Earth from the injection of the three elements heavier than iron (left) and the three "conventional" elements (right) with the sharp injection cutoffs indicated in the panel titles, grouped by luminosity distance to the sources