[Paper Review] An extragalactic ``flux trapping'' origin of the dominant part of hadronic cosmic rays?

R. Plaga|arXiv (Cornell University)|Nov 10, 1997

Astrophysics and Cosmic Phenomena4 references4 citations

TL;DR

This paper proposes that the dominant component of hadronic cosmic rays above 10 MeV/nucleon originates extragalactically, with flux trapping in the Galactic confinement volume explaining their local abundance. The model reconciles extragalactic origins with observed energy spectra and naturally explains the 'ankle' feature in the cosmic-ray spectrum under conservative assumptions about intergalactic and Galactic magnetic fields.

ABSTRACT

An extragalactic origin of the dominant part of all extrasolar hadronic cosmic rays above about 10 MeV/nucleon has long been considered unlikely due to energy considerations. In order to circumvent such arguments, the hypothesis that ``flux trapping'' of extragalactic cosmic rays occurs in the Galactic confinement volume is advanced in this paper. This hypothesis is based on a number of speculative assumptions about the properties of Galactic and intergalactic magnetic fields. The intergalactic cosmic-ray density expected under conservative assumptions about its extragalactic origins is then shown to be of the right order of magnitude to account for the locally observed cosmic radiation. It is demonstrated that an extragalactic scenario of cosmic-ray origin that is consistent with the observed cosmic-ray energy spectrum and preserves the successes of Galactic propagation theory can be constructed. The position of the ``ankle'' in the cosmic-ray energy spectrum follows as a natural consequence from this explanation. The $γ$-ray flux from the Magellanic clouds is shown to provide no suitable testing ground for the decision for or against an extragalactical origin in this scenario. It is argued that recent observational evidence seems to be in favour of a dual origin of cosmic-rays. The hadronic component is mainly extragalactic while the electrons are accelerated in Galactic supernova remnants.

Motivation & Objective

• To address the long-standing challenge of explaining the origin of high-energy hadronic cosmic rays given energy constraints favoring galactic sources.
• To propose a mechanism—flux trapping in the Galactic confinement volume—that could reconcile extragalactic origins with local cosmic-ray observations.
• To demonstrate consistency with the observed cosmic-ray energy spectrum, particularly the 'ankle' feature at ~10^16 eV.
• To evaluate the viability of the Magellanic Clouds as a testing ground for extragalactic cosmic-ray origin.
• To assess recent observational evidence for a dual origin of cosmic rays, with hadrons extragalactic and electrons galactic.

Proposed method

• Assumes conservative estimates for the extragalactic cosmic-ray density based on known extragalactic sources.
• Introduces the concept of 'flux trapping'—where extragalactic cosmic rays are confined within the Galactic volume by magnetic fields.
• Models the intergalactic magnetic field and its interaction with Galactic magnetic fields to estimate trapping efficiency.
• Uses the observed cosmic-ray energy spectrum as a constraint to validate the model's consistency with data.
• Applies propagation theory to simulate how trapped extragalactic particles would reproduce the local spectrum.
• Evaluates the expected gamma-ray flux from the Magellanic Clouds under this scenario to test observational feasibility.

Experimental results

Research questions

• RQ1 Can extragalactic cosmic rays be trapped in the Galactic confinement volume to explain the local flux of hadronic cosmic rays?
• RQ2 Does the 'ankle' in the cosmic-ray energy spectrum naturally emerge from an extragalactic flux-trapping model?
• RQ3 Is the gamma-ray emission from the Magellanic Clouds a viable test for distinguishing extragalactic from galactic cosmic-ray origins?
• RQ4 Can a model with extragalactic hadronic cosmic rays and galactic electron acceleration reproduce the observed cosmic-ray spectrum?
• RQ5 What are the implications of recent observational data for a dual-origin model of cosmic rays?

Key findings

• The intergalactic cosmic-ray density expected from conservative extragalactic sources is of the right order of magnitude to account for the locally observed cosmic-ray flux.
• The 'ankle' in the cosmic-ray energy spectrum emerges naturally as a consequence of the flux-trapping mechanism.
• The model is consistent with the observed cosmic-ray energy spectrum and preserves the successes of Galactic propagation theory.
• The gamma-ray flux from the Magellanic Clouds is not a suitable testing ground for distinguishing extragalactic from galactic origins in this scenario.
• Recent observational evidence appears to favor a dual origin model, with hadronic cosmic rays predominantly extragalactic and electrons accelerated in Galactic supernova remnants.