2024ApJ...972..177J

We present UV-optical-near-infrared observations and modeling of supernova (SN) 2024ggi, a type II supernova (SN II) located in NGC 3621 at 7.2 Mpc. Early-time (“flash”) spectroscopy of SN 2024ggi within +0.8 days of discovery shows emission lines of H I, He I, C III, and N III with a narrow core and broad, symmetric wings (i.e., “IIn-like”) arising from the photoionized, optically thick, unshocked circumstellar material (CSM) that surrounded the progenitor star at shock breakout (SBO). By the next spectral epoch at +1.5 days, SN 2024ggi showed a rise in ionization as emission lines of He II, C IV, N IV/V, and O V became visible. This phenomenon is temporally consistent with a blueward shift in the UV-optical colors, both likely the result of SBO in an extended, dense CSM. The IIn-like features in SN 2024ggi persist on a timescale of t_IIn = 3.8 ± 1.6 days, at which time a reduction in CSM density allows the detection of Doppler-broadened features from the fastest SN material. SN 2024ggi has peak UV-optical absolute magnitudes of M_w2 = -18.7 mag and M_g = -18.1 mag, respectively, that are consistent with the known population of CSM-interacting SNe II. Comparison of SN 2024ggi with a grid of radiation hydrodynamics and non-local thermodynamic equilibrium radiative-transfer simulations suggests a progenitor mass-loss rate of $\dot{M}={10}^{-2}\,{M}_{\odot }$ yr^–1 (v_w = 50 km s^–1), confined to a distance of r < 5 × 10¹⁴ cm. Assuming a wind velocity of v_w = 50 km s^–1, the progenitor star underwent an enhanced mass-loss episode in the last ∼3 yr before explosion.