  From late September 2017 to February 2018, the Be X-ray binary (BeXB) Swift
J0243.6+6124 underwent an unprecedently bright giant outburst. The reported
X-ray luminosities were so high that the system was classified as an
Ultraluminous X-ray source (ULX). It was also the first BeXB pulsar showing
radio jet emission. The source was not only bright in X-rays and radio, but
also in optical and UV wavelenghts. In this paper we aim to understand the
origin of the observed optical/UV fluxes simultaneous to the X-ray emission. We
studied the optical/UV light curves in comparison with the X-ray fluxes along
the outburst, considering the main mechanisms that can explain the optical/UV
emission in X-ray binaries. Due to the tight correlation observed between the
optical/UV and X-ray light curves, reprocessing of X-rays seems to be the most
plausible explanation. We calculated the timescales of the light curves decays
and studied the correlation indexes between the optical and X-ray emission.
Finally, we built a physical model considering X-ray heating of the surface of
the donor star, irradiation of the accretion disk, and emission from a
viscously heated accretion disk, in order to reproduce the observed optical/UV
SEDs along the outburst. We considered in our model that the Be circumstellar
disk was co-planar to the orbit, and then we neglected its irradiation in the
current model. As an input of the model, we used as incident X-ray luminosities
those calculated from the bolometric X-ray fluxes obtained from the spectral
fit of the Swift/XRT and BAT observations. We conclude that reprocessing of
X-rays as X-ray heating of the Be star surface and irradiation of the accretion
disk are the two main mechanisms that can reproduce the observed optical/UV
emission during the 2017-2018 giant outburst of Swift J0243.6+6124.
