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.
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