2022ApJ...934..186N

We present extensive radio monitoring of a Type IIb supernova (SN IIb), SN 2016gkg during t ∼ 8-1429 days postexplosion at frequencies ν ∼ 0.33-25 GHz. The detailed radio light curves and spectra are broadly consistent with self-absorbed synchrotron emission due to the interaction of the SN shock with the circumstellar medium. The model underpredicts the flux densities at t ∼ 299 days postexplosion by a factor of 2, possibly indicating a density enhancement in the circumstellar medium due to a nonuniform mass loss from the progenitor. Assuming a wind velocity v_w ∼ 200 km s^–1, we estimate the mass-loss rate to be $\dot{M}\sim $ (2.2, 3.6, 3.8, 12.6, 3.7, and 5.0) ×10^–6M_⊙ yr^–1 during ∼8, 15, 25, 48, 87, and 115 yr, respectively, before the explosion. The shock wave from SN 2016gkg is expanding from R ∼ 0.5 × 10¹⁶ to 7 × 10¹⁶ cm during t ∼ 24-492 days postexplosion indicating a shock deceleration index, m ∼ 0.8 (R ∝ t^m), and mean shock velocity v ∼ 0.1c. The radio data are inconsistent with a free-free absorption model and higher shock velocities are in support of a relatively compact progenitor for SN 2016gkg.