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 \sim 27 years .

Results : The maser emission showed moderate ( 25-30 % ) variability on timescales of months to years over \sim 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 \sim 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 .
The duration of the burst at the extreme redshifted velocities of 7.1 to 8.7 km s ^ { -1 } was from 0.9 to 1.9 years , and its lower limit for the other features was \sim 3.9 years .

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 .