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We have performed a molecular line search towards the flaring 6.7-GHz masers G 24.33+0.13, and G 359.6−0.24 using the Australia Telescope Compact Array. We present spectra of the 6.7-GHz class II methanol and 22.2-GHz water masers towards these sources and provide a comparison with other recent flaring events these sources have experienced. We also detect the fourth example of a 23.4-GHz class I methanol maser and the 11th example of a 4.8-GHz formaldehyde maser towards G 24.33+0.13. Alongside these results, we also observe the previously detected ammonia (3,3) emission and report upper limits on the presence of various other cm-wavelength methanol, ammonia, and OH transitions. Our results are consistent with the flaring of G 24.33+0.13 being driven by a variable accretion rate in the host high mass young stellar object. [ABSTRACT FROM AUTHOR]

This paper reports observations of a 22 GHz water maser 'superburst' in the G25.65+1.05 massive star-forming region, conducted in response to an alert from the Maser Monitoring Organisation (M2O). Very long baseline interferometry (VLBI) observations using the European VLBI Network (EVN) recorded a maser flux density of 1.2 × 104 Jy. The superburst was investipgated in the spectral, structural, and temporal domains and its cause was determined to be an increase in maser path length generated by the superposition of multiple maser emitting regions aligning in the line of sight to the observer. This conclusion was based on the location of the bursting maser in the context of the star-forming region, its complex structure, and its rapid onset and decay. [ABSTRACT FROM AUTHOR]

Discovery of periodic maser emission was an unexpected result from monitoring observations of methanol transitions in high-mass young stellar objects. We report on the detection of five new periodic sources from a monitoring program with the Torun 32 m telescope. Variability with a period of 149 to 540 d and different patterns from sinusoidal-like to intermittent was displayed. Three-dimensional structure of G59.633−0.192 determined from the time delays of burst peaks of the spectral features and high angular resolution map implies that the emission traces a disc. For this source the 6.7 GHz light curve followed the infrared variability supporting a radiative scheme of pumping. An unusual time delay of ∼80 d occurred in G30.400−0.296 could not be explained by the light travel time and may suggest a strong differentiation of physical conditions and excitation in this deeply embedded source. Our observations suggest the intermittent variability may present a simple response of maser medium to the underlying variability induced by the accretion luminosity while other variability patterns may reflect more complex changes in the physical conditions. [ABSTRACT FROM AUTHOR]

G111.256−0.770 is a high-mass young stellar object associated with a weak 6.7 GHz methanol maser showing strong variability. We present results of a multi-epoch monitoring program of the target, conducted with the Torun 32 m telescope for more than a decade. We found that the isotropic maser luminosity varied by a factor 16 on a time-scale of 5–6 yr and individual features showed small amplitude short-lived (∼0.2 yr) bursts superimposed on higher amplitude slow (>5 yr) variations. The maser integrated flux density appears to be correlated with the near-infrared flux observed with the (NEO)WISE, supporting radiative pumping of the maser line. [ABSTRACT FROM AUTHOR]

We present new evidence for superradiance (SR) in the methanol 6.7 GHz spectral line for three different star-forming regions: S255IR-NIRS3, G24.329+0.144, and Cepheus A. Our analysis shows that some of the flux–density flares exhibiting fast rise times and asymmetric light curves reported in these sources can naturally be explained within the context of SR. When a threshold for the inverted population column density is exceeded in a maser-hosting region, the radiation mode switches from one regulated by stimulated emission (maser) to SR. Superradiance, as a more efficient energy release mechanism, manifests itself through strong bursts of radiation emanating from spatially compact regions. Elevated inverted population densities and the initiation of SR can be due to a change in radiative pumping. Here, we show that an increase in the pump rate and the inverted population density of only a factor of a few results in a significant increase in radiation. While the changes in the pump rate can take place over a few hundred days, the rise in radiation flux density when SR is initiated is drastic and happens over a much shorter time-scale. [ABSTRACT FROM AUTHOR]

We report results of 6.7 GHz methanol maser monitoring of 139 star-forming sites with the Torun 32-m radio telescope from 2009 June to 2013 February. The targets were observed at least once a month, with higher cadences of two to four measurements per week for circumpolar objects. Nearly 80 per cent of the sources display variability greater than 10 per cent on a timescale between a week and a few years, but about three quarters of the sample have only one to three spectral features that 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. Synchronized and anticorrelated 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 three to five 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 anticorrelation between intensity and line-width that is expected for unsaturated amplification. There is a weak anticorrelation 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. [ABSTRACT FROM AUTHOR]

The results of long-term monitoring of the Galactic 22.2 GHz water masers associated with G25.65+1.05 using several telescopes are presented. We report a powerful double flare with peak flux densities of ∼60 and 130 kJy in 2017 September and 2017 November, respectively. We propose a linear relationship between the inverse of the square of the line width and the natural logarithm of the flux density of the flaring water maser feature. We infer from this that the flaring feature is unsaturated. [ABSTRACT FROM AUTHOR]