Aspherical Shock Breakout Inferred from the First Light of Supernova 2024ggi

Shock breakout emission represents the first light enlightening the dying star, which carries essential information probing nature of progenitor, ambient environment, and explosion physics. For instance, compared to a spherical explosion, an aspherical explosion can leave adiscernible imprint on the early-time spectral energy distribution characterized by a delayed shockbreakout and a possibly complex thermal emission. However, such early-time emis-sion has scarcely been detected, especially for hydrogen-rich type II supernovae, and hastherefore been far less explored in detailed modeling than the more readily observed shock-coolingphase. Here we report the ultraviolet, optical and X-ray observations of a type II supernova2024ggi (at a distance of 6.7 Mpc), beginning at a few hours after the explosion. Its spectral en-ergy distribution can be described only by a hotter component with a blackbody temperature ofabout 100,000 K and a cooler component with a temperature of 10,000 K. Such a two-componentthermal emission, together with the rapid blueward color evolution and increased ionization statevariation of emission lines observed within about one day after the explosion, are consistent with ascenario in which the supernova shock partially breaks out of a dense circumstellar material (CSM).We develop a semi-analytical shock-breakout radiation model which can well reproduce the earlymultiwavelength emission of SN 2024ggi and predict a disk-like CSM configuration around the ex-ploding star. These results provide independent and crucial evidence for aspherical explosion andunstable mass ejection of massive stars.