Constraining the cosmic ray propagation halo thickness using Fermi-LAT observations of high-latitude clouds * *Supported by the National Key R&D Program of China (2018YFA0404202) and the National Natural Science Foundation of China (11635011, 11875264, 11722328, 11851305, U1738205, U2031110)

Abstract

The diffusive halo is a basic characteristic of cosmic ray (CR) propagation and can advance our understanding of many CR-related phenomena and indirect dark matter. The method used to derive the halo size often has degeneracy problems and is thus affected by large uncertainties. The diffuse rays from high-latitude clouds might shed light on the halo size independently. Because predictions using the spatially dependent propagation (SDP) model have better agreement with the observed CRs than those of the conventional propagation model, in this work, we investigated halo thickness based on the SDP model using Fermi-LAT -ray observations of high- and intermediate-velocity clouds. We found that to avoid exceeding the relative -ray emissivity in high-latitude clouds, the halo thickness should be in the range of 3.3-9 kpc. Moreover, the spatial morphology of -rays estimated based on the SDP model for different values of the halo thickness are distinctive, which provides us with a tool to determine the halo size. This newly developed model can be tested and tuned using multi-wavelength observations in future studies.