Banados, Silk, and West showed that a Kerr black hole can act as a particle accelerator allowing for near horizon collisions of arbitrarily high center of mass energy when the black hole is near maximally rotating. In their work, they surmised that the ejecta from such collisions, which if observed could provide a unique probe of physics at scales beyond those of terrestrial accelerators, would be highly redshifted.
In this regime, we examine the specific case of proton-electron bremsstrahlung and calculate properties of the photons that constitute the collision ejecta. We calculate the photon escape probability and the redshift-dependent flux received on the celestial sphere. We note that much of the observable emission can be blueshifted.