Limits On The Core Radii Of JVAS/CLASS Gravitational Lenses

CLASS (Cosmic Lens All Sky Survey) is a large radio survey consisting of 11685 flat spectrum radio sources with the purpose of identifying multiple image gravitational lensing, which was undertaken following the success of the Jodrell Bank VLA Astrometric survey (JVAS). To date a total of 18 multiple image gravitational lenses have been discovered in the combined JVAS and CLASS sample. It has been shown that gravitational lensing by transparent extended matter distributions should produce an odd number of images. However, gravitational lenses typically consist of either two or four images (doubles and quads) of the background source. Detection of the central `odd image' would allow one to constrain the central mass distribution of the lensing galaxy, in particular allowing a value for the core radius of the lensing galaxy to be derived. However, the `odd image' forms close to the centre of the lensing galaxy and is highly de-magnified, due to the high surface mass density at the lens centre. An upper limit for the flux of the `odd image' can be obtained using high sensitivity radio observations, such as 5GHz MERLIN (Multi-Element Radio Linked Interferometer Network) data. Approximating the lensing mass to a Non-Singular Isothermal Ellipsoid (NSIE), limits on the `odd image' flux can then be converted into an upper limit on the core radius of the lensing galaxy.