Extended Shock Breakout and Early Circumstellar Interaction in SN 2024ggi

Abstract

We present high-cadence photometric and spectroscopic observations of supernova (SN) 2024ggi, a Type II SN with flash spectroscopy features which exploded in the nearby galaxy NGC 3621 at $\sim$7 Mpc. The light-curve evolution over the first 30 hours can be fit by two power law indices with a break after 22 hours, rising from $M_V\approx -12.95$ mag at +0.66 days to $M_V\approx -17.91$ mag after 7 days. In addition, the densely sampled color curve shows a strong blueward evolution over the first few days and then behaves as a normal SN II with a redward evolution as the ejecta cool. Such deviations could be due to interaction with circumstellar material (CSM). Early high- and low-resolution spectra clearly show high-ionization flash features from the first spectrum to +3.42 days after the explosion. From the high-resolution spectra, we calculate the CSM velocity to be 37 $\pm 4\mathrm{km}{\mathrm{s}}^{-1}$. We also see the line strength evolve rapidly from 1.22 to 1.49 days in the earliest high-resolution spectra. Comparison of the low-resolution spectra with CMFGEN models suggests that the pre-explosion mass-loss rate of SN 2024ggi falls in a range of $10^{-3}$ to $10^{-2}$ M${}_{\odot }$ yr${}^{-1}$, which is similar to that derived for SN 2023ixf. However, the rapid temporal evolution of the narrow lines in the spectra of SN 2024ggi ($R_{\mathrm{CSM}}\sim 2.7\times 10^{14}\mathrm{cm}$) could indicate a smaller spatial extent of the CSM than in SN 2023ixf ($R_{\mathrm{CSM}}\sim 5.4\times 10^{14}\mathrm{cm}$) which in turn implies lower total CSM mass for SN 2024ggi.

23 pages, 15 figures, 4 tables, accepted for publication in ApJL