We present ultraviolet, optical and near-infrared observations of the interacting transient SN 2009ip, covering the period from the start of the outburst in 2012 October until the end of the 2012 observing season. The transient reached a peak magnitude of MV = −17.7 mag, with a total integrated luminosity of 1.9 × 1049 erg over the period of 2012 August–December. The light curve fades rapidly, dropping by 4.5 mag from the V-band peak in 100 d. The optical and near-infrared spectra are dominated by narrow emission lines with broad electron scattering wings, signalling a dense circumstellar environment, together with multiple components of broad emission and absorption in H and He at velocities in the range 0.5–1.2 × 104 km s−1. We see no evidence for nucleosynthesized material in SN 2009ip, even in late-time pseudo-nebular spectra. We set a limit of <0.02 M⊙ on the mass of any possible synthesized 56Ni from the late-time light curve. A simple model for the narrow Balmer lines is presented and used to derive number densities for the circumstellar medium in the range ∼109–1010 cm−3. Our near-infrared data do not show any excess at longer wavelengths, and we see no other signs of dust formation. Our last data, taken in 2012 December, show that SN 2009ip has spectroscopically evolved to something quite similar to its appearance in late 2009, albeit with higher velocities. It is possible that neither of the eruptive and high-luminosity events of SN 2009ip were induced by a core collapse. We show that the peak and total integrated luminosity can be due to the efficient conversion of kinetic energy from colliding ejecta, and that around 0.05–0.1 M⊙ of material moving at 0.5–1 × 104 km s−1 could comfortably produce the observed luminosity. We discuss the possibility that these shells were ejected by the pulsational pair instability mechanism, in which case the progenitor star may still exist, and will be observed after the current outburst fades. The long-term monitoring of SN 2009ip, due to its proximity, has given the most extensive data set yet gathered of a high-luminosity interacting transient and its progenitor. It is possible that some purported Type IIn supernovae are in fact analogues of the 2012b event and that pre-explosion outbursts have gone undetected.
Bibliographical note28 pages, submitted to MNRAS. Abstract abridged for arXiv