When the radiation of a star passes near a massive object, its intensity is magnified due to the microlensing phenomenon. Similar effects should also apply to pulsar signals. However, because of the pulsating nature of pulsar radiation, the flux magnification will be accompanied by a time varying travel-time delay (Krauss and Small 1991, Larchenkova and Doroshenko 1995, Ohnishi et al. 1995, Wexet al. 1996).
Let us denote the epoch of observation by t
⊕. A travel-time delay caused by a Schwarzschild-lens is characterized by four parameters: M, the mass of the lens, q, the ratio of the transverse velocity of the lens with respect to the line of sight and the minimum impact parameter, fm
, the ratio of the minimum impact parameter and the Einstein radius, and T
0, the time of maximum delay. If we denote the ‘intrinsic’ frequency and its derivative with respect to time (corresponding to the barycentric arrival time t = t
⊕) by ν and then, we find for the observed phase of the pulsar at the barycentric time t: