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