  We study the sudden optical and ultraviolet (UV) brightening of 1ES 1927+654,
which until now was known as a narrow-line active galactic nucleus (AGN). 1ES
1927+654 was part of the small and peculiar class of "true Type-2" AGN, which
lack broad emission lines and line-of-sight obscuration. Our high-cadence
spectroscopic monitoring captures the appearance of a blue, featureless
continuum, followed several weeks later by the appearance of broad Balmer
emission lines. This timescale is generally consistent with the expected light
travel time between the central engine and the broad-line emission region in
(persistent) broad-line AGN. Hubble Space Telescope spectroscopy reveals no
evidence for broad UV emission lines (e.g., CIV1549, CIII]1909, MgII2798),
probably owing to dust in the broad-line emission region. To the best of our
knowledge, this is the first case where the lag between the change in continuum
and in broad-line emission of a "changing-look" AGN has been temporally
resolved. The nature and timescales of the photometric and spectral evolution
disfavor both a change in line-of-sight obscuration and a change of the overall
rate of gas inflow as driving the drastic spectral transformations seen in this
AGN. Although the peak luminosity and timescales are consistent with those of
tidal disruption events seen in inactive galaxies, the spectral properties are
not. The X-ray emission displays a markedly different behavior, with frequent
flares on timescales of hours to days, and will be presented in a companion
publication.
