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

Abstract

We present photometric and spectroscopic observations of SN 2023ixf covering from day one to 442 days after explosion. SN 2023ixf reached a peak $V$-band absolute magnitude of $-18.2\pm 0.07$, and light curves show that it is in the fast-decliner (IIL) subclass with a relatively short ``plateau'' phase (fewer than $\sim 70$ days). 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 200-400 d) match best with the model spectra from a 15 ${\mathrm{M}}_{\odot }$ progenitor which experienced enhanced mass loss a few years before explosion. A last-stage mass-loss rate of $\dot{M}=0.01{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$ from the r1w6 model matches best with the early-time spectra, higher than $\dot{M}\approx 2.4\times 10^{-3}{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$ derived from the ionized H$\hat{I}\pm$ luminosity at 1.58 d. 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.

Submitted to AAS