Radio Evolution of a Type IIb Supernova SN 2016gkg

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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 (2.2, 3.6, 3.8, 12.6, 3.7, and 5.0) ×10−6 M ⊙ 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.