The Radio and X‐Ray Luminous SN 2003bg and the Circumstellar Density Variations around Radio Supernovae

Abstract

We report extensive radio and X-ray observations of SN 2003bg, whose spectroscopic evolution shows a transition from a broad-lined Type Ic to a hydrogen-rich Type II, and later to a typical hydrogen-poor Type Ibc. We show that the extraordinarily luminous radio emission is well described by a self-absorption-dominated synchrotron spectrum, while the observed X-ray emission at t ≈ 30 days is adequately fit by inverse Compton scattering of the optical photons off of the synchrotron-emitting electrons. Our radio model implies a subrelativistic ejecta velocity, ≈ 0.24c, at t0 ≈ 10 days after the explosion, which emphasizes that broad optical absorption lines do not imply relativistic ejecta. We find that the total energy of the radio-emitting region evolves as E ≈ 7.3 × 1048(t/t0)0.4 ergs, assuming equipartition of energy between relativistic electrons and magnetic fields (e = B = 0.1). The circumstellar density is well described by a stellar wind profile, with modest (factor of ~2) episodic density enhancements that produce abrupt achromatic flux variations. We estimate an average mass-loss rate of ≈ 3 × 10-4 M☉ yr-1 (assuming a wind velocity of vw = 103 km s-1) for the progenitor, consistent with the observed values for Galactic Wolf-Rayet stars. Comparison with other events reveals that ~50% of radio supernovae show similar short-timescale flux variations, attributable to circumstellar density irregularities. Specifically, the radio light curves of SN 2003bg are strikingly similar to those of the Type IIb SN 2001ig, suggestive of a common progenitor evolution for these two events. Based on the relative intensity of the inferred density enhancements, we conclude that the progenitors of SNe 2003bg and 2001ig experienced quasi-periodic mass-loss episodes just prior to the SN explosion. Finally, this study emphasizes that abrupt radio light-curve variations cannot be used as a reliable proxy for an engine-driven explosion, including off-axis gamma-ray bursts.