Gamma-Ray Emission from Molecular Clouds Generated by Penetrating Cosmic Rays

TL;DR

This paper investigates how cosmic-ray penetration into molecular clouds affects gamma-ray emission, highlighting energy-dependent depletion effects that influence gamma-ray observations and interpretations.

Contribution

It introduces a model of cosmic-ray depletion inside molecular clouds due to self-excited turbulence, impacting gamma-ray flux predictions at energies below 10 GeV.

Findings

CR depletion occurs at energies below ~10 GeV in molecular clouds.

Depletion affects gamma-ray flux below ~2 GeV, relevant for Galactic Center observations.

Local clouds show signs of gamma-ray depletion, indicating CR self-modulation.

Abstract

We analyze the processes governing cosmic-ray (CR) penetration into molecular clouds and the resulting generation of gamma-ray emission. The density of CRs inside a cloud is depleted at lower energies due to the self-excited MHD turbulence. The depletion depends on the effective gas column density ("size") of the cloud. We consider two different environments where the depletion effect is expected to be observed. For the Central Molecular Zone, the expected range of CR energy depletion is $E\lesssim 10$ GeV, leading to the depletion of gamma-ray flux below $E_\gamma\approx 2$ GeV. This effect can be important for the interpretation of the GeV gamma-ray excess in the Galactic Center, which has been revealed from the standard model of CR propagation (assuming the CR spectrum inside a cloud to be equal to the interstellar spectrum). Furthermore, recent observations of some local molecular clouds suggest the depletion of the gamma-ray emission, indicating possible self-modulation of the penetrating low-energy CRs.