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

¹ Department of Physics, Tsinghua University, Beijing 100084, China
² Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
³ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, (LC), Italy
⁵ Dipartimento di Fisica e Astronomia, Università Degli Studi di Padova, 35121 Padova, PD, Italy
⁶ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, China
⁷ International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, China
⁸ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, China
⁹ INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹⁰ CISAS G. Colombo, University of Padova, Via Venezia 15, 35131 Padova, Italy
¹¹ UNESCO Chair “Environment, Resources and Sustainable Development”, Department of Science and Technology, Parthenope University of Naples, Naples, Italy
¹² INAF, Osservatorio Astronomico di Capodimonte, Salita Moiariello, 16, Naples I-80131, Italy
¹³ Max-Planck-Institut fur Astrophysik, Karl-Schwarzschild Straße 1, 85748 Garching, Germany
¹⁴ Beijing Planetarium, Beijing Academy of Sciences and Technology, Beijing 100044, China
¹⁵ University of Chinese Academy of Sciences, Beijing 100049, China
¹⁶ Ulugh Beg Astronomical Institute, Astronomy Street 33, Tashkent 100052, Uzbekistan
¹⁷ National University of Uzbekistan, Tashkent 100174, Uzbekistan
¹⁸ School of Space Science and Technology, Shandong University, Weihai, Shandong 264209, China
¹⁹ Xingming Observatory, Urumqi 830000, China
²⁰ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China

^⋆ Corresponding author: wang_xf@mail.tsinghua.edu.cn

Received: 4 April 2025
Accepted: 8 August 2025

Abstract

Context. We present a comprehensive photometric and spectroscopic study of the nearby Type II supernova (SN) 2023ixf; our extensive observations span the phases from ∼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 different 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 Hα 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 ∼8000 km s⁻¹) that can arise from interactions of the outermost ejecta with a circumstellar shell at a distance of ∼6.2 × 10¹⁵ 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_⊙. A distance of 6.35^+0.31_−0.39 Mpc is estimated for M101 based on the expanding photosphere method.