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