Gamma-ray Production in the Extended Halo of the Galaxy and Possible Implications for the Origin of Galactic Cosmic Rays

TL;DR

This study models gamma-ray production in the Galaxy's extended halo due to cosmic ray interactions, providing constraints on cosmic ray properties and implications for the baryon content in the circumgalactic medium.

Contribution

It offers a detailed calculation of cosmic ray distribution and gamma-ray flux in the Galactic halo, linking observations to cosmic ray and baryon properties.

Findings

Current gamma-ray background measurements constrain cosmic ray luminosity to ≤10^41 erg/s.

Halo interactions could explain Fermi bubbles and IceCube neutrinos.

Future telescopes may improve constraints by resolving more sources.

Abstract

Various studies have implied the existence of a gaseous halo around the Galaxy extending out to 100 kpc. Galactic cosmic rays (CRs) that propagate to the halo, either by diffusion or by convection with the possibly existing large-scale Galactic wind, can interact with the gas therein and produce gamma-rays via proton-proton collision. We calculate the cosmic ray distribution in the halo and the gamma-ray flux, and explore the dependence of the result on model parameters such as diffusion coefficient, CR luminosity, CR spectral index. We find that the current measurement of isotropic gamma-ray background at $\lesssim$TeV with Fermi Large Area Telescope already approaches a level that can provide interesting constraints on the properties of Galactic cosmic ray (e.g., with CR luminosity $L_{CR}\leq 10^{41}$erg/s). We also discuss the possibilities of the Fermi bubble and IceCube neutrinos originating from the proton-proton collision between cosmic rays and gas in the halo, as well as the implication of our results for the baryon budget of the hot circumgalactic medium of our Galaxy. Given that the isotropic gamma-ray background is likely to be dominated by unresolved extragalactic sources, future telescopes may extract more individual sources from the IGRB, and hence put even more stringent restriction on the relevant quantities (such as Galactic cosmic ray luminosity and baryon budget in the halo) in the presence of a turbulent halo that we consider.