[Paper Review] Evidence for UHECR origin in starburst galaxies

Luis A. Anchordoqui, Jorge F. Soriano|arXiv (Cornell University)|May 30, 2019

Astrophysics and Cosmic Phenomena64 references4 citations

TL;DR

This paper proposes that ultra-high-energy cosmic rays (UHECRs) originate in starburst galaxies, where diffusive shock acceleration in superwind-driven, multiple parallel shocks produces a flatter particle spectrum ($f_{\infty}(p)\propto p^{-3}$) that matches observational data. The model explains the ankle in the spectrum, flux suppression near $10^{10.6}$ GeV, and composition trends, with starburst galaxies providing a viable source population consistent with Auger and Telescope Array data.

ABSTRACT

The quest for the origin(s) of ultra-high-energy cosmic rays (UHECRs) continues to be a far-reaching pillar of high energy astrophysics. The source scrutiny is mostly based on three observables: the energy spectrum, the nuclear composition, and the distribution of arrival directions. We show that each of these three observables can be well reproduced with UHECRs originating in starburst galaxies.

Motivation & Objective

• To resolve the long-standing puzzle of UHECR origins by identifying starburst galaxies as viable sources.
• To explain the observed UHECR energy spectrum, including the ankle and flux suppression, through a physically motivated acceleration mechanism.
• To reconcile discrepancies between Telescope Array and Pierre Auger Observatory data on composition and spectral shape.
• To address criticisms of earlier models by showing that multiple shocks reduce time constraints and produce flatter spectra.
• To demonstrate that source-level GZK interactions can explain the observed maximum energy cutoff, distinct from the standard GZK cutoff interpretation.

Proposed method

• Modeling UHECR acceleration via diffusive shock acceleration at multiple parallel shocks in starburst galaxy superwinds, with magnetic fields aligned parallel to shock fronts.
• Using Rankine-Hugoniot jump conditions for magnetohydrodynamic shocks to describe plasma behavior across shocks, assuming ideal gas equation of state and constant γ.
• Calculating particle injection and re-acceleration at each shock, with adiabatic decompression between shocks to reduce acceleration time constraints.
• Summing over an infinite distribution of identical shocks with fresh injection and decompression to derive a net $f_{\infty}(p)\propto p^{-3}$ momentum distribution.
• Relating the differential energy spectrum $dN/dE \propto E^{-\gamma}$ to momentum space distribution via $dN = 4\pi p^2 f_{\infty}(p)\,dp$ to fit observational data.
• Incorporating source-level energy losses via GZK interactions to constrain maximum UHECR energy, dependent on nuclear charge $Z$ and mass $A$.

Experimental results

Research questions

• RQ1 Can the UHECR energy spectrum, including the ankle and flux suppression, be explained by acceleration in starburst galaxy superwinds?
• RQ2 Does the observed composition trend—lighter at lower energies and heavier at higher energies—align with predictions from multiple shock acceleration in starburst environments?
• RQ3 Can the observed discrepancy in flux suppression energies between Telescope Array ($10^{10.73}$ GeV) and Auger ($10^{10.6}$ GeV) be reconciled within a single source model?
• RQ4 Is the observed spectral hardening at the ankle consistent with a source spectrum $\propto E^{-\gamma}$ with $1.0 \lesssim \gamma \lesssim 1.5$?
• RQ5 Can source-level GZK interactions explain the maximum UHECR energy without requiring a universal GZK cutoff?

Key findings

• The model produces a flatter particle spectrum $f_{\infty}(p)\propto p^{-3}$ due to multiple shocks, which better matches the observed UHECR spectrum than a single-shock model ($f(p)\propto p^{-4}$).
• The predicted source spectral index $\gamma \approx 1.2$ to $1.5$ is consistent with the joint analysis of Telescope Array and Pierre Auger Observatory data, particularly the differential energy spectrum $dN/dE \propto E^{-\gamma}$.
• The flux suppression observed at $E \sim 10^{10.6}$ GeV by Auger and $E \sim 10^{10.73}$ GeV by TA is well reproduced by the model, with source-level GZK losses setting the maximum energy.
• The model explains the composition trend: light primaries (protons, helium) below $10^{9.3}$ GeV and a transition to heavier nuclei above $10^{10.5}$ GeV, consistent with Auger's findings.
• The model is compatible with the observed correlation between UHECRs above $10^{10.6}$ GeV and nearby starburst galaxies, with a post-trial probability of $p \approx 0.005$.
• The inclusion of multiple shocks and adiabatic decompression reduces the required acceleration time, alleviating a key criticism of earlier models.