Giant burst of methanol maser in S255IR-NIRS3

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. 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 ∼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.

Near-infrared monitoring of the accretion outburst in the massive young stellar object S255-NIRS3

“…Sudden increase in flux and its following moderate fading are also clearly seen in the light curves of the most and the next brightest maser velocity components (6.42 and 5.83 km/s). The same trend in the maser variability was also reported in a previous radio monitoring work of Szymczak et al (2018). Table 1 summarizes the properties of flux variations in both the maser and NIR emissions.…”

Detection of new methanol maser transitions associated with G358.93−0.03

“…Direct evidence of an accretion event was first reported by Caratti o Garatti et al (2017) in the HMYSO S255IR-NIRS3; they presented flaring in the near-infrared (NIR) and mid-infrared continuum, while Liu et al (2018) found submillimeter flaring. Fujisawa et al (2015) found an associated 6.668 GHz methanol maser flaring event in S255IR-NIRS3, which was characterized in detail by Szymczak et al (2018). Hunter et al (2017) proposed an accretion event as the progenitor in NGC 6334I from their detection of a sudden increase in the (sub)millimeter dust emission luminosity (a factor of ∼ 70).…”

New evidence for Dicke’s superradiance in the 6.7 GHz methanol spectral line in the interstellar medium

“…For one of the examples analyzed in this paper, flaring events in the 6.7 GHz methanol line are known to coincide with an IR outburst in the central source (Caratti o Garatti et al 2017;Szymczak et al 2018b). We will model such outbursts with a pumping pulse of a given amplitude and duration and show how they can initiate SR.…”

“…An important feature of the flaring events discussed in this paper is their significant flux density increases over short time-scales. For instance, in the case of the S255IR-NIRS3 flaring event, an increase in flux of more than a factor of 1000 with a rise time of less than 100 days was seen for some velocity components (Szymczak et al 2018b). To explain such steep flux density variation within the context of maser theory, a significant increase in the pump rate over a short time-scale would be necessary.…”