SN 2023xgo: Helium-rich type Icn or carbon-flash type Ibn supernova?

Abstract

We present observations of SN 2023xgo, a transitional Type Ibn/Icn SN, from −5.6 to 63 d relative to r-band peak. Early spectra show C III λ5696 emission like Type Icn SNe, shifting to Type Ibn features. The He I velocities (1800–10 000 km s−1) and pseudo-equivalent widths are among the highest in the Ibn/Icn class. The light curve declines at 0.14 mag d−1 until 30 d, matching SNe Ibn/Icn but slower than fast transients. SN 2023xgo is the faintest in our SN Ibn sample (Mr = −17.65 ± 0.04) but shows typical colour and host properties. Semi-analytical modelling of the light curve suggests a compact CSM shell (∼ 1012–1013 cm), mass-lossrate between 10−4 and 10−3 M yr−1 with CSM and ejecta masses of ∼0.22 and 0.12 M, respectively. Post-maximum light-curve, spectral modelling favours a ∼3 M helium star progenitor with extended (∼ 1015 cm), stratified CSM (density exponent of 2.9) and mass-loss rate of 0.1 − 2.7 M yr−1. These two mass-loss regimes imply a radially varying CSM, shaped by asymmetry or changes in the progenitor’s mass-loss over time. This mass-loss behaviour fits both binary and single-star evolution. Early Icn-like features stem from hot carbon ionization, fading to Ibn-like with cooling. SN 2023xgo thus offers rare insight into the connection between SNe Icn, Ibn, and SNe Ibn with ejecta signatures.