SN 2024igg: A super-Chandrasekhar/03fg-like supernova exhibiting C II-dominated spectra after 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 Università degli Studi di Padova, Dipartimento di Fisica e Astronomia, Vicolo dell’Osservatorio 2 35122 Padova, Italy
⁵ Centro di Ateneo di Studi e Attività Spaziali “Giuseppe Colombo”, Via Venezia 1, I-35131 Padova, Italy
⁶ INFN – Sezione di Padova, Via Marzolo 8, I-35131 Padova, Italy
⁷ INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate (LC), Italy
⁸ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, China
⁹ International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, China
¹⁰ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, China
¹¹ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, China
¹² Institute of Astrophysics, Central China Normal University, Wuhan 430079, China
¹³ Education Research and Application Center, National Astronomical Data Center, Wuhan 430079, China
¹⁴ Key Laboratory of Quark and Lepton Physics (Central China Normal University), Ministry of Education, Wuhan 430079, China

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Received: 27 January 2026
Accepted: 2 June 2026

Abstract

Aims. We present and analyze photometric and spectroscopic observations of the Type Ia supernova (SN Ia) 2024igg, another super-Chandrasekhar (or 03fg-like) SN whose strong C IIλ6580 feature was initially misidentified as Hα, thereby constraining its progenitor system, explosion parameters, and physical scenario.

Methods. We conducted a comparative analysis of SN 2024igg and other 03fg-like SNe. The radiative-transfer code TARDIS was run to identify the features in the first spectrum at t ≈ −13.9 days. A pseudo-bolometric light curve was constructed using ultraviolet and optical photometry and was then modeled with fitting tools.

Results. SN 2024igg shows many characteristics in common with other 03fg-like objects, such as high ultraviolet flux, slowly declining light curves (Δm₁₅(B) = 0.90 ± 0.08 mag), low expansion velocities, and strong and persistent C II absorption. On the other hand, it exhibits some remarkable properties within this subgroup, including a moderately low optical luminosity (M_max(B) = − 18.99 ± 0.15 mag), a rise time shorter than 18.5 days, and the strongest C IIλ6580 ever detected. The bolometric analysis yields a ⁵⁶Ni mass of M_Ni = 0.547 ± 0.082 M_⊙ and an ejecta mass of 1.54^+0.22_−0.19 M_⊙, which marginally exceeds the Chandrasekhar mass. Our TARDIS result indicates that most of the features in the earliest spectrum might be attributed to C II, which is consistent with a model in which an SN explodes within a carbon-rich circumstellar medium (CSM). The CSM interaction would produce a density peak in the ejecta, offering a natural explanation for the slowly evolving line velocities near −8000 km s⁻¹. The CSM may stem from the debris of a secondary white dwarf in a white dwarf merger or the envelope of an asymptotic giant branch star. Combined with the unshifted forbidden lines in the spectrum taken at t ≈ +135 days, we suggest that SN 2024igg comes from a symmetric explosion on a secular timescale after the merger.