Spectroscopic observations of progenitor activity 100 days before a Type Ibn supernova

Obtaining spectroscopic observations of the progenitors of core-collapse supernovae is often unfeasible, due to an inherent lack of knowledge as to what stars experience supernovae and when they will explode. In this Letter we present photometric and spectroscopic observations of the progenitor activity of before the He-rich progenitor explodes as a Type Ibn supernova. The progenitor of shows an exponential rise in flux prior to core collapse. Complex He I emission line features are observed in the progenitor spectra, with a P Cygni-like profile, as well as an evolving broad base with velocities of the order of 10,000 The luminosity and evolution of is consistent with a Type Ibn, reaching a peak $r$-band magnitude of $-18.8$ mag, although there is some uncertainty regarding the distance to the host, NGC 4388, which is located in the Virgo cluster. We present additional evidence of asymmetric He-rich material being present both prior to and after the explosion of which suggests that this material survived the ejecta interaction. Broad O I C I and the Ca II triplet lines are observed at late phases, confirming that was a genuine supernova, rather than a non-terminal interacting transient. provides insight into the final moments of a massive star's life, demonstrating that the progenitor is likely highly unstable before core collapse.