Inverse bremsstrahlung in shocked astrophysical plasmas

There has recently been interest in the role of inverse bremsstrahlung, the emission of photons by fast suprathermal ions in collisions with ambient e lectrons possessing relatively low velocities, in tenuous plasmas in variou s astrophysical contexts. This follows a long hiatus in the application of suprathermal ion bremsstrahlung to astrophysical models since the early 197 0s. The potential importance of inverse bremsstrahlung relative to normal b remsstrahlung, i.e., where ions are at rest, hinges upon the underlying vel ocity distributions of the interacting species. Ln this paper, we identify the conditions under which the inverse bremsstrahlung emissivity is signifi cant relative to that for normal bremsstrahlung in shocked astrophysical pl asmas. We determine that, since both observational and theoretical evidence favors electron temperatures almost comparable to, and certainly not very deficient relative to, proton temperatures in shocked plasmas, these enviro nments generally render inverse bremsstrahlung at best a minor contributor to the overall emission. Hence, inverse bremsstrahlung can be safely neglec ted in most models invoking shock acceleration in discrete sources such as supernova remnants. However, on scales greater than or similar to 100 pc di stant from these sources, Coulomb collisional losses can deplete the cosmic -ray electrons, rendering inverse bremsstrahlung, and perhaps bremsstrahlun g from knock-on electrons, possibly detectable.