Optical and near-infrared observations of SN 2023ixf for over 600 days after the explosion

Context. We present a comprehensive photometric and spectroscopic study of the nearby Type II supernova (SN) 2023ixf; our extensive observations span the phases from similar to 3 to over 600 days after the first light. Aims. The aim of this study is to obtain key information on the explosion properties of SN 2023ixf and the nature of its progenitor. Methods. The observational properties of SN 2023ixf were compared with those of a representative sample of Type IIP and IIL SNe to investigate commonalities and diversities. We conducted a detailed analysis of the temporal evolution of major spectral features observed throughout differ-ent phases of the SN 2023ixf explosion. Several key interpretations are addressed through a comparison between the data and the model spectra predicted by nonlocal thermodynamic equilibrium (non-LTE) radiative-transfer calculations for progenitor stars within a range of zero-age main sequence (ZAMS) masses. Results. Our observations indicate that SN 2023ixf is a transitional SN that bridges the gap between the Type IIP and IIL subclasses of H-rich SNe; it is characterized by a relatively short plateau (70 d) in the light curve. It shows a rather prompt spectroscopic evolution toward the nebular phase; emission lines of Na, O, H, and Ca in nebular spectra all exhibit multi-peak profiles, which might be due to a bipolar distribution of the ejecta. In particular, the H alpha profile can be separated into two central peaked components (with velocities of about 1500 km s(-1)) that are likely due to nickel-powered ejecta and two outer box components (with velocities of up to similar to 8000 km s-1) that can arise from interactions of the outermost ejecta with a circumstellar shell at a distance of similar to 6.2 x 10(15) cm. The nebular-phase spectra of SN 2023ixf show good agreement with those predicted by a non-LTE radiative-transfer code for progenitor stars with ZAMS masses ranging from 15 to 19 M-circle dot A distance of 6.35(+0.31-)0.39 Mpc is estimated for M101 based on the expanding photosphere method.