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SN 2024iss: A Multi-Wavelength Exposé of a Type IIb Supernova with an Early-Time Ultraviolet Spectrum and Shock Breakout Constraints
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SN 2024iss: A Multi-Wavelength Exposé of a Type IIb Supernova with an Early-Time Ultraviolet Spectrum and Shock Breakout Constraints

Rujula Yete, Wynn Jacobson-Galan, Ferdinand Ferdinand, Luc Dessart, Mansi Kasliwal, Kyle Davis, Anthony Piro, Victoria Villar, Igor Andreoni, Katie Auchettl, …
ArXiv.org
30 Mar 2026
DOI: https://doi.org/10.48550/arxiv.2603.20375
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https://doi.org/10.48550/arxiv.2603.20375View
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Abstract

Physics - High Energy Astrophysical Phenomena Physics - Solar and Stellar Astrophysics
We present multi-wavelength observations and a comprehensive analysis of the nearby (D ∼ 14 Mpc) Type IIb supernova (SN IIb) 2024iss. Observations of SN2024iss include an early ZTF detection at∼ 40 minutes after first light and the earliest Hubble Space Telescope UV spectrum for a SN IIb to date at 7 days after first light. With the bolometric light curve and He-star models, we estimate an ejecta mass range of∼ 1.1-3.3 M_(⊙)and a⁵⁶\textrm{Ni}{}{m}ass of0.11 ± 0.01 M_(⊙) . We fit shock-cooling emission models to the first peak in the light curve and estimate a progenitor radius of100-320 R_(⊙)and a H-rich envelope mass of0.07-0.46 M_(⊙) . We also compared optical/UV spectra to binary progenitor model spectra, which indicate a stripped H-rich envelope mass of0.19-0.28 M_(⊙) . We use early-time X-ray detections to calculate CSM densities that are consistent with a progenitor mass-loss rate of5×10⁻⁴ M_(⊙)( v_(w) = 100 km/s), corresponding to a period of significant mass ejection in the final 2-5 years before core collapse. In the UV spectrum, we observe strong Mg II emission extending to∼15,000 km/s as well as weak P-Cygni profiles of iron-group elements (e.g., Fe, Ti, Al, Ni) present in the outer SN ejecta during the end of shock cooling phase. We find that the overall spectroscopic evolution of SN2024iss is comparable to other SNe IIb, but that the increased brightness following the initial light curve peak is likely influenced by SN ejecta-CSM interaction. Finally, optical/NIR nebular spectroscopy of SN2024iss at∼ 260-412 days reveals multi-peaked forbidden line profiles of O I and Mg I] indicative of inner ejecta asymmetry and/or clumping. We demonstrate the utility of a rich, multi-wavelength dataset for constraining the progenitor systems and explosion dynamics of SNe IIb.

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