We present a detailed analysis of the spectral and timing characteristics of
a 7-Hz type-A quasi-periodic oscillation (QPO) detected in NICER observations
of the black hole X-ray binary MAXI J1348-630 during its high-soft state. The
QPO is broad and weak, with an integrated fractional rms amplitude of 0.9 per
cent in the 0.5-10 keV band. Thanks to the large effective area of NICER,
combined with the high flux of the source and a relatively long accumulative
exposure time, we construct the first rms and phase-lag spectra for a type-A
QPO. Our analysis reveals that the fractional rms amplitude of the QPO
increases with energy from below 1 per cent at 1 keV to 3 per cent at 6 keV.
The shape of the QPO spectrum is similar to that of the Comptonised component,
suggesting that the Comptonised region is driving the variability. The phase
lags at the QPO frequency are always soft taking the lowest energy as
reference. By jointly fitting the time-averaged spectrum of the source and the
rms and phase-lag spectra of the QPO with the time-dependent Comptonisation
model vkompthdk, we find that the radiative properties of the type-A QPO can be
explained by a vertically extended Comptonised region with a size of 2300 km.Comment: 7 pages, 5 figures, accepted for publication in MNRA