2015MNRAS.448.2284S

We report the discovery of 6.7 GHz methanol maser periodic flares in four massive star-forming regions and the updated light curve for the known periodic source G22.357+0.066. The observations were carried out with the Torun 32-m radio telescope between 2009 June and 2014 April. Flux density variations with period of 120 to 245 d were detected for some or all spectral features. A variability pattern with a fast rise and relatively slow fall on time-scale of 30-60 d dominated. A reverse pattern was observed for some features of G22.357+0.066, while sinusoidal-like variations were detected in G25.411+0.105. A weak burst lasting ∼ 520 d with the velocity drift of 0.24 km/s/yr occurred in G22.357+0.066. For three sources for which high-resolution maps are available, we found that the features with periodic behaviour are separated by more than 500 au from those without any periodicity. This suggests that the maser flares are not triggered by large-scale homogeneous variations in either the background seed photon flux or the luminosity of the exciting source and a mechanism which is able to produce local changes in the pumping conditions is required.