  Observing a supernova explosion shortly after it occurs can reveal important
information about the physics of stellar explosions and the nature of the
progenitor stars of supernovae (SNe). When a star with a well-defined edge
explodes in vacuum, the first photons to escape from its surface appear as a
brief shock-breakout flare. The duration of this flare can extend to at most a
few hours even for nonspherical breakouts from supergiant stars, after which
the explosion ejecta should expand and cool. Alternatively, for stars exploding
within a distribution of sufficiently dense optically thick circumstellar
material, the first photons escape from the material beyond the stellar edge,
and the duration of the initial flare can extend to several days, during which
the escaping emission indicates photospheric heating. The difficulty in
detecting SN explosions promptly after the event has so far limited data
regarding supergiant stellar explosions mostly to serendipitous observations
that, owing to the lack of ultraviolet (UV) data, were unable to determine
whether the early emission is heating or cooling, and hence the nature of the
early explosion event. Here, we report observations of SN 2023ixf in the nearby
galaxy M101, covering the early days of the event. Using UV spectroscopy from
the Hubble Space Telescope (HST) as well as a comprehensive set of additional
multiwavelength observations, we trace the photometric and spectroscopic
evolution of the event and are able to temporally resolve the emergence and
evolution of the SN emission.
