A shock crashing into confined dense circumstellar matter brightens the nascent SN 2023ixf

Red supergiants may experience a short-lived period of episodic mass loss rather than steady winds before their core collapses, leading to dense circumstellar matter (CSM) close to core-collapse supernovae (SNe). Interaction of SN ejecta with such nearby CSM can generate additional radiation, appending to the cooling radiation from the shock breakout of the progenitor envelope, to brighten the nascent SN explosion. This phenomenon is conspicuous for SN 2023ixf as its V-band brightness showed a rapid increase of about three magnitudes from the first to the third day after the explosion, which is distinctive among type II SNe with flash ionized signatures. In this paper, we employ a Monte Carlo method to simulate the radiative diffusion process in the unshocked CSM. Considering a wide range of mass-loss rates from 10^-5 to 10^-2 Msun/yr, we confirmed that the fast-rising light curve of SN 2023ixf can be fitted by the interaction of the SN ejecta with a CSM having a mass-loss rate of about 10^-2 Msun/yr located within 10^15 cm to the progenitor.