Hamilton's Object -- a clumpy galaxy straddling the gravitational
  caustic of a galaxy cluster : Constraints on dark matter clumping
  We report the discovery of a 'folded' gravitationally lensed image,
'Hamilton's Object', found in a HST image of the field near the AGN SDSS
J223010.47-081017.8 ($z=0.62$). The lensed images are sourced by a galaxy at a
spectroscopic redshift of 0.8200$\pm0.0005$ and form a fold configuration on a
caustic caused by a foreground galaxy cluster at a photometric redshift of
0.526$\pm0.018$ seen in the corresponding Pan-STARRS PS1 image and marginally
detected as a faint ROSAT All-Sky Survey X-ray source. The lensed images
exhibit properties similar to those of other folds where the source galaxy
falls very close to or straddles the caustic of a galaxy cluster. The folded
images are stretched in a direction roughly orthogonal to the critical curve,
but the configuration is that of a tangential cusp. Guided by morphological
features, published simulations and similar fold observations in the
literature, we identify a third or counter-image, confirmed by spectroscopy.
Because the fold-configuration shows highly distinctive surface brightness
features, follow-up observations of microlensing or detailed investigations of
the individual surface brightness features at higher resolution can further
shed light on kpc-scale dark matter properties. We determine the local lens
properties at the positions of the multiple images according to the
observation-based lens reconstruction of Wagner et al. (2019). The analysis is
in accordance with a mass density which hardly varies on an arc-second scale (6
kpc) over the areas covered by the multiple images.
