Probing the Sea of Cosmic Rays by Measuring Gamma-Ray Emission from Passive Giant Molecular Clouds with HAWC

TL;DR

This paper uses HAWC data to set the first upper limits on gamma-ray emission from passive giant molecular clouds at high galactic latitudes, constraining the cosmic-ray density in these regions.

Contribution

It provides the first constraints on multi-TeV gamma-ray emission from passive GMCs, testing the cosmic-ray sea paradigm in distant parts of the Galaxy.

Findings

No significant gamma-ray excess detected from GMCs.

Upper limits on gamma-ray flux are established at 95% credible intervals.

Limits are consistent with cosmic-ray densities similar to those near Earth.

Abstract

The study of high-energy gamma rays from passive Giant Molecular Clouds (GMCs) in our Galaxy is an indirect way to characterize and probe the paradigm of the "sea" of cosmic rays in distant parts of the Galaxy. By using data from the High Altitude Water Cherenkov (HAWC) observatory, we measure the gamma-ray flux above 1 TeV of a set of these clouds to test the paradigm. We selected high-galactic latitude clouds that are in HAWC's field-of-view and which are within 1~kpc distance from the Sun. We find no significant excess emission in the cloud regions, nor when we perform a stacked log-likelihood analysis of GMCs. Using a Bayesian approach, we calculate 95\% credible intervals upper limits of the gamma-ray flux and estimate limits on the cosmic-ray energy density of these regions. These are the first limits to constrain gamma-ray emission in the multi-TeV energy range ($>$1 TeV) using passive high-galactic latitude GMCs. Assuming that the main gamma-ray production mechanism is due to proton-proton interaction, the upper limits are consistent with a cosmic-ray flux and energy density similar to that measured at Earth.