The unprecedented 2012 outburst of SN 2009ip: a luminous blue variable star becomes a true supernova

Some reports of supernova (SN) discoveries turn out not to be true\ncore-collapse explosions. One such case was SN 2009ip, which was recognized to\nbe a luminous blue variable (LBV) eruption. This source had a massive hot\nprogenitor star identified in pre-explosion data, it had documented evidence of\npre-outburst variability, and it was subsequently discovered to have a 2nd\noutburst in 2010. This same source rebrightened again in 2012, and early\nspectra showed the same narrow-line profiles as before, suggesting another\nLBV-like eruption. We present new photometry and spectroscopy of SN 2009ip,\nindicating that it has transitioned into a true SN. The most striking discovery\nin these data is that unlike previous reports, the spectrum exhibited Balmer\nlines with very broad P-Cygni profiles characteristic of normal Type II\nsupernovae (SNe II), in addition to narrow emission lines seen in SNe IIn and\nLBVs. Emission components have FWHM 8000 km/s, while the P-Cygni absorption\ncomponent has blue wings extending to -13,000 km/s. These velocities are\ntypical of SNe II, but have never been associated with emission lines from a\nnonterminal LBV-like eruption. Initially, the peak absolute magnitude seemed\nfainter than that of normal SNe. However, after a brief period of fading, the\nsource quickly brightened again to M_R=-17.5 mag over a couple days. The broad\nlines mostly disappeared, and the spectrum began to resemble the early\noptically thick phases of SNe IIn. Two weeks later the source leveled off near\n-18 mag, after which broad emission lines again developed in the spectrum as\nthe source faded. We conclude that the 2012 outburst of SN 2009ip was the\nresult of a true core-collapse SN IIn that occured when the progenitor star was\nin an LBV-like outburst phase, and where the SN was initially faint and then\nrapidly brightened due to interaction with circumstellar material (abridged).\n