Cosmic Ray Transport and Gamma-Ray Signatures in the Interstellar Medium

TL;DR

This work addresses the challenge of explaining suppressed cosmic-ray diffusion and 90-degree scattering near sources by highlighting mirror diffusion as a non-resonant transport mechanism in MHD turbulence. It combines a theoretical overview with Monte Carlo CR propagation in a realistic Young Massive Star Cluster environment using CRPropa, incorporating a composite isotropic photon field to model interactions. The results indicate that slow diffusion can substantially enhance secondary production within cluster scales (e.g., $R \sim 40$ pc) and yield gamma-ray and CR spectra that resemble observations, albeit with limited statistics and pending inclusion of additional backgrounds. The findings support the relevance of mirror diffusion for understanding VHE emission and motivate further, more complete simulations.

Abstract

The interaction of cosmic rays (CRs) with magnetic fields and the interstelar medium (ISM) leads to the production of nonthermal radiation. Although this has been a topic of study for many years, it still poses many challenges to the understanding of these processes. In this work we present a short review of recent advances in the understanding of CR propagation in magnetohydrodynamical (MHD) turbulence, in particular the process of mirror diffusion, and how it can help explain recent observational constraints for CR diffusion away from sources. We also present preliminary results from Monte Carlo simulations of CR cascading and propagation within a young massive stellar cluster (YMSC), aimed at probing the origin of very-high-energy (VHE) emission from these sources.

Figures (4)

• Figure 1: Photon energy density for the different background photon fields. Blue line shows the contribution for the CMB, orange for the background interstellar radiation field, green for the stars inside the YMSC, and red for the dust present in the cluster.
• Figure 2: The inverse mean free path (MFP) for two interaction processes. Top: Pair production. Bottom: Inverse Compton scattering. Solid colored lines represent the contribution from different radiation fields: interstellar radiation field (ISRF, blue), cosmic microwave background (CMB, orange), and stellar blackbody emission from the cluster (green). Dashed lines indicate key spatial scales: the 40 pc cluster size (red), the Galactic scale (black), and the effective confinement scales from mirror diffusion for a central source (purple) and a uniform source distribution (pink).
• Figure 3: Inverse mean free path (MFP) for proton-proton (pp) interactions, calculated following the parameterization from 2006PhRvD..74c4018K. The blue and orange lines indicate the interaction MFP for the lowest and highest gas densities found in any cell of the magnetohydrodynamic simulation, respectively. These values represent the range of expected interaction lengths within the young massive star cluster environment. Dashed lines indicate key spatial scales: the 40 pc cluster size (red), the Galactic scale (black), and the effective confinement scales from mirror diffusion for a central source (purple) and a uniform source distribution (pink).
• Figure 4: Total spectra of $\gamma$-rays and cosmic rays (CRs) from the cascading simulation in arbitrary units. The black solid line represents the injected CR spectrum, measured at a distance of 1 pc from the central source. The orange solid line and orange dashed line show the resulting CR and $\gamma$-ray spectra, respectively, observed at a distance of 30 pc from the source.