  High-cadence observations of the Galactic bulge by the microlensing surveys
led to the discovery of a handful of extremely short-timescale microlensing
events that can be attributed to free-floating or wide-orbit planets. Here, we
report the discovery of another strong free-floating planet candidate, which
was found from the analysis of the gravitational microlensing event
OGLE-2019-BLG-0551. The light curve of the event is characterized by a very
short duration (<3 d) and a very small amplitude (< 0.1 mag). From modeling of
the light curve, we find that the Einstein timescale, tE = 0.381 +/- 0.017 d,
is much shorter, and the angular Einstein radius, thetaE = 4.35 +/- 0.34 uas,
is much smaller than those of typical lensing events produced by stellar-mass
lenses (tE ~ 20 d, thetaE ~ 0.3 mas), indicating that the lens is very likely
to be a planetary-mass object. We conduct an extensive search for possible
signatures of a companion star in the light curve of the event, finding no
significant evidence for the putative host star. For the first time, we also
demonstrate that the angular Einstein radius of the lens does not depend on
blending in the low-magnification events with strong finite source effects.

Earth and Planetary Astrophysics;Astrophysics of Galaxies;Instrumentation and Methods for Astrophysics;Solar and Stellar Astrophysics