Type-A quasi-periodic oscillation in the black hole transient MAXI J1348-630

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 percent in the 0.5-10keV 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 percent at 1keV to similar to 3 percent at 6keV. The shape of the QPO spectrum is similar to that of the Comptonized component, suggesting that the Comptonized 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 Comptonization model vkompthdk, we find that the radiative properties of the type-A QPO can be explained by a vertically extended Comptonized region with a size of similar to 2300km.