Gravitational Lensing of Anisotropic Sources

TL;DR

This paper investigates how anisotropic emission from sources affects gravitational lensing observations, revealing that source anisotropy can influence image detection probabilities and flux ratios, especially in cluster lensing scenarios.

Contribution

It provides a detailed analysis of the impact of source anisotropy on strong gravitational lensing, including the dependence on lens profile and redshift, and discusses observational implications.

Findings

Source anisotropy can cause missed images with probability up to 7%.

Anisotropy effects are significant in cluster lensing but negligible in galaxy-scale lensing.

The angle between light rays depends on lens profile and redshift, affecting lensing observations.

Abstract

In strong gravitational lensing, the multiple images we see correspond to light rays that leave the source in slightly different directions. If the source emission is anisotropic, the images may differ from conventional lensing predictions (which assume isotropy). To identify scales on which source anisotropy may be important, we study the angle delta between the light rays emerging from the source, for different lensing configurations. If the lens has a power law profile M propto R^gamma, the angle delta initially increases with lens redshift and then either diverges (for a steep profile gamma<1), remains constant (for an isothermal profile gamma=1), or vanishes (for a shallow profile gamma>1) as zl approaches zs. The scaling with lens mass is roughly delta propto M^(1/(2-gamma)). The results for an NFW profile are qualitatively similar to those for a shallow power law, with delta peaking at about half the redshift of the source (not half the distance). In practice, beaming could modify the statistics of beamed sources lensed by massive clusters: for an opening angle theta_jet, there is a probability as high as P ~ 0.02-0.07 (theta_jet/{0.5} deg)^-1 that one of the lensed images may be missed (for 2 < zs < 6). Differential absorption within Active Galactic Nuclei could modify the flux ratios of AGNs lensed by clusters; a sample of AGNs lensed by clusters could provide further constraints on the sizes of absorbing regions. Source anisotropy is not likely to be a significant effect in galaxy-scale strong lensing.