Blueshift and spectral features of the gravitational searchlight

It is shown that a photon emitted in the forward direction by a charged particle moving in an equatorial circular orbit centred on a highly collapsed mass M, the radius being slightly in excess of one and a half times the Schwarzschild radius, is strongly blueshifted when it arrives at a distant receiver. A ring shaped emitting region composed of such orbiting particles has a power law spectrum of the form dν/ν as seen by a distant stationary observer. (author)