Identifying the mechanisms of water maser variability during the accretion burst in NGC6334I

Context. High-mass young stellar objects gain most of their mass in short intense bursts of accretion. Maser emission is an invaluable tool in discovering and probing these accretion bursts. Aims. Our aim was to observe the 22 GHz water maser response induced by the accretion burst in NGC6334I-MM1B and to identify the underlying maser variability mechanisms. Methods. We report seven epochs of very long baseline interferometry (VLBI) observations of 22 GHz water masers in NGC6334I with the VLBI Exploration of Radio Astrometry (VERA) array, from 2014 to 2016, spanning the onset of the accretion burst in 2015.1. We also report 2019 Atacama Large Millimeter/submillimeter Array (ALMA) observations of 321 GHz water masers and 22 GHz single-dish maser monitoring by the Hartebeesthoek Radio Astronomical Observatory (HartRAO). We analysed long-term variability patterns and used proper motions with the 22 GHz to 321 GHz line ratio to distinguish between masers in non-dissociative C-shocks and dissociative J-shocks. We also calculated the burst-to-quiescent variance ratio of the single-dish time series. Results. We detected a water maser distribution resembling a bipolar outflow morphology. The constant mean proper motion before and after the burst indicates that maser variability is due to excitation effects from variable radiation rather than jet ejecta. For the whole region, we find that the flux density variance ratio in the single-dish time series can identify maser efficiency variations in 22 GHz masers. The northern region, CM2-W2, is excited in C-shocks and showed long-term flaring with velocity-dependent excitation of new maser features after the onset of the burst. We propose that radiative heating of H 2 due to high-energy radiation from the accretion burst be the main mechanism for the flaring in CM2-W2. The southern regions are excited by J-shocks, which have shown short-term flaring and dampening of water masers. We attribute the diverse variability patterns in the southern regions to the radiative transfer of the burst energy in the complex source geometry. Conclusions. Our results indicate that the effects of source geometry, shock type, and incident radiation spectrum are fundamental factors affecting 22 GHz maser variability. Investigating water masers in irradiated shocks will improve their use as a diagnostic in time-variable radiation environments, such as accretion bursting sources.