2025ApJ...995L..49B

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 (∼10⁶ρ_ISM) at a radius of 10¹⁶ cm, corresponding to a mass-loss rate of ∼4 × 10^–3M_⊙ 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 3σ 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 $\dot{M}$< 2.5 × 10^–3M_⊙ yr^–1. These results suggest a shell-like CSM from at most 4 × 10¹⁵ to 2 × 10¹⁶ cm (∼10⁵R_⊙), 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.