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

Understand

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).