Monitoring observations of 6.7 GHz methanol masers

Abstract

We report results of 6.7 GHz methanol maser monitoring of 139 star-forming sites with the Torun 32 m radio telescope from June 2009 to February 2013. The targets were observed at least once a month, with higher cadences of 2-4 measurements per week for circumpolar objects. Nearly 80 per cent of the sources display variability greater than 10 per cent on a time-scale between a week and a few years but about three quarters of the sample have only 1-3 spectral features which vary significantly. Irregular intensity fluctuation is the dominant type of variability and only nine objects show evidence for cyclic variations with periods of 120 to 416 d. Synchronised and anti-correlated variations of maser features are detected in four sources with a disc-like morphology. Rapid and high amplitude bursts of individual features are seen on 3-5 occasions in five sources. Long (>50 d to 20 months) lasting bursts are observed mostly for individual or groups of features in 19 sources and only one source experienced a remarkable global flare. A few flaring features display a strong anti-correlation between intensity and line-width that is expected for unsaturated amplification. There is a weak anti-correlation between the maser feature luminosity and variability measure, i.e. maser features with low luminosity tend to be more variable than those with high luminosity. The analysis of the spectral energy distribution and continuum radio emission reveals that the variability of the maser features increases when the bolometric luminosity and Lyman flux of the exciting object decreases. Our results support the concept of a major role for infrared pumping photons in triggering outburst activity of maser emission.

6.7 GHz variability characteristics of new periodic methanol maser sources

“…The sources with the lower limit (>4.7) of relative amplitude have less luminous (< 2 × 10 3 L ) central stars. This finding is consistent with that reported in Szymczak et al (2018) for another sample of methanol masers; this supports a scenario where the periodic maser activity is closely related to the infrared radiation field provided by a HYMSO which periodically heats up its dusty envelope (Parfenov & Sobolev 2014).…”

“…Long-term observations revealed that many 6.7 GHz maser sources are variable on various timescales and magnitudes (Goedhart et al 2004;Szymczak et al 2018). One of the unexpected results was detection of periodic variability in a number of HMYSOs (Goedhart et al 2003(Goedhart et al , 2005(Goedhart et al , 2009(Goedhart et al , 2014Szymczak et al 2011Szymczak et al , 2015Szymczak et al , 2016Fujisawa et al 2014a;Maswanganye et al 2015Maswanganye et al , 2016Sugiyama et al 2017).…”

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

“…The morphology of this source was studied using the European VLBI Network (EVN) in June 2009 (Bartkiewicz, Szymczak & van Langevelde 2016) and a weak emission at 6.7 GHz line was reported, corresponding to a maser brightness temperature < 2 × 10 7 K. The comparison to detected flux levels from previous observations with the EVLA suggested strong variability in this source. Szymczak et al (2018a) monitored G24.329+0.144 between MJD 55011 to MJD 56386, and detected a synchronized outburst in all spectral features.…”

A highly variable methanol maser in G111.256–0.770

“…The target is one of the most variable 6.7 GHz masers in the sample studied by Szymczak et al (2018); the relative amplitude of individual features was up to 5.5 on timescales of between two months and a few years. The maser emission of isotropic luminosity 8.5 × 10 −8 L (Wu et al 2010) spatially coincides with the 22 GHz H2O masers and the radio continuum source of spectral index 0.64 between 1.3 and 3.6 cm wavelength is consistent with free-free emission from a thermal jet or with a partially thick HII region (Trinidad et al 2006).…”

“…Time series for the velocity-integrated flux density of the 6.7 GHz methanol maser line. The black squares and magenta circles denote the archival Torun 32 m data and fromSzymczak et al (2018), respectively. The blue triangle refers to the first detection(Szymczak et al 2000) and the black star marks the VLA observation at MJD 55990(Hu et al 2016).…”