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The bright, blue, rapidly evolving AT2018cow is a well-studied peculiarextragalactic transient. Despite an abundance of multi-wavelength data, therestill is no consensus on the nature of the event. We present our analysis ofthree epochs of Hubble Space Telescope (HST) observations spanning the periodfrom 713-1474 days post burst, paying particular attention to uncertainties ofthe transient photometry introduced by the complex background in whichAT2018cow resides. Photometric measurements show evident fading in the UV andmore subtle but significant fading in the optical. During the last HSTobservation, the transient's optical/UV colours were still bluer than those ofthe substantial population of compact, young, star-forming regions in the hostof AT2018cow, suggesting some continued transient contribution to the light.However, a compact source underlying the transient would substantially modifythe resulting spectral energy distribution, depending on its contribution inthe various bands. In particular, in the optical filters, the complex, diffusebackground poses a problem for precise photometry. An underlying cluster isexpected for a supernova occurring within a young stellar environment or atidal-disruption event (TDE) within a dense older one. While many recent workshave focused on the supernova interpretation, we note the substantialsimilarity in UV light-curve morphology between AT2018cow and several tidaldisruption events around supermassive black holes. Assuming AT2018cow arisesfrom a TDE-like event, we fit the late-time emission with a disc model and find$M_{BH} = 10^{3.2{\pm}0.8}$ M$_{\odot}$. Further observations are necessary todetermine the late-time evolution of the transient and its immediateenvironment.

Late time HST UV and optical observations of AT~2018cow: extracting a cow from its background