SN 2024iss: A double-peaked Type IIb supernova with evidence of circumstellar interaction

¹ Department of Physics, Tsinghua University, Beijing 100084, China
² National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
³ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049,1408, China
⁴ Las Cumbres Observatory, 6740 Cortona Drive, Suite 102 Goleta, CA 93117, USA
⁵ Department of Physics, University of California Santa Barbara, Santa Barbara, California 93106, USA
⁶ Università degli Studi di Padova, Dipartimento di Fisica e Astronomia, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
⁷ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁸ INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, (LC), Italy
⁹ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
¹⁰ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, China
¹¹ International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, China
¹² Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, China
¹³ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA
¹⁴ Gemini Observatory, 670 North A‘ohoku Place, Hilo, HI 96720-2700, USA
¹⁵ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi, Xinjiang 830011, China
¹⁶ Università degli Studi di Padova, Dipartimento di Fisica e Astronomia, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁷ Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
¹⁸ Department of Astronomy, University of Texas, Austin, TX 78712, USA
¹⁹ Kavli Institute for Theoretical Physics, University of California, Santa Barbara, 552 University Road, Goleta, CA 93106-4030, USA
²⁰ Johns Hopkins University, Baltimore, MD, USA
²¹ School of Physics and Astronomy, Monash University, Clayton, Australia
²² OzGrav: The ARC Center of Excellence for Gravitational Wave Discovery, Hawthorn, VIC 3122, Australia
²³ Adler Planetarium, 1300 S. DuSable Lake Shore Dr., Chicago, IL 60605, USA
²⁴ HUN-REN CSFK Konkoly Observatory, MTA Center of Excellence, Konkoly Thege ut 15-17 Budapest 1121, Hungary
²⁵ Department of Experimental Physics, University of Szeged, Dóm tér 9 Szeged 6720, Hungary
²⁶ ELTE Eötvös Loránd University, Institute of Physics and Astronomy, Pázmány Péter sétány 1/A, Budapest 1117, Hungary
²⁷ Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
²⁸ Beijing Planetarium, Beijing Academy of Sciences and Technology, Beijing 100044, China

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Received: 27 October 2025
Accepted: 5 February 2026

Abstract

Aims. We present optical, ultraviolet, and X-ray observations of supernova (SN) 2024iss, a Type IIb SN that shows a prominent double-peaked light curve.

Methods. We modeled the first peak with a semianalytical shock-cooling model and the X-ray emission with a free-free model. We also compared the envelope radius and mass-loss rate with those of other Type IIb SNe to explore the relationships between the progenitor envelope and the circumstellar material.

Results. The shock-cooling peak in the V-band light curve reached M_V = −17.33 ± 0.26 mag, while the ⁵⁶Ni-powered second peak attained M_V = −17.43 ± 0.26 mag. Early spectra show a photospheric velocity of approximately 19 400 km s⁻¹ at 3.82 days from the Hα P Cygni profile. The Balmer lines persist for at least more than 87 days after the explosion, which is characteristic of hydrogen-rich ejecta. Modeling the first light-curve peak with the shock-cooling model suggests an extended hydrogen envelope with a mass of 0.11 ± 0.04 M_⊙ and a radius of 244 ± 43 R_⊙. Fitting the second light-curve peak with an Arnett-like model indicates a typical ⁵⁶Ni mass of 0.117 ± 0.013 M_⊙ and a relatively low ejecta mass of 1.27 ± 0.34 M_⊙. X-ray observations revealed bright thermal bremsstrahlung emission and indicate a mass-loss rate of 1.6 × 10⁻⁵ M_⊙ yr⁻¹, which is similar to that of SN 1993J.

Conclusions. Supernova 2024iss occupies a transitional position between the two subclasses of extended and compact Type IIb SNe. Its envelope radius and preexplosion mass-loss rate appear to be consistent with the correlation observed in the broader sample. The observational properties of SN 2024iss are compatible with a binary-interaction scenario being the dominant mechanism for envelope stripping.