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We present the densely sampled early light curve of the Type II supernova(SN) 2023ixf, first observed within hours of explosion in the nearby PinwheelGalaxy (Messier 101; 6.7 Mpc). Comparing these data to recently updated modelsof shock-cooling emission, we find that the progenitor likely had a radius of$410 \pm 10\ R_\odot$. Our estimate is model dependent but consistent with ared supergiant. These models provide a good fit to the data starting about 1day after the explosion, despite the fact that the classification spectrumshows signatures of circumstellar material around SN 2023ixf during that time.Photometry during the first day after the explosion, provided almost entirelyby amateur astronomers, does not agree with the shock-cooling models or asimple power-law rise fit to data after 1 day. We consider the possible causesof this discrepancy, including precursor activity from the progenitor star,circumstellar interaction, and emission from the shock before or after itbreaks out of the stellar surface. The very low luminosity ($-11\mathrm{\ mag}> M > -14\mathrm{\ mag}$) and short duration of the initial excess lead us toprefer a scenario related to prolonged emission from the SN shock travelingthrough the progenitor system.

Shock Cooling and Possible Precursor Emission in the Early Light Curve of the Type II SN 2023ixf