Oblique Shocks at Supernova Remnants in Massive Star Clusters: A Model for the Cosmic-Ray Knee Observed by LHAASO

TL;DR

This paper proposes a model where oblique shocks in Massive Star Clusters accelerate cosmic rays to PeV energies, explaining the cosmic-ray knee observed by LHAASO and predicting associated gamma-ray and neutrino signals.

Contribution

It introduces a novel model emphasizing shock obliquity in MSCs as key to reaching high energies, unifying cosmic-ray features with cluster acceleration mechanisms.

Findings

Oblique shocks enhance particle acceleration efficiency.

The model reproduces the observed cosmic-ray spectrum and composition.

Predicted gamma-ray and neutrino emissions are detectable.

Abstract

This work establishes oblique shocks in Massive Star Clusters (MSC) as a primary mechanism for accelerating cosmic rays (CR) up to the knee of the energy spectrum. We develop a model that incorporates the combined contribution of supernova and collective wind shocks, emphasizing the critical role of the shock obliquity angle in determining the maximum particle energy. We illustrate, within our model that oblique shocks can significantly enhance acceleration efficiency, allowing particles to reach multi-PeV energies in a rigidity-dependent manner. Our preferred model, which incorporates oblique shocks, reproduces the all-particle spectrum and composition observed by The Large High Altitude Air Shower Observatory (LHAASO), interpreting the knee as arising from a sequence of rigidity-dependent cutoffs. The model also predicts subdominant but detectable gamma-ray and neutrino emissions. This study provides an attempt at building a unified framework connecting MSC particle acceleration to the observed features of the cosmic-ray knee.