Probing the galactic cosmic-ray density with current and future $\gamma$-ray instruments

TL;DR

This paper explores how current and future gamma-ray instruments can expand our understanding of cosmic-ray distribution across the Galaxy by detecting gamma rays from molecular clouds at various energies.

Contribution

It assesses the potential of existing and upcoming gamma-ray detectors to measure cosmic-ray densities in different parts of the Galaxy at multiple energy ranges.

Findings

Fermi-LAT's current observations are limited to nearby, massive molecular clouds.

Enhanced sensitivity in future detectors could significantly increase the number of observable molecular clouds.

LHAASO is expected to probe cosmic rays at PeV energies within five years.

Abstract

Cosmic Rays (CRs) propagating through dense molecular clouds (MCs) produce gamma rays which carry direct information about the CR distribution throughout the Galaxy. Observations of gamma rays in different energy bands allow exploration of the average CR density in the Galactic Disk, the so-called level of the "CR Sea". Fermi-LAT observations have demonstrated the method's feasibility based on two dozen MCs in our Galaxy. However, the potential of Fermi-LAT is limited by the most massive and relatively nearby MCs; thus, the current observations cover only a tiny fraction of the Milky Way. In this paper, we study the prospects of expanding the CR measurements to very and ultra-high energies and remote parts of the Galaxy with the current and next-generation detectors. Based on calculations of fluxes expected from MCs, we formulate the requirements to the sensitivity of the post-Fermi-LAT detectors to map GeV-TeV CRs in the Galactic Disk. We also explore the potential of the current and future air-shower and atmospheric Cherenkov telescope arrays for the extension of CR studies to multi-TeV and PeV energy bands. We demonstrate that the improvement of the Fermi-LAT sensitivity by a factor of a few would allow a dramatic increase in the number of detectable MCs covering almost the entire Galaxy. The recently completed LHAASO should be able to take the first CR probes at PeV energies in the coming five years or so.