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

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.