2025PASJ...77L..31Y

We report the multi-band photometric observations of the Type IIb supernova (SN) 2024iss at ultraviolet (UV), optical, and near-infrared (NIR) wavelengths starting one day after the explosion. The UV and optical light curves show the first peak two days after the explosion date. Following the first peak, a secondary maximum is observed in the optical and NIR bands, similar to SNe IIb with double-peaked light curves. The quasi-bolometric light curve shows fast decay for a week after the explosion. From the analysis of the bolometric light curve, the ejecta mass and kinetic energy are estimated to be $M_{\rm ej}=2.8\pm 0.6~M_{\odot }$ and $E_{\rm kin}=9.4\pm 4.1\times 10^{50}$ erg. The mass of the radioactive $^{56}$Ni is estimated to be $M(^{56}\mathrm{Ni})=0.2~M_{\odot }$. Fitting a blackbody function to the spectral energy distribution reveals that the photospheric temperature exhibits a rapid exponential decline during the first week after the explosion. An analytic model describing the cooling emission after shock breakout provides a reasonable explanation for the observed temperature evolution. From these ejecta parameters, we calculated the progenitor radius to be $R_{\rm pro}=50$-$340~R_{\odot }$. We conclude that these explosion properties are consistent with a core-collapse explosion from a yellow supergiant progenitor.