Left panel: location of AT 2020hat in the HST/ACS + F606W pre-discovery image. The dashed white circle is centred on the transformed position of the transient and has a radius of 3× our positional uncertainty. Right panel: grey density plot shows the colour-magnitude diagram for all sources detected in both F606W and F814W filters at a combined S/N > 5. A foreground reddening A_V = 0.29 mag and distance modulus μ = 28.56 mag have been corrected for. The sharp diagonal detection cut is due to limiting magnitude, as determined by the S/N threshold. Pink points are sources within 1″ of AT 2020hat as indicated in the left panel, for which the S/N > 5 detection threshold has been relaxed (as a consequence, they are plotted with large error bars). The green point is the progenitor candidate for AT 2020hat.