Gravitational lensing in the presence of plasma scattering with application to Fast Radio Bursts

Abstract

We describe how gravitational lensing of fast radio bursts (FRBs) is affected by a plasma screen in the vicinity of the lens or somewhere between the source and the observer. Wave passage through a turbulent medium affects gravitational image magnification, lensing probability (particularly for strong magnification events), and the time delay between images. The magnification is suppressed because of the broadening of the angular size of the source due to scattering by the plasma. The time delay between images is modified as the result of different dispersion measure (DM) along photon trajectories for different images. Each of the image lightcurve is also broadened due to wave scattering so that the images could have distinct temporal profiles. The first two effects are most severe for stellar and sub-stellar mass lens, and the last one (scatter broadening) for lenses and plasma screens at cosmological distances from the source/observer. This could limit the use of FRBs to measure their cosmic abundance. On the other hand, when the time delay between images is large, such that the lightcurve of a transient source has two or more well separated peaks, the different DMs along the wave paths of different images can probe density fluctuations in the IGM on scales $\lesssim 10^{-6}$ rad and explore the patchy reionization history of the universe using lensed FRBs at high redshifts. Different rotation measure (RM) along two image paths can convert linearly polarized radiation from a source to partial circular polarization.