Identifying the Mechanisms of Water Maser Variability During the Accretion Burst in NGC6334I

Abstract

HMYSOs gain most of their mass in short bursts of accretion. Maser emission is an invaluable tool in discovering and probing accretion bursts. We observed the 22 GHz water maser response induced by the accretion burst in NGC6334I-MM1B and identified the underlying maser variability mechanisms. We report seven epochs of VLBI observations of 22 GHz water masers in NGC6334I with the VERA array, from 2014 to 2016, spanning the onset of the accretion burst in 2015.1. We also report 2019 ALMA observations of 321 GHz water masers and 22 GHz maser monitoring by HartRAO. We analyze variability patterns and use proper motions with the 22 GHz to 321 GHz line ratio to distinguish between masers in C-shocks and J-shocks. We also calculated the burst-to-quiescent variance ratio of the single-dish time series. 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. 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. We propose that radiative heating of H2 due to high-energy radiation from the accretion burst be the mechanism for the flaring in CM2-W2. The southern regions are excited by J-shocks and have short-term flaring and dampening of water masers. We attributed the diverse variability patterns in the southern regions to the radiative transfer of the burst energy in the source. 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.