Time Delays and Frequency Shiftsof Strongly

Accepted ABSTRACT Large light deeection angles are produced in the strong gravitational eld regions around neutron stars and black holes. In the case of binary systems, part of the photons emitted from the companion star towards the collapsed object are expected to be deeected in the direction of the earth. Based on a semi-classical approach we calculate the characteristic time delays and frequency shifts of these photons as a function of the binary orbital phase. The intensity of the strongly deeected light rays is reduced by many orders of magnitude, therefore making the observations of this phenomenon extremely diicult. Relativistic binary systems containing a radio pulsar and a collapsed object are the best available candidates for the detection of the strongly deeected photons. Based on the accurate knowledge of their orbital parameters, these systems allow to predict accurately the delays of the pulses along the highly deeected path, such that the sensitivity to very weak signals can be substantially improved through coherent summation over long time intervals. We discuss in detail the cases of PSR 1913+16 and PSR 1534+12 and nd that the system geometry is far more promising for the latter. The observation of the highly deeected photons can provide a test of general relativity in an unprecedented strong eld regime as well as a tight constraint on the radius of the collapsed object.