Self-modulation of cosmic rays in molecular clouds: Imprints in the radio observations

TL;DR

This paper proposes a theory of cosmic ray self-modulation in molecular clouds, predicting spectral hardening in radio emissions that matches observations, and emphasizes the need for low-frequency data to test these predictions.

Contribution

It introduces a new theory of cosmic ray self-modulation in dense molecular clouds and links it to observable radio spectral features in the Galactic Center region.

Findings

Spectral hardening in radio emissions matches the theory's predictions.

Self-modulation causes depletion of sub-GeV CR electrons in dense clouds.

Thermal emission dominates at GHz frequencies, requiring low-frequency observations for validation.

Abstract

We analyze properties of non-thermal radio emission from the Central Molecular Zone (CMZ) and individual molecular clouds, and argue that the observed features can be interpreted in the framework of our recent theory of self-modulation of cosmic rays (CRs) penetrating dense molecular regions. For clouds with gas column densities of $\sim10^{23}$ cm$^{-2}$, the theory predicts depletion of sub-GeV CR electrons, occurring due to self-modulation of CR protons and leading to harder synchrotron spectra in the sub-GHz range. The predicted imprints of electron depletion in the synchrotron spectra agree well with the spectral hardening seen in available radio observations of the CMZ. A similar, but even stronger effect on the synchrotron emission is predicted for individual (denser) CMZ clouds, such as the Sgr B2. However, the emission at frequencies above $\sim$ GHz, where observational data are available, is completely dominated by the thermal component, and therefore new observations at lower frequencies are needed to verify the predictions.