a) Running-difference narrowband maps of the umbral oscillations signals obtained at 304 Å, integrated from 02:30 to 03:30 UT, at the frequencies 11.11 mHz (1.5 min), 8.3 mHz (2.0 min), 6.7 mHz (2.5 mHz), and 5.2 mHz (3.2 min). b) Dependence of the narrowband power distribution across the umbra (in the horizontal direction via the sunspot centre, see the dashed arrow in the 5.2 mHz map in panel a)) on the oscillation period. The vertical dashed lines show the umbral boundaries. The digits indicate the spectral power enhancements that coincide with the spectral peaks in Fig. 2b.