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 μG)(N H /0.3 cm −3) −1/3κ 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 supernova remnant in the rarified, so-called hot ISM.