SN 2006gy: Was it Really Extraordinary?

We present the photometric and spectroscopic study of the very luminous type IIn SN 2006gy for a time period spanning more than one year. The evolution of multiband light curves, the pseudobolometric (BVRI) light curve, and an extended spectral sequence is used to derive constraints on the origin and evolution of the supernova (SN). A broad, bright (M_R ~ -21.7) peak characterizes all monochromatic light curves. Afterward, rapid luminosity fading (γ _R ~ 3.2 mag(100 days)^-1) is followed by a phase of slow luminosity decline (γ _R ~ 0.4 mag(100 days)^-1) between days ~ 170 and ~ 237. At late phases (>237 days), because of the large luminosity drop (>3 mag), only upper visibility limits are obtained in the B, R, and I bands. In the near-infrared, two K-band detections on days 411 and 510 open new issues about dust formation or infrared echo scenarios. At all epochs, the spectra are characterized by the absence of broad P-Cygni profiles and a multicomponent Hα profile, which are the typical signatures of type IIn SNe. Hα velocities of FWHM ≈ 3200 km s^-1 and FHWM ≈ 9000 km s^-1 are measured around the maximum phase for the intermediate and high velocity components, respectively, and they slowly evolve with time. After maximum, spectroscopic and photometric similarities are found between SN 2006gy and bright, interaction-dominated SNe (e.g., SN 1997cy, SN 1999E, and SN 2002ic). This suggests that ejecta-circumsteller material (CSM) interaction plays a key role in SN 2006gy about six to eight months after maximum, sustaining the late-time light curve. Alternatively, the late luminosity may be related to the radioactive decay of ~ 3 M _sun of ⁵⁶Ni. Models of the light curve in the first 170 days suggest that the progenitor was a compact star (R ~ (6-8) × 10¹² cm, M _ej ~ 5-14 M _sun) and that the SN ejecta collided with massive (6-10 M _sun), opaque clumps of previously ejected material. These clumps do not completely obscure the SN photosphere, such that at its peak, the luminosity is due both to the decay of ⁵⁶Ni and to interaction with CSM. Neither an extraordinarily large explosion energy nor a supermassive star is required to explain the observational data.