2006 gy : was it really extra - ordinary ?

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 pseudo-bolometric (BVRI) light curve and an extended spectral sequence are used to derive constraints on the origin and evolution of the SN. A broad, bright (MR ∼ −21.7) peak characterizes all monochromatic light curves. Afterwards, a rapid luminosity fading (γR ∼ 3.2 mag (100 d) ) is followed by a phase of slow luminosity decline (γR ∼ 0.4 mag (100 d) ) between day ∼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 IR echoes 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 and FHWM ≈ 9000 km s are measured around maximum phase for the intermediate and high velocity components, respectively, and they evolve slowly with time. After maximum, spectroscopic and photometric similarities are found between SN 2006gy and bright, interaction-dominated SNe (e.g. SN1997cy, SN1999E and SN2002ic). This suggests that ejecta-CSM interaction plays a key role in SN2006gy about 6 to 8 months after maximum, sustaining the late-time-light curve. Alternatively, the late luminosity may be related to the radioactive decay of ∼ 3M⊙ of Ni. Models of the light curve in the first 170 days suggest that the progenitor was a compact star (R ∼ 6− 8 · 10cm, Mej ∼ 5− 14M⊙), and that the SN ejecta INAF-OAPD, Vicolo dell’Osservatorio 5, I-35122, Padova, Italy INAF Osservatorio Astronomico di Capodimonte, Salita Moiariello, 16 80131 Napoli, Italy Center for Relativistic Astrophysics Network, Piazza della Repubblica 10, I-65122 Pescara, Italy