Radio observations reveal a smooth circumstellar environment around the extraordinary type Ib supernova 2012au

We present extensive radio and X-ray observations of SN 2012au, an energetic, radio-luminous supernova of Type Ib that exhibits multi-wavelength properties bridging subsets of hydrogen-poor superluminous supernovae, hypernovae, and normal core-collapse supernovae. The observations closely follow models of synchrotron emission from a shock-heated circumburst medium that has a wind density profile (ρr ^-2). We infer a sub-relativistic velocity for the shock wave v 0.2 c and a radius of r 1.4 × 10¹⁶cm at 25 days after the estimated date of explosion. For a wind velocity of 1000 km s^-1, we determine the mass-loss rate of the progenitor to be , consistent with the estimates from X-ray observations. We estimate the total internal energy of the radio-emitting material to be E 10⁴⁷ erg, which is intermediate to SN 1998bw and SN 2002ap. The evolution of the radio light curve of SN 2012au is in agreement with its interaction with a smoothly distributed circumburst medium and the absence of stellar shells ejected from previous outbursts out to r 10¹⁷ cm from the supernova site. We conclude that the bright radio emission from SN 2012au was not dissimilar from other core-collapse supernovae despite its extraordinary optical properties, and that the evolution of the SN 2012au progenitor star was relatively quiet, marked with a steady mass loss, during the final years preceding explosion.