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

TL;DR

SN 2024iss is a nearby Type IIb supernova exhibiting a clear double-peaked optical light curve, enabling constraints on the progenitor envelope and circumstellar interaction. By combining optical/UV photometry and spectroscopy with X-ray observations, the authors model the first peak as shock-cooling emission and the second peak via an Arnett-like nickel-decay framework, while treating the X-ray emission as thermal bremsstrahlung from ejecta-CSM interaction. They derive an extended hydrogen envelope with $M_{ m env}\approx0.11\,M_{\\odot}$ and $R_{ m env}\approx244\,R_{\\odot}$, a typical $^{56}$Ni mass of $M_{\rm Ni}\approx0.117\,M_{\\odot}$, and a relatively low ejecta mass $M_{\rm ej}\approx1.27\,M_{\\odot}$ with $E_k\approx0.43\times10^{51}$ erg, while the X-ray data imply a mass-loss rate of $\dot{M}\approx1.6\times10^{-5}\,M_{\odot}\,{ m yr^{-1}}$ and a compact CSM extent of $R\lesssim1.3\times10^{14}$ cm, suggesting eruptive mass loss within ~4 years of explosion. The results place SN 2024iss between the extended eIIb and compact cIIb subclasses, support a binary-interaction envelope-stripping scenario, and demonstrate the value of prompt X-ray observations for probing SESN environments and progenitor mass-loss histories.

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 $\sim19,400\,km\,s^{-1}$ at 3.82days from the H$α$ 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 $0.11\pm0.04\,M_{\odot}$ and a radius of $244\pm43~R_{\odot}$. Fitting the second light-curve peak with an Arnett-like model indicates a typical $^{56}$Ni mass of $ 0.117\pm0.013~M_{\odot}$ and a relatively low ejecta mass of $1.272\pm0.343\,M_{\odot}$. X-ray observations reveal bright thermal bremsstrahlung emission and indicate a mass-loss rate of $1.6\times10^{-5}\ M_{\odot} \ \rm{yr}^{-1}$. 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 $\lesssim1.3\times10^{14}$ cm, indicating that the progenitor star experienced eruptive mass loss within $\sim4\,yr$ of its terminal explosion.

Figures (14)

• Figure 1: Optical light curves of SN 2024iss. Left panel: the early-time multiband photometry within the first ten days. Right panel: the multiband light curves of SN 2024iss within the first $\sim$100 days. All magnitudes were corrected for the Galactic extinction. For better display, light curves in different bandpasses were shifted vertically by arbitrary numbers as labeled on the right. Facilities used to obtain the photometry are indicated by the legend. The last non-detection from ASAS-SN is marked by the green inverted triangle.
• Figure 2: The $V$-band light curve of SN 2024iss compared to those of well-sampled cases, i.e., the Type IIb SNe 1993J, 2011dh, 2013df, 2016gkg and 2020acat. The light curves of the comparison SNe have been shifted in both magnitude and time to align with the peak magnitude and the time of the $V$-band light curve peak of SN 2024iss. The black dashed line represents the expected decline rate of [56]Co Woosley1989. The upper-right inset shows a zoom-in of the first 30 days.
• Figure 3: Galactic reddening-corrected $U-B$, $B-V$, and $g-r$ color curves of SN 2024iss, compared with Galactic reddening-corrected color curves of SNe 1993J, 2011fu, 2011dh, 2013df, and 2016gkg. For the purpose of display, the $V-R$ color curve of SNe 1993J, 2011fu, 2013df and 2016gkg were converted to $g-r$ using the transformation from Jordi2006.
• Figure 4: Top: Pseudo-bolometric light curve of SN 2024iss. The purple line shows the best-fit Arnett model to the secondary peak. Bottom: Evolution of the blackbody temperature and the photospheric radius of SN 2024iss, as indicated by the left- and right-hand ordinates, respectively.
• Figure 5: Spectral sequence of SN 2024iss, spanning the first 7 days after the explosion. Phases are marked on the right. Different colors distinguish the different spectrographs used in the observations, as shown at the top. A log of the spectroscopy of SN 2024iss is given in Table \ref{['table:Log of spectra']}.