Gamma rays from reaccelerated cosmic rays in high velocity clouds colliding with the Galactic disk

Abstract

High velocity clouds moving toward the disk will reach the Galactic plane and will inevitably collide with the disk. In these collisions a system of two shocks is produced, one propagating through the disk and the other develops within the cloud. The shocks produced within the clouds in these interactions have velocities of hundreds of kilometers per second. When these shocks are radiative they may be inefficient in accelerating fresh particles, however they can reaccelerate and compress Galactic cosmic rays from the background. In this work we investigate the interactions of Galactic cosmic rays within a shocked high velocity cloud, when the shock is induced by the collision with the disk. This study is focused in the case of radiative shocks. We aim to establish under which conditions these interactions lead to significant nonthermal emission, especially gamma rays. We model the interaction of cosmic ray protons and electrons reaccelerated and further energized by compression in shocks within the clouds, under very general assumptions. We also consider secondary electron-positron pairs produced by the cosmic ray protons when colliding with the material of the cloud. We conclude that nearby clouds reaccelerating Galactic cosmic rays in local shocks can produce high-energy radiation that might be detectable with existing and future gamma-ray detectors. The emission produced by electrons and secondary pairs is important at radio wavelengths, and in some cases it may be relevant at hard X-rays. Concerning higher energies, the leptonic contribution to the spectral energy distribution is significant at soft gamma rays.