SN 2024iss: A Double-peaked Type IIb Supernova with Evidence of Circumstellar Interaction

Abstract

We present optical, ultraviolet, and X-ray observations of supernova (SN) 2024iss, a Type IIb SN that shows a prominent double-peaked light curve. We modeled the first peak with a semianalytical shock-cooling model and the X-ray emission with a free-free model. We compare the envelope radius and mass-loss rate with other Type IIb SNe to explore the relationships between the progenitor envelope and the circumstellar material (CSM). The shock-cooling peak in the $V$-band light curve reached $M_V=-17.33\pm 0.26$mag, while the ${}^{56}$Ni-powered second peak attained $M_V=-17.43\pm 0.26$mag. Early spectra show an photospheric velocity of $\sim 19,400km{s}^{-1}$ at 3.82days from the H$\hat{I}\pm$ P~Cygni profile. The Balmer lines persist at least +87 days after the explosion, characterizing hydrogen-rich ejecta. Modeling the first light-curve peak suggests an extended envelope with a mass of $0.11\pm 0.04M_{\odot }$ and a radius of $244\pm 43R_{\odot }$. Fitting the second light-curve peak with an Arnett-like model indicates a typical ${}^{56}$Ni mass of $0.117\pm 0.013M_{\odot }$ and a relatively low ejecta mass of $1.272\pm 0.343M_{\odot }$. X-ray observations reveal bright thermal bremsstrahlung emission and indicate a mass-loss rate of $1.6\times 10^{-5}{M}_{\odot }{\mathrm{yr}}^{-1}$. 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 as theoretically predicted. 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 $\lesssim 1.3\times 10^{14}$ cm, indicating that the progenitor star experienced eruptive mass loss within $\sim 4yr$ of its terminal explosion.

24 pages, 14 figures, submitted to A&A