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We present a radio-quiet quasar at z=0.237 discovered "turning on" by theintermediate Palomar Transient Factory (iPTF). The transient, iPTF 16bco, wasdetected by iPTF in the nucleus of a galaxy with an archival SDSS spectrum withweak narrow-line emission characteristic of a low-ionization emission lineregion (LINER). Our follow-up spectra show the dramatic appearance of broadBalmer lines and a power-law continuum characteristic of a luminous(L_bol~10^45 erg/s) type 1 quasar 12 years later. Our photometric monitoringwith PTF from 2009-2012, and serendipitous X-ray observations from theXMM-Newton Slew Survey in 2011 and 2015, constrain the change of state to haveoccurred less than 500 days before the iPTF detection. An enhanced broad Halphato [OIII]5007 line ratio in the type 1 state relative to other changing-lookquasars also is suggestive of the most rapid change of state yet observed in aquasar. We argue that the >10 increase in Eddington ratio inferred from thebrightening in UV and X-ray continuum flux is more likely due to an intrinsicchange in the accretion rate of a pre-existing accretion disk, than an externalmechanism such as variable obscuration, microlensing, or the tidal disruptionof a star. However, further monitoring will be helpful in better constrainingthe mechanism driving this change of state. The rapid "turn on" of the quasaris much shorter than the viscous infall timescale of an accretion disk, andrequires a disk instability that can develop around a ~10^8 M_sun black hole ontimescales less than a year.

iPTF Discovery of the Rapid "Turn On" of a Luminous Quasar