The nearby He-rich superluminous supernova SN 2021bnw during photospheric phases

Technical Summary: The nearby He-rich superluminous supernova SN 2021bnw during photospheric phases

Problem and Context
Superluminous supernovae (SLSNe) challenge the classical neutrino-driven core-collapse paradigm, as synthesizing the required mass of $^{56}$Ni to power their extreme luminosities is difficult. While viable explanations include the spin-down radiation of a newborn magnetar and the interaction of ejecta with circumstellar material (CSM), the dominant power source for SLSNe I remains elusive. Specifically, the nature of helium-rich SLSNe (SLSNe Ib) is poorly constrained due to a small sample size. It is unclear whether He-line detections correlate with specific light curve (LC) features, such as rebrightenings, or with CSM interaction. SN 2021bnw, a hydrogen-deficient but helium-rich SLSN Ib, presents an opportunity to expand this sample and constrain progenitor masses and powering mechanisms.

Methodology
The authors present an extensive spectro-photometric follow-up campaign of SN 2021bnw, discovered by the Zwicky Transient Facility (ZTF) and classified by ePESSTO+.

• Observations: Optical-to-near-infrared (NIR) photometry was obtained using multiple facilities (ZTF, ATLAS, LCO, NOTCam, SOFI, etc.), and optical/NIR spectra were collected from pre-maximum to late phases (up to 97 days post-maximum).
• Data Reduction: Imaging data were reduced using dedicated pipelines or ecsnoopy. Spectra were reduced with standard IRAF routines or dedicated pipelines.
• Analysis Techniques:
  • Light Curves: The epoch of maximum luminosity was determined via polynomial fitting. Pseudo-bolometric light curves were constructed by integrating spectral energy distributions (SEDs) derived from multicolor photometry.
  • Spectral Identification: The NIR spectrum (87 days post-maximum) was analyzed to identify helium features. The authors employed the Monte Carlo radiative-transfer code TARDIS to generate synthetic spectra, testing line identifications (specifically He I $\lambda$1.083 $\mu$m and $\lambda$2.058 $\mu$m) against the observed data.
  • Physical Modeling: To constrain explosion parameters, the authors utilized the STELLA code, which couples hydrodynamics and radiation transport. They tested a $^{56}$Ni + CSM interaction scenario, comparing model outputs against the observed bolometric light curve.

Key Results

• Photometry: SN 2021bnw reached an absolute peak magnitude of $M_g = -20.70 \pm 0.10$ mag. The light curve exhibited a wedge-shaped rise and decline, followed by a linear decline phase lasting 80 days. Notably, two deviations (rebrightenings) were detected in the optical bands: a first bump peaking at ~35 days and a second around 70–80 days. The pseudo-bolometric decline rate (0.016 mag d$^{-1}$) was faster than the $^{56}$Co decay slope assuming full $\gamma$-ray trapping.
• Spectroscopy: Early spectra showed hot temperatures (~14,000–17,000 K) with O II and Ca II features. The NIR spectrum taken 87 days after maximum revealed distinct absorption features. TARDIS modeling confirmed the presence of helium, estimating a helium mass of $\sim$0.10–0.15 $M_\odot$. The identification of He I $\lambda$1.083 $\mu$m and $\lambda$2.058 $\mu$m was supported by the synthetic spectra, despite the blending of features due to high expansion velocities.
• Physical Parameters: The STELLA modeling of a $^{56}$Ni + CSM scenario converged on an ejecta mass of $\approx$15–20 $M_\odot$, a CSM mass of $\approx$7 $M_\odot$, an explosion energy of $\approx$5 $\times$ 10$^{51}$ erg, and a $^{56}$Ni mass of $\approx$1.7 $M_\odot$.
• Comparisons: SN 2021bnw shares remarkable similarities in light curve shape and spectral evolution with SN 2017egm. However, it deviates from the standard $t_{\text{decline}}$ vs. $t_{\text{rise}}$ correlation observed in other SLSNe, a behavior the authors attribute to CSM interaction.

Conclusions and Significance
The paper concludes that SN 2021bnw was likely powered by a combination of a central energy source (potentially $^{56}$Ni decay, though a magnetar is not ruled out) and the interaction of ejecta with a previously lost, helium-rich circumstellar material. The detection of helium lines in the NIR, coupled with the specific light curve morphology (rebrightenings), supports the hypothesis that CSM interaction plays a significant role in shaping the evolution of SLSNe Ib.

The significance of this work lies in its contribution to the expanding sample of SLSNe Ib. By providing detailed constraints on the progenitor mass and explosion physics for a single-progenitor scenario, the study helps narrow down the nature of SLSN I(b) progenitors. The authors note that while their derived parameters (high $^{56}$Ni mass and kinetic energy) suggest a departure from ordinary core collapse—potentially pointing toward collapsar or magneto-rotational scenarios—these estimates require further confirmation through thorough stellar evolution studies. The paper emphasizes that the absence of clear spectroscopic signatures of CSM interaction in the optical spectra may indicate asymmetry in the ejecta-CSM configuration, a feature naturally arising in rotationally driven explosion channels.