An enigma within a mystery revealed an explanation to the peculiar emission lines seen in a supernovae as bright as an entire galaxy – SN 2006gy, a hypernova or quark-nova (a hypothetical supernova that gives birth to a quark star instead of a neuron star) inside galaxy NGC 1260, some 250 million light-years away, first discovered on September 18, 2006- as well as an explanation for its previously unknown origin.
Spectral lines provide the clue
Superluminous supernovae are the most luminous explosions in the cosmos. SN 2006gy is one of the most studied such events, but researchers have been uncertain about its origin. Astrophysicists at Stockholm University have, together with Japanese colleagues, now discovered large amounts of iron in the superluminous supernova –among the brightest in the cosmos–through spectral lines that have never previously been seen either in supernovae or in other astrophysical objects. That has led to a new explanation for how the supernova arose.
“No-one had tested to compare spectra from neutral iron, i.e. iron with all electrons retained, with the unidentified emission lines in SN 2006gy, because iron is normally ionized (one or more electrons removed). We tried it and saw with excitement how line after line lined up just as in the observed spectrum”, says astrophysicist Anders Jerkstrand at Stockholm University.
“It became even more exciting when it quickly turned out that very large amounts of iron was needed to make the lines – at least a third of the Sun’s mass – which directly ruled out some old scenarios and instead revealed a new one.”
Double Star Consisting of a White Dwarf
The progenitor to SN 2006gy was, according to the new model, a double star consisting of a white dwarf — a very dense star of the same size as the Earth but comparable in mass to a small star — and a hydrogen-rich massive star as large as our solar system in close orbit. As the hydrogen- rich star expanded its envelope, which happens when new fuel is ignited in the late stages of evolution, the white dwarf was caught in the envelope and spiraled in towards the center of the companion. When it reached the center the unstable white dwarf exploded and a so-called Type Ia (read one-a) supernova was born. This supernova then collided with the ejected envelope, which is flung out during the inspiral, and this gigantic collision gave rise to the light of SN 2006gy.
“That a Type Ia supernova appears to be behind SN 2006gy turns upside down what most researchers have believed”, says Jerkstrand. “That a white dwarf can be in close orbit with a massive hydrogen-rich star, and quickly explode upon falling to the center, gives important new information for the theory of double star evolution and the conditions necessary for a white dwarf to explode.”
Image credit: The Chandra X-ray Observatory took this image of hypernova SN 2006gy (upper right) and galaxy NGC 1260 in 2006. Light from this brilliant explosion took some 250 million years to reach us. NASA/CXC/UC Berkeley/N.Smith, et al. Inset image via Fox, Ori D. et al. Mon.Not.Roy.Astron.Soc. 454 (2015) no.4