Kilpatrick, Charles D.Foley, Ryan JDrout, Maria R.Pan, Yen-ChenPanther, Fiona HelenCoulter, David A.Filippenko, Alexei V.Marion, G HPiro, Anthony LRest, A.Seitenzahl, IvoStrampelli, GWang, Xi2020-01-122020-01-120035-8711http://hdl.handle.net/1885/196962We present multi-epoch, multicolour pre-outburst photometry and post-outburst light curves and spectra of the luminous blue variable (LBV) outburst Gaia16cfr discovered by the Gaia satellite on 2016 December 1 UT. We detect Gaia16cfr in 13 epochs of Hubble Space Telescope imaging spanning phases of 10 yr to 8 months before the outburst and in Spitzer Space Telescope imaging 13 yr before outburst. Pre-outburst optical photometry is consistent with an 18 M⊙ F8 I star, although the star was likely reddened and closer to 30 M⊙. The pre-outburst source exhibited a significant near-infrared excess consistent with a 120 au shell with 4 × 10−6 M⊙ of dust. We infer that the source was enshrouded by an optically thick and compact shell of circumstellar material from an LBV wind, which formed a pseudo-photosphere consistent with S Dor-like variables in their ‘maximum’ phase. Within a year of outburst, the source was highly variable on 10–30  d time-scales. The outburst light curve closely matches that of the 2012 outburst of SN 2009ip, although the observed velocities are significantly slower than in that event. In H α, the outburst had an excess of blueshifted emission at late times centred around −1500 km s−1, similar to that of double-peaked Type IIn supernovae and the LBV outburst SN 2015bh. From the pre-outburst and post-outburst photometry, we infer that the outburst ejecta are evolving into a dense, highly structured circumstellar environment from precursor outbursts within years of the 2016 December event.The work of AVF was conducted in part at the Aspen Center for Physics, which is supported by NSF grant PHY-1607611; the author thanks the Center for its hospitality during the neutron stars workshop in 2017 June and July. AVF is grateful for financial assistance from the TABASGO Foundation, the Christopher R. Redlich Fund, the Miller Institute for Basic Research in Science (U.C. Berkeley) and HST grants GO-13646 and AR-14295 from the Space Telescope Science Institute (STScI), which is operated by AURA under NASA contract NAS 5-26555.application/pdfen-AU© 2017 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society201810.1093/mnras/stx26752019-08-25