2018A&A...617A..80S

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 6.7GHz methanol masers in S255IR-NIRS3 during the 2015-2016 burst.
Methods. 8.5-yr monitoring data with an average sampling interval of 5 days were obtained with the Torun 32m radio telescope. Archival data were added, extending the time series to ∼27yr.
Results. The maser emission showed moderate (25-30%) variability on timescales of months to years over ∼23yr since its discovery. The main burst was preceded by a 1yr increase of the total flux density by a factor of 2.5, then it grew by a factor of 10 over ∼0.4yr and declined by a factor of 8 during the consecutive 2.4yr. 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 (<4.7km/s) velocities when the redshifted emission rapidly grew and new emission features appeared at velocities >5.8km/s. 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 7.1-8.7km/s was from 0.9 to 1.9yr, and its lower limit for the other features was ∼3.9yr.
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.