A spectrally stratified hot accretion flow in the hard state of MAXI J1820+070

We study the structure of the accretion flow in the hard state of the black-hole X-ray binary MAXI J1820+070 with NICER data. We use the power spectra to reconstruct the energy spectra of the variability components peaking at four different time scales. We find that the spectrum changes as a function of time scales. The two variability components peaking at longer time scales have similar shape, while the two peaking at the shorter time scales differ significantly. In particular, the one corresponding to the shortest time scales has the hardest spectrum. Both the variability spectra and the time-averaged spectrum are well-modelled by a disc blackbody and thermal Comptonization, but the presence of (at least) two Comptonization zones with different temperatures and optical depths is required. The disc blackbody component is highly variable, but only in the variability components peaking at the longest time scales ($\geq1$ s). The seed photons for the spectrally harder zone come predominantly from the softer Comptonization zone. Our results require the accretion flow in this source to be structured, and cannot be described by a single Comptonization region upscattering disc blackbody photons, and reflection from the disc.