A Type Ia supernova at the heart of superluminous transient SN 2006gy

Superluminous supernovae radiate up to 100 times more energy than normal supernovae. The origin of this energy and the nature of their stellar progenitors are poorly understood. We identify neutral iron lines in the spectrum of one such transient, SN 2006gy, and show that they require a large mass of iron (>~0.3 Msun) expanding at 1500 km/s. We demonstrate that a model of a standard Type Ia supernova hitting a shell of circumstellar material produces a light curve and late-time iron-dominated spectrum that match SN 2006gy. In such a scenario, common envelope evolution of the progenitor system can synchronize envelope ejection and supernova explosion and may explain these bright transients.