SN 2024iss: A Multiwavelength Exposé of a Type IIb Supernova with an Early-time Ultraviolet Spectrum and Shock-breakout Constraints

We present multiwavelength observations and analysis of SN 2024iss, a Type IIb supernova (SN IIb) located at ∼14 Mpc. The first Zwicky Transient Facility detection of SN 2024iss is ∼40 minutes after first light (one of the earliest detections of a SN IIb to date) and a Hubble Space Telescope (HST) UV spectrum was obtained at 7 days (the earliest HST UV SN IIb spectrum to date). We estimate an ejecta mass range of 1.1–3.3 M ⊙ and a 56 Ni mass of 0.11 ± 0.01 M ⊙ from bolometric light-curve modeling and He-star model comparisons. We fit shock-cooling emission models to estimate a progenitor radius of 100–320 R ⊙ and an envelope mass of 0.07–0.46 M ⊙ . We compare optical/UV spectra to binary progenitor models, which indicate a stripped H-rich envelope mass of 0.19–0.28 M ⊙ . We use early-time X-ray detections to calculate circumstellar material (CSM) densities that are consistent with a progenitor mass-loss rate of 5 × 10 −4 M ⊙ yr −1 ( v w = 100 km s −1 ), corresponding to 2–5 yr preexplosion. In the UV, we observe strong Mg ii emission extending to 15,000 km s −1 as well as weak P Cygni profiles of iron-group elements (e.g., Fe, Ti, Al, Ni) present in outer SN ejecta. The overall spectroscopic evolution of SN 2024iss is comparable to that of SNe IIb, but its increased brightness at peak is influenced by SN ejecta–CSM interaction. Nebular spectroscopy of SN 2024iss at ∼260–412 days reveals multipeaked forbidden-line profiles of O i and Mg i ] indicative of inner ejecta asymmetry and/or clumping. We demonstrate the utility of a rich, multiwavelength data set for constraining the progenitor systems and explosion dynamics of SNe IIb.