2011A&A...531L...3S

We report the discovery of periodic flares of 6.7GHz methanol maser in the young massive stellar object G22.357+0.066. The target was monitored in the methanol maser line over 20 months with the Torun 32m telescope. The emission was also mapped at two epochs using the EVN. The 6.7GHz methanol maser shows periodic variations with a period of 179days. The periodic behavior is stable for the last three densely sampled cycles and has even been stable over ∼12years, as the archival data suggest. The maser structure mapped with the EVN remains unchanged at two epochs just at the putative flare maxima separated by two years. The time delays of up to ∼16days seen between maser features are combined with the map of spots to construct the 3-dimensional structure of the maser region. The emission originating in a single ∼100AU layer can be modulated by periodic changes in the infrared pumping radiation or in the free-free background emission from an HII region.