TeV Gamma Rays Expected from Supernova Remnants in Different Uniform Interstellar Media

Abstract

Calculations of the expected TeV $γ$-ray emission, produced by accelerated cosmic rays (CRs) in nuclear collisions, from supernova remnants evolving in a uniform interstellar medium (ISM) are presented. The aim is to study the sensitivity of $γ$-ray production to a physical parameter set. Apart from its general proportionality to N_H, it is shown that the $γ$-ray production essentially depends upon the ratio of the CR diffusion coefficient $κ$ to a critical value $κ_{crit}=10(B_0/5 μ{G})(N_H/0.3 {cm}^{-3})^{-1/3}κ_B$, where B_0 and N_H are the magnetic field and hydrogen number density of the ISM, and $κ_B$ denotes the Bohm diffusion coefficient. If $κ$ is of the same order or lower than $κ_{crit}$, then the peak TeV $γ$-ray flux in the Sedov evolutionary phase, normalized to a distance of 1 kpc, is about 10^{-10}(N_H/0.3 {cm}^{-3}) photons cm^{-2} s^{-1}. For a CR diffusion coefficient that is significantly larger than $κ_{crit}$, the CR cutoff energy is less than 10 TeV and the expected $γ$-ray flux at 1 TeV is considerably below the presently detectable level of 10^{-12} photons cm^{-2} s^{-1}. The same is of course true for a SNR in the rarified, so-called hot ISM.