Title: Subaru/FOCAS late time spectroscopy of the
extremely luminous Type IIn SN 2006gy

Authors: K. S. Kawabata, M. Tanaka, K. Maeda, T. Hattori,
K. Nomoto, N. Tominaga, and M. Yamanaka

Abstract:

SN 2006gy is extremely luminous Type IIn supernova,
characterized by the largest radiation energy
(~10^51 erg) comparable to the kinetic energy of
typical supernovae.
The mechanism of its huge luminosity is unclear.
We performed optical spectroscopy and photometry
of SN 2006gy at late time, ~400 days after the
explosion, with the Subaru/FOCAS.
We performed optical spectroscopy and photometry
of SN 2006gy at late time, ~400 days after the
explosion, with the Subaru/FOCAS.
The SN was clearly detected, although other groups
failed to detect the SN in optical wavelengths at
similar epochs.
We found that the SN faded by ~5 mag from peak to
~400 days after the explosion 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 and
showed many intermediate width (~2000 km/s FWHM)
emission lines of [Fe II], [Ca II], Ca II and
several unusual emission lines.
Some of the unidentified emission lines might be
due to Ti or Ni synthesized at the explosion.
The absence of the broad [O I] 6300, 6364 line and
weak [Fe II] and [Ca II] lines would be explained
by a moderately high electron density in the line
emitting region.
The H-alpha line luminosity was as small as
~1x10^39 erg/s, being comparable with those
of normal Type II SNe at similar epochs.
Our observation indicates that the strong CSM
interaction had almost finished by ~400 days.
If the late time optical flux is purely powered by
radioactive decay, at least M_Ni ~ 3 M_sun should
be produced at the SN explosion.