Extended Data Fig. 5: The spectra of SN 2020eyj do not match Type Ic or Ibn SNe at early epochs.

The SEDM classification spectrum of SN 2020eyj compared with Type Ibn SN 2006jc and Type Ic SN1994I at similar epochs, about 12 days after peak. At 15 days post-peak, SN 2006jc already showed strong He emission lines and developed the quasi-continuum typical for CSM-interaction-dominated spectra. These features are not observed in SN 2020eyj at 12 days post-peak but do become prominent at late phases (Fig. 2). At 13 days post-peak, SN 1994I has grown redder compared with its peak spectrum shown in Fig. 1 and the O i 7,774 Å absorption feature has become more prominent, whereas in SN 2020eyj, this feature is not present.