SN 2023ixf in the Pinwheel Galaxy M101: From Shock Breakout to the Nebular Phase

Abstract We present photometric and spectroscopic observations of supernova SN 2023ixf covering from day 1 to 442 days after explosion. SN 2023ixf reached a peak V -band absolute magnitude of −18.2 ± 0.07, and light curves show that it has a relatively short “plateau” phase (∼72 days), and can be classified either as an SN IIL or a transitional event between IIP and IIL. Early-time spectra of SN 2023ixf exhibit strong, very narrow emission lines from ionized circumstellar matter (CSM), possibly indicating a Type IIn classification. But these flash/shock-ionization emission features faded after the first week and the spectrum evolved in a manner similar to that of typical Type II SNe, unlike the case of most genuine SNe IIn in which the ejecta interact with CSM for an extended period of time and develop intermediate-width emission lines. We compare observed spectra of SN 2023ixf with various model spectra to understand the physics behind SN 2023ixf. Our nebular spectra (between 200 and 400 days) match best with the model spectra from a 15 M ⊙ progenitor that experienced enhanced mass loss a few years before explosion. A last-stage mass-loss rate of M ̇ = 0.01 M ⊙ yr − 1 from the r1w6 model matches best with the early-time spectra, higher than M ̇ ≈ 2.4 × 1 0 − 3 M ⊙ yr − 1 derived from the ionized H α luminosity at 1.58 days. We also use SN 2023ixf as a distance indicator and fit the light curves to derive the Hubble constant by adding SN 2023ixf to the existing sample; we obtain H 0 = 73 . 1 − 3.50 + 3.68 km s −1 Mpc −1 , consistent with the results from SNe Ia and many other independent methods.