A unique small-scale gravitational arc in A1201

We present a snapshot Hubble Space Telescope (HST) image of the galaxy cluster A1201 (z = 0.169), revealing a tangential arc 2" from the brightest cluster galaxy (BCG). Keck Echelle Spectrograph and Imager (ESI) spectroscopy confirms that the arc is gravitational in nature and that the source galaxy lies at z = 0.451. We construct a model of the gravitational potential of the cluster that faithfully reproduces the observed arc morphology. Despite the relaxed appearance of the cluster in the HST frame, the best-fit ellipticity of the total matter distribution is epsilon(total) greater than or equal to 0.5, in contrast to the light distribution of the BCG (epsilon(BCG) = 0.23 +/- 0.03) on 2" scales. Further deep optical observations and pointed X-ray spectro-imaging observations with Chandra are required to determine whether this elongation is due to a single elongated dark matter halo or a more complex distribution of matter in the cluster core. We compare the arc with a sample drawn from the published literature and confirm that it is unique among tangential systems in the small physical scales that it probes (similar to6 kpc). In anticipation of a more thorough investigation of this cluster across a broad range of physical scales, we use our fiducial lens model to estimate the projected mass and mass-to-light ratio of the cluster within a radius of 6 kpc, obtaining M = (5.9(-0.7)(+0.9)) x 10(11) M., M/L-V = 9.4(-2.1)(+2.4) (M/L).. Overall our results confirm the importance of HST snapshot surveys for identifying rare lensing constraints on cluster mass distributions. In combination with follow-up optical and X-ray observations, the arc in A1201 should help to increase our understanding of the physics of cluster cores.