  Spectral-timing analysis of the fast variability observed in X-rays is a
powerful tool to study the physical and geometrical properties of the
accretion/ejection flows in black-hole binaries. The origin of type-B
quasi-periodic oscillations (QPO), predominantly observed in black-hole
candidates in the soft-intermediate state, has been linked to emission arising
from the relativistic jet. In this state, the X-ray spectrum is characterised
by a soft-thermal blackbody-like emission due to the accretion disc, an iron
emission line (in the 6-7 keV range), and a power-law like hard component due
to Inverse-Compton scattering of the soft-photon source by hot electrons in a
corona or the relativistic jet itself. The spectral-timing properties of MAXI
J1348-630 have been recently studied using observations obtained with the NICER
observatory. The data show a strong type-B QPO at ~4.5 Hz with increasing
fractional rms amplitude with energy and positive lags with respect to a
reference band at 2-2.5 keV. We use a variable-Comptonisation model that
assumes a sinusoidal coherent oscillation of the Comptonised X-ray flux and the
physical parameters of the corona at the QPO frequency, to fit simultaneously
the energy-dependent fractional rms amplitude and phase lags of this QPO. We
show that two physically-connected Comptonisation regions can successfully
explain the radiative properties of the QPO in the full 0.8-10 keV energy
range.
