Constraining the cosmic ray propagation halo thickness using Fermi-LAT observations of high-latitude clouds

TL;DR

This study uses Fermi-LAT gamma-ray observations of high-latitude clouds within a spatially dependent propagation model to constrain the cosmic ray halo thickness to approximately 3.3 to 9 kiloparsecs, offering a new method to determine halo size.

Contribution

The paper introduces a novel approach using gamma-ray data and a spatially dependent propagation model to independently constrain the cosmic ray halo thickness.

Findings

Halo thickness constrained to 3.3-9 kpc range.

Distinct gamma-ray morphology linked to halo size.

Method provides a new tool for cosmic ray halo measurement.

Abstract

As a basic characteristic of cosmic ray (CR) propagation, the diffusive halo can advance our understanding of many CR-related studies and indirect dark matter. The method to derive the halo size usually has degeneracy problems thus affected by large uncertainties. The diffusion gamma ray from high-latitude clouds might shed light on the halo size independently. Since the spatially dependent propagation (SDP) model has a better agreement with the observed CRs, compared with conventional propagation model, in this work, we investigate the halo thickness based on SDP model with Fermi-LAT $\rm\gamma$-ray observation on the high- and intermediate-velocity clouds. As a result, in order not to exceed the relative $\gamma$-ray emissivity in the high-latitude clouds, halo thickness should be in the range of $\rm ~3.3\sim9~ kpc$. Moreover, the spatial morphology of $\rm\gamma$-rays estimated based on SDP model under different values of halo thickness are distinctive, which provides us a tool to determine the halo size. We hope that our model could be tested and tuned by multi-wavelength observations in the future.