MHD decomposition explains diffuse $\gamma$-ray emission in Cygnus X

TL;DR

This paper models cosmic-ray diffusion in Cygnus X using MHD turbulence modes to explain observed gamma-ray emission, linking micro-physics of particle transport with astrophysical observations.

Contribution

It introduces a 3D CR transport model considering different MHD turbulence modes within a two-zone framework for Cygnus X.

Findings

Successfully explains gamma-ray emission with MHD mode-based CR transport

Highlights the importance of turbulence mode distribution in diffusion

Connects micro-physics of turbulence with gamma-ray observations

Abstract

Cosmic-ray (CR) diffusion is the result of the interaction of such charged particles against magnetic fluctuations. These fluctuations originate from large-scale turbulence cascading towards smaller spatial scales, decomposed into three different modes, as described by $magneto-hydro-dynamics$ (MHD) theory. As a consequence, the description of particle diffusion strongly depends on the model describing the injected turbulence. Moreover, the amount of energy assigned to each of the three modes is in general not equally divided, which implies that diffusion properties might be different from one region to another. Here, motivated by the detection of different MHD modes inside the Cygnus-X star-forming region, we study the 3D transport of CRs injected by two prominent sources within a two-zone model that represents the distribution of the modes. Then, by convolving the propagated CR-distribution with the neutral gas, we are able to explain the $\gamma$-ray diffuse emission in the region, observed by the Fermi-LAT and HAWC Collaborations. Such a result represents an important step in the long-standing problem of connecting the CR observables with the micro-physics of particle transport.