Giant burst of methanol maser in S255IR-NIRS3

TL;DR

This paper reports on a significant methanol maser burst in S255IR-NIRS3, linking it to an infrared burst and suggesting radiative pumping as the mechanism, with detailed long-term monitoring revealing complex emission dynamics.

Contribution

It provides the first detailed analysis of a giant methanol maser burst in S255IR-NIRS3, connecting it to an infrared burst and supporting radiative pumping models.

Findings

The maser burst was preceded by a 2.5-fold flux increase over a year.

Peak maser luminosity was 24.5 times higher than pre-burst levels.

The burst involved complex velocity-dependent emission changes.

Abstract

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.