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.
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机译：我们报道了SN 2003bg的大量无线电和X射线观察结果，其光谱演变显示了从宽泛的Ic型转变为富氢的II型，后来又转变为典型的贫氢Ibc。我们表明，超吸收的无线电辐射由自吸收主导的同步加速器光谱很好地描述了，而在t≈30天观察到的X射线发射通过同步加速器发射的光子的反向康普顿散射就完全适合了电子。我们的射电模型暗示在爆炸后的t0≈10天时，亚相对论的喷射速度为≈0.24c，这强调宽的光吸收线并不意味着相对论的喷射。我们发现，假设相对论电子和磁场之间的能量分配相等（e = B = 0.1），则无线电发射区的总能量将演化为E≈7.3×1048（t / t0）0.4 ergs。恒星风廓线很好地描述了星际密度，适度（约2倍）的情节性密度增强会产生突然的消色差通量变化。我们估计该祖先的平均质量损失率为≈3×10-4M☉yr-1（假设风速为vw = 103 km s-1），与银河Wolf-Rayet星的观测值一致。与其他事件的比较表明，约50％的超新星显示出相似的短时标通量变化，这归因于星际密度不规则性。特别是，SN 2003bg的无线电光曲线与IIb SN 2001ig类型的无线电光曲线非常相似，这暗示了这两个事件的共同祖细胞进化。基于推断密度增强的相对强度，我们得出结论，SNe 2003bg和2001ig的祖先在SN爆炸之前经历了准周期性质量损失事件。最后，这项研究强调，突然的无线电光曲线变化不能用作发动机驱动爆炸（包括离轴伽马射线暴）的可靠替代。
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