Yet modern transient surveys have begun to uncover a handful of nearby PISN candidates, and SN 2018ibb stands out as one of the best identified to date. This Type Ic supernova exhibited a rise time exceeding 100 days and a peak luminosity around 10^44 erg s^-1, both consistent with theoretical PISN light-curve predictions. Its spectroscopic properties also matched expectations, including the strong [O III] emission characteristic of pair-instability explosions. However, late-phase spectra revealed an unexpected blue flux excess below approximately 5000 Å, a feature not predicted in standard PISN models. This excess was suggested to originate from the interaction between the supernova ejecta and dense circumstellar matter (CSM) surrounding the progenitor, implying an additional luminosity source beyond the radioactive decay of nickel-56.

To investigate this puzzle, a team led by Takashi J. Moriya of the National Astronomical Observatory of Japan developed synthetic light-curve models of PISNe interacting with dense CSM. The researchers adopted a PISN progenitor model computed with the KEPLER hydrodynamical stellar evolution code, using a bare helium star whose explosion energy and nickel-56 mass were determined self-consistently rather than treated as free parameters. The subsequent light curves were computed with the STELLA multi-frequency radiation hydrodynamics code, which solves hydrodynamic equations coupled with radiative transfer using the variable Eddington factor method across 100 wavelength bins. Based on the estimated metallicity of the host galaxy, they identified a model whose bolometric light curve matched the overall behavior of SN 2018ibb around peak luminosity, with properties consistent with previous independent estimates.

The team attached a wind-like CSM density structure on top of the ejecta, characterized by a mass-loss rate and CSM velocity. They assumed a CSM velocity of 1,000 km s^-1, corresponding to typical terminal wind speeds of Wolf-Rayet stars, and varied the mass-loss rate to adjust the CSM density. Within the investigated range, the CSM interaction had little effect on the light curve near peak. However, after approximately 150 days since explosion, the synthetic light curves with CSM began to deviate from those without, with increasing CSM density producing higher luminosity at later times.

The critical breakthrough came from matching the observed luminosity evolution of SN 2018ibb from 150 days after peak. The researchers found that a mass-loss rate of 0.01 (v_CSM/1,000 km s^-1) M_sun yr^-1 reproduced the bolometric luminosity well in the 300-day window following that epoch. More strikingly, SN 2018ibb showed a distinct luminosity break at around 300 days from peak. By varying the CSM radius while keeping the mass-loss rate fixed, the team demonstrated that this break is determined by the CSM radius, and the observed break could be reproduced by setting the dense CSM radius at 8.5×10^16 cm.

This radius carries profound implications for the progenitor’s final moments. Assuming a wind velocity of 1,000 km s^-1, the mass-loss rate must have increased to the inferred value for a period of 28 (v_CSM/1,000 km s^-1)^-1 years before explosion to place the CSM at the required distance. After the luminosity break, the synthetic light curves returned to the CSM-free prediction, indicating that the ejecta had swept past the dense material. However, the observed bolometric light curve of SN 2018ibb showed a flattening after the break, suggesting the supernova began interacting with less dense CSM beyond that boundary.

The team validated their numerical results using an analytic model of interaction-powered supernova luminosity, approximating the ejecta density structure with two power-law components. They found that the combined luminosity of the CSM-free PISN and the analytic interaction contribution reproduced the overall evolution until the break, with a kinetic-to-radiative conversion efficiency that matched the numerical model well. Notably, the CSM interaction luminosity contributed negligibly around peak, meaning PISN properties and CSM properties can be independently constrained from different phases of the light curve. The photospheric velocity evolution was also largely unaffected by CSM interaction, with the model’s g-band velocities roughly consistent with Fe II absorption-line measurements from SN 2018ibb.

The inferred mass-loss rate is far higher than expected from hot stellar winds of hydrogen-free stars, and if sustained over long timescales, the progenitor would have been stripped below the mass threshold required for a PISN explosion. The enhancement must therefore have occurred only in the final short period before detonation, consistent with the light-curve-derived timescale. Intriguingly, the progenitor model was found to trigger carbon shell burning after core contraction approximately 30 years before explosion, matching the estimated enhancement timescale. The core contraction with shell burning initiates progenitor expansion, which may drive the mass-loss enhancement. Alternatively, as the star approaches the dynamically unstable configuration triggered by pair creation, it may also enter a state conducive to enhanced mass loss.

Other mechanisms remain possible. The CSM could have formed through eruptive mass loss or binary mass transfer rather than a steady wind. Another intriguing possibility is that the mass-loss rate was not actually enhanced, but material lost from the progenitor accumulated near the stellar surface due to strong magnetic fields confining the stellar wind. Such magnetically confined material could remain close to the star and affect the PISN light curve in the manner observed. Further investigation is needed to determine whether magnetically confined CSM can extend to the inferred radius.

As PISN candidates accumulate, signatures of unexpected CSM interaction are emerging repeatedly. SN 2023vbw, another recent PISN candidate, also shows CSM interaction features. Because PISN progenitors require low overall mass loss to sustain their enormous masses, they were not expected to be surrounded by dense CSM. However, a significant fraction of supernovae from massive stars are now recognized to experience mass-loss enhancement within decades before explosion. PISN progenitors may undergo similar short-term enhanced mass loss, affecting their observational properties. Most predicted PISN characteristics to date ignore dense CSM, but models likely require updating. The presence of dense CSM is expected to make PISNe bluer than previously predicted, which could influence expected discovery rates at high redshifts and reshape our understanding of how the most massive stars die across cosmic time.

Subject of Research: Properties of the circumstellar matter around the pair-instability supernova candidate SN 2018ibb revealed by its light curve

Article Title: Properties of the circumstellar matter around the pair-instability supernova candidate SN 2018ibb revealed by its light curve

Article References: Moriya, T. J., Schulze, S., Heger, A., Blinnikov, S. I., & Potashov, M. S. (2026). Properties of the circumstellar matter around the pair-instability supernova candidate SN 2018ibb revealed by its light curve. Frontiers in Astronomy and Space Sciences, 13, Article 1944397. https://doi.org/10.3389/fspas.2026.1944397

Image Credits: AI Generated

DOI: 10.3389/fspas.2026.1944397

Keywords: pair-instability supernovae, SN 2018ibb, circumstellar matter, mass loss, massive stars, light curves, supernova ejecta interaction, stellar evolution, Properties, circumstellar, matter, around

The concept of a pair-instability gap arises from the physics governing massive stars approaching the end of their life cycles. When such stars become sufficiently hot and dense, they can produce electron-positron pairs from energetic photons, a process that softens the pressure within the star and triggers catastrophic instabilities. This pair formation destabilizes the star’s core, potentially leading to pair-instability supernovae—a kind of explosive event so violent that it obliterates the star entirely, leaving no remnant black hole behind. This mechanism predicts a mass gap in the remnant black holes formed from these stars, with masses unable to lie within a specific range roughly spanning 50 to 130 solar masses.

Historically, gravitational-wave astronomy has struggled to provide direct evidence for this hypothesized mass gap. Early data suggested a sharp cut-off in black-hole masses around 45 solar masses, but subsequent detections of more massive binary black-hole mergers complicated this picture. Instead of a clear void, observations appeared to support a continuum of masses, raising questions about either the theoretical models or the observational completeness. The latest analysis overturns this ambiguity by distinguishing characteristics not in the primary black holes of binaries but rather in their secondary components.

By analyzing over fifty black-hole merger events cataloged in GWTC-4, the team detected a conspicuous dearth of black holes with masses within this forbidden range when considering the secondary black hole in a merging binary system. Whereas the distribution of primary masses—defined as the more massive component of the binary—did not show a traditional gap, the secondary mass distribution unmistakably exhibited a drop consistent with the predicted pair-instability gap. Specifically, the lower boundary of the gap was pinned at approximately 44 solar masses, with credible uncertainty bounds, a figure consistent with theoretical expectations and nuclear astrophysics constraints.

Intriguingly, the study also reveals a connection between the pair-instability gap and the spin distribution of black holes in binaries. Black-hole binaries with primary masses within the gap tend to exhibit higher spin rates than those below the gap. This correlation hints at the presence of hierarchical mergers—systems where the primary component itself is the product of an earlier black-hole merger. Such mergers would naturally possess higher spins, as angular momentum is conserved and often increased through the merger process, thus populating the mass gap with objects formed through evolutionary channels different from direct stellar collapse.

These findings not only validate long-predicted theoretical work dating back to the 1960s and early 2000s but also open new avenues to probe stellar nucleosynthesis, specifically the role of nuclear reactions within massive stars. By precisely constraining the location of the pair-instability gap, the team places novel limits on the S-factor of the crucial nuclear reaction 12C(α, γ)16O at energies around 300 keV. The S-factor represents the astrophysical cross-section for this reaction and influences the internal composition and evolution of massive stars, impacting their ultimate fates and the masses of resulting black holes.

The implications of these results extend beyond black-hole astrophysics into the broader field of gravitational-wave science and stellar evolution theory. The confirmation of the pair-instability gap acts as a natural boundary condition, refining models of massive star interiors, supernova mechanisms, and remnant formation scenarios. Moreover, the existence of a subpopulation of hierarchical binary black holes highlights the complexity of black-hole demographics, suggesting that many detections might trace their origins back to dense stellar environments such as globular clusters or galactic nuclei, where repeated mergers can occur.

Beyond the astrophysical insights, these discoveries showcase the power of gravitational-wave observatories as tools for probing fundamental physics. The detection of specific mass gaps and correlations with black-hole spin offers an unprecedented experimental window to phenomena once solely accessible through theoretical frameworks or electromagnetic observations. Such capability underscores the vital role of multi-messenger astronomy in unraveling the cosmic dance of massive objects and the extreme physical processes governing their birth, evolution, and demise.

The study also contributes to the ongoing discourse about the evolution of heavy elements in the universe. The nuclear reaction of carbon and alpha particles leading to oxygen synthesis is integral to the chemical enrichment process within galaxies. By constraining the reaction’s parameters, astrophysicists refine models of the element formation chain in stars, which in turn influences interpretations of stellar populations and the formation histories of galaxies, including our own Milky Way.

In addition to these scientific milestones, the methodology employed in this research exemplifies advances in data analysis techniques and statistical modeling within gravitational-wave astronomy. Sophisticated frameworks allowed the disentanglement of overlapping signals and noise, extraction of subtle features in mass and spin distributions, and the robust quantification of uncertainties. These tools will undoubtedly enhance future explorations, especially as gravitational-wave detectors improve sensitivity and catalog sizes grow.

Looking forward, the presence of a clear pair-instability gap invites targeted searches for binary systems bridging the gap’s boundaries and for evidence of multiple-generation mergers in diverse environments. It also spurs interest in refining the nuclear physics inputs to stellar models, potentially motivating laboratory experiments to better determine reaction rates critical for stellar evolution. These efforts, in concert with enhanced gravitational-wave detections, promise to deepen our grasp on the life cycles of the universe’s most massive and enigmatic objects.

In summary, this breakthrough marks a significant stride in confirming a fundamental prediction of stellar astrophysics, revealing a mass gap aligned with pair-instability supernova theory in the secondary black-hole components observed via gravitational waves. The convergence of observational data and theoretical expectation enriches our understanding of black-hole formation, the complexity of merger populations, and underlying nuclear processes shaping the life and death of massive stars. This development solidifies gravitational-wave astronomy as a critical frontier for exploring the cosmos and decoding the mysteries encoded in the masses and spins of black holes.

Subject of Research:
Black-hole mass distribution and pair-instability supernovae in gravitational-wave observations.

Article Title:
Evidence of the pair-instability gap from black-hole masses.

Article References:
Tong, H., Fishbach, M., Thrane, E. et al. Evidence of the pair-instability gap from black-hole masses. Nature (2026). https://doi.org/10.1038/s41586-026-10359-0

Image Credits:
AI Generated

DOI:
https://doi.org/10.1038/s41586-026-10359-0