Detection of the longest periodic variability in 6.7 GHz methanol masers of G5.900-0.430

Long-term monitoring observations with the Hitachi 32 m radio telescope of the 6.7 GHz methanol masers associated with the high-mass star-forming region G5.900-0.430 are presented. A period of flux variability at approximately 1260 d is detected in the features at V_LSR = 9.77 and 10.84 km s^-1, while a secondary shorter period, 130.6 d, is determined for the 0.66 km s^-1 feature. This is only the second source which has two different periods. The period of ~1260 d is approximately twice as long as the longest known period of 6.7 GHz methanol masers. The variability pattern of the symmetric sine curves and the consistency with the expected period-luminosity relation suggest that the mechanism of the maser flux variability of the 9.77 and 10.84 km s^-1 features in this source can be explained by protostellar pulsation instability. On the other hand, because the 0.66 km s^-1 feature has an intermittent and asymmetric variability profile, we propose that this feature is explained by the colliding wind binary or spiral shock models. Obtaining the spatial distribution of the 0.66 km s^-1 feature using very long baseline interferometry will lead to a better understanding of this source.