Imaging a water maser superburst

In September 2017, during the IAU Symposium 336 "Astrophysical masers: Unlocking the mysteries of the Universe", reports surfaced of two water maser sources going "superburst". Maser Monitoring Organisation (M2O), a team of astronomers led by Ross Burns (a JIVE support scientist at the time), conducted a rapid follow up study to image the superburst in detail. Their findings clarify that the observed superburst was the result of a rare spatial alignment of two maser cloudlets.

Masers are invaluable tools for the study of star formation. Their bright, compact emission outlines disks and bowshocks associated with the accretion and ejection structures of protostars, and can be used to derive accurate source distances via annual parallax measurements. However, while maser amplification has a well established theoretical background some aspects of astrophysical maser behaviors still defy firm explanation, referring in particular to their notoriously extreme variations in brightness.

The superbursts detected with single-dish telescopes and reported during the IAU symposium 336 "Astrophysical masers: Unlocking the mysteries of the Universe", identified a sudden raising in brightness by several orders of magnitude in two maser sources, reaching flux densities of tens of thousands of Janskys (Jy). In an attempt to tease out the process by which such extreme variability can occur, a team of astronomers requested target-of-opportunity observations with the European VLBI Network (EVN) to image the superburst masers with milliarcsecond resolution.

A few days later electronic Very Long Baseline Interferometry (e-VLBI) spectral line observations were successfully conducted, with the observing session split across the two targets. The array contained both short and long baselines, comprised of Effelsberg, Jodrell Bank (MkII), Onsala (20 m), Torun, Yebes and Hartebeesthoek. By the scheduled date of the observations one of the superburst sources had already abated and rapidly returned to its pre-burst state, however, the other superburst source, a massive star forming region called G25.65+1.05, was still fully active at a flux density of almost 12,000 Jy.

The data revealed a complex distribution of maser emission, tracing two arcs presumably associated with shocks in a protostellar jet or outflow. The superburst maser was found to reside in one of the arc structures that comprised around seven other maser cloudlets which had not changed in their flux density. A comparison of the superburst flux density on the long and short EVN baselines revealed sub-milliarcsecond structure which aligned on the sky with two spatially separated regions of maser emission which flanked the position of the superburst.

The team concluded that the superburst maser had occurred as a result of a rare spatial alignment of two maser cloudlets along the observer's line of sight in which the background maser was subsequently amplified by the foreground maser. The two stage amplification lead to a spectacular surge in the measured flux density, most of which originating from a sub-milliarcsecond scale region.
Several weeks after the EVN observations the G25.65+1.05 water maser had returned to its quiescent phase, only to go superburst yet again, reaching a flux density of 130,000 Jy and making it one of the most powerful sources of maser emission in our Galaxy. The EVN investigation gleaned essential insights into the mechanism of action of the superburst class of maser sources, of which only two other maser sources are known.

The investigation, titled "VLBI observations of the G25.65+1.05 water maser superburst", was recently accepted for publication in the Monthly Notices of the Royal Astronomical Society.

R A Burns, G Orosz, O Bayandina, G Surcis, M Olech, G MacLeod, A Volvach, G Rudnitskii, T Hirota, K Immer J Blanchard, B Marcote, H J van Langevelde, J O Chibueze, K Sugiyama, Kee-Tae Kim, I Val‘tts, N Shakhvorostova, B Kramer, W A Baan, C Brogan, T Hunter, S Kurtz, A M Sobolev, J Brand, L Volvach, VLBI observations of the G25.65+1.05 water maser ‘Super-burst', Monthly Notices of the Royal Astronomical Society, , stz3172, https://doi.org/10.1093/mnras/stz3172

Image credit 

The scalar averaged, cross-power water maser spectrum measured with the EVN (solid, black line) is shown in comparison to single-dish spectra provided by the M2O on the same date (dashed red and blue lines), and (solid red and blue lines) pre-flare. The panel below highlights the low-flux density maser features. 

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