2009MNRAS.397.1084P

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 δ between the light rays emerging from the source, for different lensing configurations. If the lens has a power-law profile M ∝ R^γ, the angle δ initially increases with lens redshift and then either diverges (for a steep profile γ < 1), remains constant (for an isothermal profile γ = 1), or vanishes (for a shallow profile γ > 1) as z_l ⟶ z_s. The scaling with lens mass is roughly δ ∝ M^1/(2–γ). The results for an Navarro-Frenk-White (NFW) profile are qualitatively similar to those for a shallow power law, with δ 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 θ_jet, there is a probability as high as P ∼ 0.02-0.07(θ_jet/0.5°)^–1 that one of the lensed images may be missed (for 2 ≲ z_s≲ 6). Differential absorption within active galactic nuclei (AGNs) 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.