Pair creation at shocks: Application to the high energy emission of compact objects

We investigate the effect of pair creation on a shock structure. Particles, accelerated in the shock via the first order Fermi process, are supposed t o cool by the inverse Compton process on external soft photons, resulting i n a cut-off power law shape of the particle distribution function. The high energy photons produced are thus able to create pairs, through photon-phot on annihilation. The increase of the pair pressure may then be sufficient t o modify the shock profile. We show that there is even a limit of the pair pressure (of the order of 20% of the ram pressure of the upstream flow) abo ve which the shock can no longer exist. Conversely, significant changes of the ow velocity profile will also modify the spectral index and the high en ergy cut-off of the particle distribution function. Hence the number of par ticles able to trigger the pair creation process will change, modifying the pair creation rate accordingly. Taking into account these different proces ses, we self-consistently derive the ow velocity profile and the particle d istribution function. We show that, in some regions of the parameter space, the system can converge towards stationary states where pair creation and hydrodynamical effects balance. We discuss the application of this model to explain the high energy emission observed in compact objects. We show that hard X-ray spectra (alpha (x) < 1.) are only obtained for low pair pressur es and we don't expect any strong annihilation line in this case. We also s uggest a possible variability mechanism if the soft photon compactness itse lf depends on the pair density of the hot plasma, such as is expected in re illumination models.