  Context: High-mass young stellar objects (HMYSOs) can undergo accretion
episodes that strongly affect the star evolution, the dynamics of the disk, and
its chemical evolution. Recently reported extraordinary bursts in the methanol
maser emission may be the observational signature of accretion events in deeply
embedded HMYSOs. Aims: We analyze the light curve of 6.7 GHz methanol masers in
S255IR-NIRS3 during the 2015-2016 burst. Methods: 8.5-year monitoring data with
an average sampling interval of 5 days were obtained with the Torun 32 m radio
telescope. Archival data were added, extending the time series to ~27 years.
Results: The maser emission showed moderate (25-30%) variability on timescales
of months to years over ~23 years since its discovery. The main burst was
preceded by a one-year increase of the total flux density by a factor of 2.5,
then it grew by a factor of 10 over ~0.4 years and declined by a factor of 8
during the consecutive 2.4 years. The peak maser luminosity was a factor of
24.5 higher than the pre-burst quiescent value. The light curves of individual
features showed considerable diversity but indicated a general trend of
suppression of the maser emission at blueshifted (<4.7 km s$^{-1}$) velocities
when the redshifted emission rapidly grew and new emission features appeared at
velocities >5.8 km s$^{-1}$. This new emission provided a contribution of about
80% to the maser luminosity around the peak of the burst. Conclusions: The
onset of the maser burst exactly coincides with that of the infrared burst
estimated from the motion of the light echo. This strongly supports the
radiative pumping scheme of the maser transition. The growth of the maser
luminosity is the result of an increasing volume of gas where the maser
inversion is achieved.
