Deciphering the spectral bumps of Galactic cosmic rays through gamma-ray observations of nearby molecular clouds

TL;DR

This paper explores how gamma-ray observations of nearby molecular clouds can reveal the presence and properties of a potential nearby cosmic-ray source, offering a new method to understand cosmic-ray spectral features.

Contribution

It proposes using gamma-ray spectra of molecular clouds as probes to detect and characterize a nearby cosmic-ray source, providing a novel observational approach.

Findings

Nearby sources can imprint distinct features on gamma-ray spectra of GMCs.

Energy dependence of gamma-ray spectral index can constrain the source's distance.

Uniform spectral bump across GMCs suggests a widespread Galactic phenomenon.

Abstract

The observed spectral bump in cosmic-ray (CR) proton and helium spectra, along with the phase and amplitude evolution of CR dipole anisotropy, provide plausible yet indirect evidence for the presence of a nearby CR source. This study investigates the potential of giant molecular clouds (GMCs) located near the solar system to act as natural probes of CRs from the nearby source, with their gamma-ray emissions serving as indicators of spatial variations in CR flux within the solar neighborhood resulting from this source. We show that a nearby source, accounting for the CR data, could imprint distinct features on the $\gamma$-ray spectra of different GMCs. We expect that these features are detectable by LHAASO and upcoming high-energy $\gamma$-ray observatories, providing a powerful test for the hypothesized nearby source. Notably, we find that determining the energy dependence of the $\gamma$-ray spectral index offers a promising approach to investigate the nearby source and constrain its distance. Conversely, if the spectral bump is a widespread Galactic phenomenon, the energy dependence would exhibit uniformity across all GMCs, distinguishing the underlying mechanism accounting for the spectral bump from the scenario involving a nearby source.