Photometric observations of SN 2024iss

Modeling the first light-curve peak with the shock-cooling model suggests an extended hydrogen envelope with a mass of 0.11+/-0.04M_{sun}_ and a radius of 244+/-43R_{sun}_. Fitting the second light-curve peak with an Arnett-like model indicates a typical 56 Ni mass of 0.117+/-0.013M_{sun}_ and a relatively low ejecta mass of 1.272+/-0.343M_{sun}_. X-ray observations reveal bright thermal bremsstrahlung emission and indicate a mass-loss rate of 1.6x10^-5^M_{sun}_/yr, which is similar to that of SN 1993J.

SN 2024iss occupies a transitional position between the two subclasses of extended (eIIb) and compact (cIIb) Type IIb SNe. Its envelope radius and pre-explosion mass-loss rate appear to be correlated, in agreement with theoretical predictions. The observational properties of SN 2024iss are compatible with a binary interaction scenario being the dominant mechanism for envelope stripping.

Furthermore, the low column density of neutral hydrogen suggests a compact CSM with an outer radius of <~1.3x10^14^cm, indicating that the progenitor star experienced eruptive mass loss within ~4yr of its terminal explosion.