Rare stellar explosion challenges understanding of supernova origins

<p>We present observations of SN 2023xgo, a transitional Type Ibn/Icn SN, from −5.6 to 63 d relative to <em>r</em>-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 (<em>M_r</em> = −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-loss rate 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.</p>