Early phase simultaneous multi-band observations of Type II supernova SN 2024ggi with Mephisto

We present early-phase good cadence simultaneous multi-band ($ugi$, $vrz$--bands) imaging of nearby supernova SN 2024ggi, which exploded in the nearby galaxy, NGC~3621. A quick follow-up was conducted within less than a day after the explosion and continued ~23 days. The $uvg$-band light curves display a rapid rise (~1.4 mag day⁻¹) to maximum in ~4 days and absolute magnitude Mg~--17.75 mag. The post-peak decay rate in redder bands is ~0.01 mag day⁻¹. Different colors (e.g., $u-g$ and $v-r$) of SN~2024ggi are slightly redder than SN~2023ixf. A significant rise (~12.5 kK) in black-body temperature (optical) was noticed within ~2 days after the explosion, which successively decreased, indicating shock break out inside a dense circumstellar medium (CSM) surrounding the progenitor. Using semi-analytical modeling, the ejecta mass and progenitor radius were estimated as 1.2 M_ and ~550 R_, respectively. The archival deep images ($g,r,i,z$-bands) from the Dark Energy Camera Legacy Survey (DECaLS) were examined, and a possible progenitor was detected in each band (~22--22.5 mag) and had a mass range of 14--17 M_.