Emergence of Fresnel diffraction zones in gravitational lensing by a cosmic string

The possibility to detect cosmic strings - topological defects of early Universe, by means of wave effects in gravitational lensing is discussed. To find the optimal observation conditions, we define the hyperbolic-shaped Fresnel observation zones associated with the diffraction maxima and analyse the frequency patterns of wave amplification corresponding to different alignments. In particular, we show that diffraction of gravitational waves by the string may lead to significant amplification at cosmological distances. The wave properties we found are quite different from what one would expect, for instance, from light scattered off a thin wire or slit, since a cosmic string, as a topological defect, gives no shadow at all.