Seven-year periodic variations in the methanol maser line displayed by the massive protostar IRAS 20216+4104

Aims . We report the discovery and analysis of a periodic methanol maser in the massive protostar IRAS 20216+4104. Methods . To obtain the light curve, we used the 6.7 GHz methanol maser spectra collected between 2000–2003 and 2009–2023 with the Hartebeesthoek and Torun radio telescopes, as well as spectra from the literature reported prior to 1992. Results . The velocity-integrated flux density shows sinusoidal-like variations with a period of 6.9±0.03 yr. All but one of the features show periodic changes with a relative amplitude of 2 up to >89. A slightly variable feature displays a moderate anti-correlation between the flux density and the other significantly variable features. The maser emission appears to follow the continuum emission of the red-shifted outflow cavity. A maximum emission of 3.4 and 4.6 µm precedes the maser peak by 15% of the period and the (infrared) IR light centroids show time-dependent displacement. The periodic behaviour of the maser and IR emission is likely due to the eclipsing effect from a wobbling inner disk.