Periodicity in Class II methanol masers in high-mass star-forming regions

Abstract

We report the results of 10 years of monitoring of six regularly varying 6.7 GHz methanol masers using the Hartebeesthoek 26-m telescope. Observations were done at intervals of 1-2 weeks, with faster sampling during flaring episodes. Four of the sources were also monitored at 12.2 GHz and show correlated variations.We find the Lomb-Scargle periodogram to be the most sensitive method to search for periodicity but possibly prone to false detections. Periods range from 132.8 d (with 26 cycles observed) to 509 d (with seven cycles observed). Five of the sources show arguably periodic variations, while G331.13−0.24 shows strong periodicity in one peak, with large and variable delays in other peaks