VLBI observations of the G25.65+1.05 water maser superburst

Abstract

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.

Molecular line search toward the flaring 6.7-GHz methanol masers of G24.33+0.13 and G359.6-0.243: rare maser transitions detected

“…For G 24.33+0.13, the decrease in the flux density of all maser components (see Figure 1) from our second epoch suggests that the flaring does not result from an increase in the maser path length, where we would expect to see variation in a small number of velocity-coherent maser features (e.g. Burns et al 2020b). The similarity between this flare and that observed in 2010 by Wolak et al (2018) indicates some long-term periodic mechanism may be responsible for flaring in this source.…”

“…A link between protostellar accretion outbursts and maser flares has been made in three class II methanol maser sources to date: S255 NIRS 3 (Moscadelli et al 2017), NGC 6334I-MM1 (Hunter et al 2018) and G 358.93-0.03 (Burns et al 2020b). These events in protostars, where accretion rates burst higher than average, are thought to be necessary for the production of high-mass stars (Meyer et al 2017).…”

Long-term Variability of Class I Methanol Masers in the High-mass Star-forming Region DR21(OH)

“…Another possibility is to assume a foreground cloud moving in front of a maser cloud, as Boboltz et al (1998) did to model variation in H 2 O masers in the high-mass star-forming region W49. Yet another cause of maser variability could be through an increase in the path length due to multiple overlapping maser clouds along the line of sight (Burns et al 2020). All of these, a rotating maser cloud, a foreground cloud moving in front of a maser cloud, and overlapping maser clouds, could be initiated by turbulence, which can be caused by the very same shocks that are responsible for pumping Class I CH 3 OH masers in outflows.…”

Maser flares driven by variations in pumping and background radiation

“…A radiative mechanism is not the only explanation for H2O flaring in G025.65+1.05. VLBI observations identify the flaring object with a single compact spot of maser emission (Burns et al 2020). An image from spectral channels in the blue-shifted wing of this spot shows a double structure, leading to an interpretation as a possible line-of-sight overlap of two masing objects.…”

“…An image from spectral channels in the blue-shifted wing of this spot shows a double structure, leading to an interpretation as a possible line-of-sight overlap of two masing objects. Preliminary modelling results for spatial overlap (Gray 2018) indicate that this mechanism can produce flares of significantly higher variability index than is typical of radiative pumping mechanisms, and the overlap interpretation therefore becomes more likely if the source is at the further of the two possible distances (2.7 and 12.5 kpc) mentioned in Burns et al (2020). Another possible example of this overlap mechanism is a very high brightness H2O flare from Orion KL (Shimoikura et al 2005).…”