Mapping the Dense Circumstellar Environments of SNe Ibn, SNe Icn, and Fast Blue Optical Transients

Technical Summary: Mapping the Dense Circumstellar Environments of SNe Ibn, SNe Icn, and Fast Blue Optical Transients

Problem Statement
Large-scale time-domain surveys have revealed a growing population of rapidly evolving, blue, and luminous optical transients. While Fast Blue Optical Transients (FBOTs) are observationally defined by these traits, their physical origins remain diverse, with proposed power sources ranging from dense circumstellar material (CSM) interaction to central engines. Similarly, stripped-envelope supernovae SNe Ibn (He-rich, H-poor CSM) and SNe Icn (C/O-rich, H/He-poor CSM) are linked to CSM interaction but are often studied in isolation. Previous comparisons between these classes have been hindered by heterogeneous samples, class-specific analyses, and inconsistent light-curve models. Consequently, it remains unclear whether the apparent similarities among SNe Ibn, SNe Icn, and FBOTs reflect a genuine physical connection within a common interaction framework or merely a phenomenological overlap in the "fast, blue, and luminous" region of parameter space.

Methodology
To address this, the authors compiled a uniform sample of 25 events (10 SNe Ibn, 5 SNe Icn, and 10 FBOTs) from the literature, ensuring multi-band optical coverage and sufficient pre-maximum constraints. The study employed a two-pronged approach:

1. Empirical Light-Curve Measurements: The authors utilized Gaussian-process (GP) reconstructions to measure observables (peak luminosity, rise timescale, and decline proxies) on a common observational basis, specifically using the observed $g$-band as a reference. This allowed for a model-independent comparison of peak colors and light-curve evolution across the three classes.
2. Unified Physical Modeling: The authors applied the TransFit-CSM framework, a time-dependent radiative-diffusion model coupling thin-shell ejecta–CSM interaction dynamics with an optically thick CSM shell. The model was modified to include a unified heating prescription comprising:
   • Shock heating from the forward shock, treated as a localized volumetric source.
   • An effective inner heating component (modeled as $^{56}$Ni radioactive decay) to account for potential non-thermal or central-engine contributions.
   • A power-law CSM density profile ($\rho \propto r^{-2}$) and homologously expanding ejecta.

The model was fitted to the multiband light curves of all 25 events using identical priors for physical parameters (CSM mass $M_{\text{CSM}}$, CSM radius $R_{\text{CSM}}$, ejecta mass $M_{\text{ej}}$, explosion energy $E_{\text{SN}}$, opacity $\kappa$, shock efficiency $\epsilon_{\text{sh}}$, and effective $^{56}$Ni mass $M_{\text{Ni}}$).

Key Results

• Empirical Overlap: In the observed $g$-band peak-luminosity versus rise-time and decline-versus-rise-time planes, the three classes are not cleanly separated. FBOTs preferentially occupy the luminous, rapidly evolving end of the distribution but show limited overlap with the Ibn/Icn population. Peak colors ($g-r$) span a broadly overlapping blue region, with FBOTs extending to slightly bluer values.
• Parameter Space Continuity: When mapped into the unified TransFit-CSM parameter space, the distributions of fitted physical parameters show substantial overlap.
  • In the $R_{\text{CSM}}$–$M_{\text{CSM}}$ and $M_{\text{ej}}$–$M_{\text{CSM}}$ planes, FBOTs do not form a distinct group but are embedded within the broader distribution of SNe Ibn and Icn.
  • The primary distinction for FBOTs in this framework is a tendency toward higher characteristic ejecta velocities ($v_{\text{ej,mean}}$) and shorter diffusion timescales, rather than a fundamentally different CSM mass or radial scale.
• Modeling Success: The unified shock + effective inner heating model successfully reproduces the main optical light-curve morphologies (peak structure, band ordering, and post-peak decline) for representative events from all three classes.

Significance and Claims
The paper argues that the optical light curves of SNe Ibn, SNe Icn, and at least a subset of FBOTs can be compared within a common dense-CSM interaction framework. The authors conclude that:

1. Physical Continuity: The empirical diversity of these events can be arranged along a continuous range of optical light-curve timescales and luminosities, supported by overlapping fitted CSM and ejecta parameters.
2. Progenitor Implications: The sequence from Ibn to Icn likely represents a difference in the chemical composition and stripping depth of the CSM (He-rich vs. C/O-rich) rather than a distinct CSM mass scale. FBOTs appear to extend this interaction parameter space toward shorter diffusion times and larger specific energy input (higher ejecta velocities).
3. Limitations and Nuance: The authors explicitly state that a successful optical fit does not prove CSM interaction is the sole power source. The "effective" $^{56}$Ni mass in the model likely absorbs unmodeled inner power (e.g., central engines, fallback accretion) for the most extreme FBOTs (e.g., AT2018cow-like events). Thus, while dense CSM interaction accounts for the thermal optical emission in a subset of FBOTs, the most extreme events likely require additional non-thermal or engine-driven components.

The study provides a "common language" for comparing these transients, suggesting that while FBOTs are observationally distinct due to their extreme evolution rates, they share a physical foundation of compact, dense CSM interaction with stripped-envelope supernovae.