The First Radio View of a Type Ibn Supernova in SN 2023fyq: Understanding the Mass-loss History in the Last Decade before the Explosion

Supernovae that interact with hydrogen-poor, helium-rich circumstellar material (CSM), known as type Ibn supernovae (SNe Ibn), present a unique opportunity to probe mass-loss processes in massive stars. In this work, we report the first radio detection of an SN Ibn, SN 2023fyq, and characterize the mass-loss history of its stellar progenitor using the radio and X-ray observations obtained over 18 months post-explosion. We find that the radio emission from 58 to 185 days is best modeled by synchrotron radiation attenuated by free–free absorption from a CSM of density ∼10 −18 g cm −3 (∼10 6 ρ ISM ) at a radius of 10 16 cm, corresponding to a mass-loss rate of ∼4 × 10 −3 M ⊙ yr −1 (for a CSM velocity of 1700 km s −1 from optical spectroscopy) from 0.7 to 3 yr before the explosion. This timescale is consistent with the time frame over which pre-explosion optical outbursts were observed. However, our late-time observations at 525 days post-explosion yield nondetections, and the 3 σ upper limits (along with an X-ray nondetection) allow us to infer a drop in the progenitor mass-loss rate at 5–10 yr pre-explosion with M ̇ < 2.5 × 10 −3 M ⊙ yr −1 . These results suggest a shell-like CSM from at most 4 × 10 15 to 2 × 10 16 cm (∼10 5 R ⊙ ), with a CSM density that is roughly consistent with predictions from a merger model for this object. Future radio observations of a larger sample of SNe Ibn will provide key details on the extent and density of their helium-rich CSM.