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SN 2020zbf is a hydrogen-poor superluminous supernova (SLSN) atz = 0.1947 that shows conspicuous C IIfeatures at early times, in contrast to the majority of H-poor SLSNe. Its peak magnitude isM_g = −21.2 mag and its rise time (≲26.4 days from first light) places SN 2020zbf among the fastest rising type I SLSNe. We used spectra taken from ultraviolet (UV) to near-infrared wavelengths to identify spectral features. We paid particular attention to the C IIlines as they present distinctive characteristics when compared to other events. We also analyzed UV and optical photometric data and modeled the light curves considering three different powering mechanisms: radioactive decay of⁵⁶Ni, magnetar spin-down, and circumstellar medium (CSM) interaction. The spectra of SN 2020zbf match the model spectra of a C-rich low-mass magnetar-powered supernova model well. This is consistent with our light curve modeling, which supports a magnetar-powered event with an ejecta massM_ej = 1.5 M_⊙. However, we cannot discard the CSM-interaction model as it may also reproduce the observed features. The interaction with H-poor, carbon-oxygen CSM near peak light could explain the presence of C IIemission lines. A short plateau in the light curve around 35–45 days after peak, in combination with the presence of an emission line at 6580 Å, can also be interpreted as being due to a late interaction with an extended H-rich CSM. Both the magnetar and CSM-interaction models of SN 2020zbf indicate that the progenitor mass at the time of explosion is between 2 and 5M_⊙. Modeling the spectral energy distribution of the host galaxy reveals a host mass of 10^8.7M_⊙, a star formation rate of 0.24_−0.12^+0.41M_⊙yr⁻¹, and a metallicity of ∼0.4Z_⊙. 

Abstract We present the results from a multiyear radio campaign of the superluminous supernova (SLSN) SN 2017ens, which yielded the earliest radio detection of an SLSN to date at the age of ∼3.3 yr after explosion. SN 2017ens was not detected at radio frequencies in the first ∼300 days but reachedL_ν≈ 10²⁸erg s⁻¹cm⁻²Hz⁻¹atν∼ 6 GHz, ∼1250 days post explosion. Interpreting the radio observations in the context of synchrotron radiation from the supernova shock interaction with the circumstellar medium (CSM), we infer an effective mass-loss rate $\dot{M}\approx {10}^{-4}{M}_{☉}{\mathrm{yr}}^{-1}$atr∼ 10¹⁷cm from the explosion’s site, for a wind speed ofv_w= 50–60 km s⁻¹as measured from optical spectra. These findings are consistent with the spectroscopic metamorphosis of SN 2017ens from hydrogen poor to hydrogen rich ∼190 days after explosion reported by Chen et al. SN 2017ens is thus an addition to the sample of hydrogen-poor massive progenitors that explode shortly after having lost their hydrogen envelope. The inferred circumstellar densities, implying a CSM mass up to ∼0.5M_☉, and low velocity of the ejection suggest that binary interactions (in the form of common-envelope evolution and subsequent envelope ejection) play a role in shaping the evolution of the stellar progenitors of SLSNe in the ≲500 yr preceding core collapse. 

We present photometric and spectroscopic observations of SN 2020xga and SN 2022xgc, two hydrogen-poor superluminous supernovae (SLSNe-I) atz = 0.4296 andz = 0.3103, respectively, which show an additional set of broad Mg IIabsorption lines, blueshifted by a few thousands kilometer second⁻¹with respect to the host galaxy absorption system. Previous work interpreted this as due to resonance line scattering of the SLSN continuum by rapidly expanding circumstellar material (CSM) expelled shortly before the explosion. The peak rest-frameg-band magnitude of SN 2020xga is −22.30 ± 0.04 mag and of SN 2022xgc is −21.97 ± 0.05 mag, placing them among the brightest SLSNe-I. We used high-quality spectra from ultraviolet to near-infrared wavelengths to model the Mg IIline profiles and infer the properties of the CSM shells. We find that the CSM shell of SN 2020xga resides at ∼1.3 × 10¹⁶cm, moving with a maximum velocity of 4275 km s⁻¹, and the shell of SN 2022xgc is located at ∼0.8 × 10¹⁶cm, reaching up to 4400 km s⁻¹. These shells were expelled ∼11 and ∼5 months before the explosions of SN 2020xga and SN 2022xgc, respectively, possibly as a result of luminous-blue-variable-like eruptions or pulsational pair instability (PPI) mass loss. We also analyzed optical photometric data and modeled the light curves, considering powering from the magnetar spin-down mechanism. The results support very energetic magnetars, approaching the mass-shedding limit, powering these SNe with ejecta masses of ∼7 − 9 M_⊙. The ejecta masses inferred from the magnetar modeling are not consistent with the PPI scenario pointing toward stars > 50 M_⊙He-core; hence, alternative scenarios such as fallback accretion and CSM interaction are discussed. Modeling the spectral energy distribution of the host galaxy of SN 2020xga reveals a host mass of 10^7.8M_⊙, a star formation rate of 0.96_−0.26^+0.47M_⊙yr⁻¹, and a metallicity of ∼0.2 Z_⊙.