Probing the galactic cosmic-ray density with current and future γ-ray instruments

Abstract

Context. Cosmic rays (CRs) propagating through dense molecular clouds (MCs) produce γ-rays, which carry direct information about the CR distribution throughout the Galaxy. Observations of γ-rays in different energy bands allow for the exploration of the average CR density in the Galactic disk, the so-called level of the “CR sea”. Observations with the Fermi-Large Area Telescope (LAT) demonstrated the method’s feasibility based on two dozen MCs in our Galaxy. However, the potential of Fermi-LAT is limited to the exploration of the most massive and relatively nearby MCs; thus, the current observations cover only a tiny fraction of the Milky Way. Aims. In this work, we aim to 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. Methods. Based on calculations of fluxes expected from MCs, we formulated the requirements to the sensitivity of the post-Fermi-LAT detectors in order to map GeV-TeV CRs in the Galactic disk. We also explored 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. Results. We demonstrated 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 Large High altitude air Shower Observatory should be able to take the first CR probes at PeV energies in the coming five years or so.