Extremely luminous supernova 2006gy at late phase: Detection of optical emission from supernova

Abstract

Supernova (SN) 2006gy is an extremely luminous Type IIn SN characterized by the bright peak magnitude MR -22mag and its long duration. The mechanism giving rise to its huge luminosity is still unclear. We performed optical spectroscopy and photometry of SN 2006gy at late time, 400 days after the explosion, with the Subaru/FOCAS in a good seeing condition. We carefully extracted the SN component, although there is an ambiguity because of the contamination by bright nucleus of the host galaxy. We found that the SN faded by 3mag from 200 to 400 days after the explosion (i.e., by 5mag from peak to 400 days) in R band. The overall light curve is marginally consistent with the 56Ni heating model, although the flattening around 200 days suggests the optical flux declined more steeply between 200 and 400 days. The late time spectrum was quite peculiar among all types of SNe. It showed many intermediate width (2000 km s-1 FWHM) emission lines, e.g., [Fe II], [Ca II], and Ca II. The absence of the broad [O I] 6300, 6364 line and weakness of [Fe II] and [Ca II] lines compared with Ca II IR triplet would be explained by a moderately high electron density in the line emitting region. This high-density assumption seems to be consistent with the large amount of ejecta and low expansion velocity of SN 2006gy. The Hα line luminosity was as small as 1× 1039 erg s-1, being comparable with those of normal Type II SNe at similar epochs. Our observation indicates that the strong circumstellar medium interaction had almost finished by 400 days. If the late time optical flux is purely powered by radioactive decay, at least M( 56Ni) 3 M ⊙ should be produced at the SN explosion. In the late phase spectrum, there were several unusual emission lines at 7400-8800 and some of them might be due to Ti or Ni synthesized at the explosion. © 2009. The American Astronomical Society. All rights reserved.