SN 2023iop: An Extreme 1987A-like Supernova and the Death of Very Massive Stars

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Introduction

The observational landscape of hydrogen-rich core-collapse Type II supernovae (SNe II; [10] (R. M. Chornock 1941)) was fundamentally altered by the discovery of SN 1987A. At the time, stellar-evolution calculations and models of SN II light curves generally favored extended red-supergiant progenitors for H-rich explosions (e.g., [95, 12]). However, SN 1987A was traced to the compact blue supergiant Sanduleak −69°202 [14, 233], demonstrating that core collapse can also occur in a markedly different evolutionary state.

This event defined a distinct subclass of 87A-like SNe, characterized by a long, slow rise to a typically faint photometric peak (at $\sim100$ days) and a broad light-curve morphology that differs markedly from that of more common SNe II ([10, 326, 239, 271]), but spectroscopically they are similar to normal SNe II ([97, 110]). These properties are commonly associated with compact progenitors ($R \lesssim100 R_\odot$), although the observed light-curve evolution also depends on the ejecta mass, explosion energy, and source of luminosity [207, 79].

In the classical interpretation, a substantial fraction of the shock-deposited energy is lost to adiabatic expansion within the dense envelope, causing the early-time luminosity to remain low until energy diffusing from deeper layers becomes visible [207, 326, 239, 238]. While 87A-like SNe remain relatively rare ($\sim1.4 \times10^{-6}\ \mathrm{Mpc}^{-3}\ \mathrm{yr}^{-1}$, corresponding to $\sim1.4\%$ of the total core-collapse SN rate; [271]), $\sim30$ such events have now been identified through systematic surveys [291, 271]. This growing population provides a valuable laboratory for understanding the evolutionary pathways of massive stars, progenitor structure and mixing, and the diverse outcomes of H-rich explosions [326].

Recent years have seen the discovery of extreme transients that share the defining photometric characteristics of the 87A-like subclass while extending its properties to much higher luminosities and longer characteristic timescales. Three well-studied examples of this extreme population are OGLE-2014-SN-073 (hereafter called OGLE-73; [294]), SN 2020faa [323, 250], and SN 2023vbw [130]. OGLE-73 displayed an unusually broad light curve and a high peak luminosity, suggesting a remarkably large ejecta mass and explosion energy [294]. Similarly, SNe 2020faa and 2023vbw exhibited sustained high luminosity that challenged conventional radioactive-decay models, implying that an additional, more potent energy source was at play [323, 250, 130]. Whether the three extreme events represent the high-luminosity tail of the classical 87A-like population or a physically distinct class of explosions remains unclear.

These extraordinary objects have necessitated the exploration of powering mechanisms beyond standard explosion mechanisms and radioactive decay [294, 159, 218, 201, 79, 250]. One candidate is the pair-instability supernova (PISN) mechanism, thought to occur in stars with helium core masses between $\sim65$ and $130\,M_{\odot}$, where the production of electron-positron pairs leads to thermonuclear disruption and ${}^{56}\mathrm{Ni}$ yields of up to several $10\,M_{\odot}$ [100, 17, 241, 159]. Alternatively, a magnetar central engine can inject rotational energy into the ejecta via magnetic dipole radiation [153, 76, 79, 294, 218, 217]. Fallback accretion provides another potential engine, where gravitational potential energy is released as material accretes onto the central compact remnant following a failed or partial shock revival [201, 294]. Finally, “hidden” circumstellar-matter (CSM) interaction might also boost luminosity if the interaction occurs within an asymmetric disk or torus embedded beneath the optically thick photosphere. In this scenario, the CSM interaction zone is within the optically thick photosphere and is not directly observed, masking the characteristic narrow emission lines of Type IIn SNe [275, 210, 250].

It is currently unclear which combinations of progenitor radii, ejecta masses, and explosion energies are required to produce the high luminosities observed in the extreme 87A-like events. Furthermore, distinguishing observationally between PISN, magnetar-powered, fallback-powered, and hidden-CSM-interaction scenarios remains a primary challenge owing to the lack of unique spectral signatures in the photospheric phase and the degeneracy of light-curve models. Resolving these questions is vital for mapping the upper end of the core-collapse mass distribution and the final fates of the most massive stars.

Here, we present SN 2023iop, the most extreme 87A-like event observed to date. It exhibits a light-curve morphology that mirrors the SN 1987A prototype, but on an unprecedented scale. Its light curve is broader and $\sim0.9$ mag more luminous at peak than that of the previous record holder, OGLE-73. When modeled using radioactive decay, the required radioactive ${}^{56}\mathrm{Ni}$ mass exceeds what is achievable in neutrino-driven explosion models, necessitating either a PISN explosion or the presence of an additional energy source (e.g., energy injection from a compact stellar object, and CSM interaction). SN 2023iop therefore provides a useful test of the models that have been proposed for the most luminous 87A-like events.

This paper is structured as follows. We report the discovery of SN 2023iop in § 2 and describe the observations in § 3. In § 4, we derive the photometric, spectroscopic, spectropolarimetric, and host-galaxy properties of SN 2023iop. § 5 constrains the ejecta composition, evaluates different powering mechanisms, compares SN 2023iop with other 87A-like SNe, and outlines a multiwavelength strategy for identifying the power source of such extreme events. We summarize our conclusions in § 6. Throughout the paper, we provide all uncertainties at the $1\sigma$ level. The photometry is reported in the AB system [211]. Phase information is reported in the rest frame with respect to the time of $r$-band maximum brightness at MJD $=$ 60169.63. UTC dates are used throughout.

Discovery

SN 2023iop, located at $\alpha$, $\delta$ (J2000) $=$ $00^{\mathrm{h}}14^{\mathrm{m}}42.699^{\mathrm{s}}$, $+21^\circ51^\prime33.77^{\prime\prime}$, was detected in the transient alert stream of the Zwicky Transient Facility (ZTF; [21, 118]) on 11 May 2023 by C. Fremling [103] (internal name: ZTF23aajucmw). Other surveys also reported detections: the Asteroid Terrestrial-impact Last Alert System (ATLAS; [296]) survey on 29 May 2023 (internal name: ATLAS23lqe) and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) Survey for Transients ([135]) on 23 August 2023 (internal name: PS23hhw). A false-color image of the field, built from our observations with the ESO Very Large Telescope, is shown in Figure 1.

J. Sollerman et al. [277] classified SN 2023iop as a Type II SN at redshift $z = 0.095$ on 26 July 2023 using the SED Machine ([27]), based on a comparison with spectra of SNe II. Late-time spectra that are presented in this paper reveal galaxy emission lines at $z = 0.1020$, which we adopt as the SN redshift (§ 4.1). This converts the peak apparent magnitude of $r = 18.33$ to a K-corrected absolute magnitude of $M_r = -20.14$ (Table 1), placing SN 2023iop in the regime of H-rich superluminous supernovae (SLSNe$^{1}$; [109, 137, 111, 151, 227]).

Observations and data reduction

Our follow-up campaign of SN 2023iop combines optical, near-infrared (NIR), ultraviolet (UV), radio, and X-ray observations obtained between discovery and more than 600 rest-frame days after maximum light. A summary of all observations and their data reduction is provided in Section A of the Appendix. Table 5 in the Appendix summarizes the SN photometry.