Liyang Chen, Xiaofeng Wang, Qinyu Wu, Moira Andrews, Joseph Farah, P. Ochner, Andrea Reguitti, Alexei V. Filippenko, Thomas G. Brink, Jujia Zhang, Cuiying Song, Jialian Liu, David J. Sand, Irene Albanese, Kate D. Alexander, Jennifer Andrews, K. Azalee Bostroem, Y-Z Cai, Collin T. Christy, Ali Esamdin, Andrea Farina, Noah Franz, D. Andrew Howell, Brian Hsu, Maokai Hu, Abdusamatjan Iskandar, Liping Li, Gaici Li, Dongyue Li, Wenxiong Li, Jinzhong Liu, Yiming Mao, Curtis McCully, Jun Mo, Megan Newsome, Yuan Qi Ni, Andrea Pastorello, Estefania Padilla Gonzalez, Jeniveve Pearson, Haowei Peng, C. L. Ransome, Manisha Shrestha, Nathan Smith, Bhagya Subrayan, G. Terreran, Giorgio Valerin, J. Vinkó, Sergiy S. Vasylyev, Letian Wang, Zhenyu Wang, Hao Wang, J. Craig Wheeler, Kathryn Wynn, Danfeng Xiang, Shengyu Yan, Weimin Yuan, Juan Zhang, WeiKang Zheng, Yu Zhang
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 56 Ni-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 H α 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 56 Ni 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.