Supernovas often briefly outshine their entire galaxy, but the universe seems determined to keep upping the stakes. Every now and then, there is an event that releases a hundred times more energy than an “ordinary” supernova. We now have an explanation for one of these, known as SN 2006gy.
Astrophysicists think they understand what causes both major types of ordinary supernova, but superluminous supernovae are a different matter. When SN 2006gy's light reached us after 238 million years, astronomers were not even sure what sort of supernova it was, but a new paper in Science claims to have settled that.
According to Dr Anders Jerkstrand of the Max Plank Institute, material ejected by a standard Type Ia supernova encountered a pre-existing shell, and the light created by this impact was orders of magnitude brighter than the initial explosion. The shell had been thrown off 10-200 years before the explosion and may have had a mass 10 times that of the Sun.
Type Ia supernovas occur when a white dwarf draws enough material from a neighboring star to unleash a fusion outburst, or when two white dwarfs collide. Other supernovae types, while differing in the spectrum of light they produce, involve the collapse of a massive star's core, leading to a spectacular rebound.
SN 2006gy was one of the first superluminous supernovas detected. The narrow hydrogen lines in its spectrum initially made astronomers think it was a type IIn supernova, a type of core collapse event marked by its narrow spectral lines. Several theories were presented to explain how such an event could release so much more energy than normal, but none have proven convincing.
More than a year after the explosion, Jerkstrand and co-authors detected a set of unfamiliar emission lines in the infrared part of its spectrum. They were eventually identified as being from unusually slow-moving (for a supernova) iron ions, resulting from the shell's friction slowing the explosion's material down.
The authors conclude SN 2006gy produced at least 30 percent of the Sun's mass in iron. Much of this was initially nickel-56, which decays first to cobalt and then iron. High nickel-56 production is a feature of Type Ia supernovae, but it is seen more rarely in other supernova types. Combined with a few other distinguishing features of the event's light curve, this was enough for the authors to rule out all other known supernova types.
It's common for very large stars to throw off a lot of material prior to their final demise – as Eta Carinae did in the 19th Century – which is part of the reason astronomers initially thought they were looking at a core collapse event. A white dwarf certainly couldn't produce a similar shell on its own, but the paper describes a model by which interactions between it and a companion could throw off material from the larger star, setting the scene for what has been observed.