Lomb-Scargle periodogram in the 3677−3686 μHz frequency range where a doublet and an independent frequency are above the 5.6σ detection threshold of local noise level. The bold black vertical segment and vertical dashed line indicate the position of one possible third component for the doublet, as shown in the bottom panel. The two top panels are the expanded view of the two components as indicated by the vertical arrows in the bottom panel. Both the fine structures of the two components show well-resolved closed peaks and have the same frequency separation of 0.044 μHz.