Gravitational lensing by cosmic strings

The possible role of a cosmological distribution of vacuum strings in the gravitational imaging of quasars is discussed. Lensing properties are described for open strings and for large and small closed loops. A simplified model shows that small loops typically produce a pair of magnified merging images. This effect introduces an observational 'amplification bias' which causes magnified quasars to appear frequently in a flux-limited sample even though they are intrinsically rare. Highly magnified merging images are sensitive to small changes in the lens, and it is shown that loop oscillations can lead to observable variations in image size, configuration, and total luminosity. (author)