Long-term monitoring of 6.7-GHz methanol masers

A sample of 54 6.7-GHz methanol masers was monitored using the Hartebeesthoek 26-m telescope during the period 1999 January – 2003 March. The observations were taken at 1–2 week intervals, with daily observations when possible if a source was seen to be varying rapidly. It was found that the majority of the sources display a significant level of variability. The time-range of variations range from a few days up to several years. The types of behaviour observed included non-varying, monotonic increases or decreases, as well as aperiodic, quasi-periodic and periodic variations. Seven sources show clear evidence of periodicity, with periods ranging from 132 d up to 520 d.