Exotic Image Formation in Strong Gravitational Lensing by Clusters of Galaxies -- IV. Elliptical NFW Lenses and Hyperbolic Umbilics

Abstract

A source lying near hyperbolic umbilic (HU) leads to a ring-like image formation, constituting four images with high magnification factors and lying in a small region of the lens plane. Since (based on our earlier work) the observed number of HU image formations in cluster lenses is expected to increase in future, it is timely to investigate them in more detail. Like fold and cusp, HU also satisfies the magnification relation, i.e., the signed magnification sum of the four images equals zero. This work presents a detailed study of HU magnification relation ($R_{\mathrm{hu}}$) considering the elliptical Navarro-Frenk-White (eNFW) lens profile suitable for cluster scale dark matter halos. Our results show that for an isolated eNFW lens, $R_{\mathrm{hu}}$ is more sensitive to ellipticity than its mass or concentration parameter. An ellipticity greater than 0.3 results in $R_{\mathrm{hu}}$ lying close to zero with a small scatter around it. A substructure near the HU image formation causes the average $R_{\mathrm{hu}}$ value to deviate from zero and increases the scatter, with the amount of deviation depending on the image type near which the substructure lies. However, a population of substructures in the lens plane (equivalent to the galaxy lenses inside the cluster) does not significantly shift the average $R_{\mathrm{hu}}$ value from zero but increases the scatter around it. We find that $R_{\mathrm{hu}}\simeq 0$ for HU image formation in the Abell 1703 cluster. Repeating this test in other clusters where HU formations are discovered can be a useful indicator of substructure in cluster halos.

18 pages, 17 figures. Accepted in MNRAS

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