On the Broadening of the Characteristic Frequency Range towards Higher Photon Energies in the X-ray Variability of the Black Hole Transient MAXI J1820+070

Energy-dependence of X-ray Fourier power spectral states and the characteristic frequencies of the Band-Limited Noise (BLN) components have been seen in the hard state and intermediate states of black hole X-ray binaries. Here we report our analysis of the \emph{Insight}-HXMT observations of the black hole transient MAXI J1820$+$070 during its 2018 outburst when the source was brightest in hard X-rays. We found opposite trends of the low-frequency ($<$ 0.1 Hz) and the high-frequency ($>$ 10 Hz) BLN components, i.e., decreasing vs. increasing in frequency with increasing photon energy up to beyond 200 keV, respectively. This establishes an apparent two-way broadening of the power plateau formed by multiple BLNs in the power spectra towards higher photon energies. The trend of increasing characteristic frequency of the highest BLN component with increasing photon energy has been interpreted as due to that the corresponding seed photons which are up-scatted to relatively higher energies originate in a region relatively more central in the corona previously. Following the same framework, the decreasing trend of the characteristic frequency of the low-frequency BLN component with increasing photon energy can be interpreted as due to that the corresponding seed photons which are up-scattered to higher photon energies originate from further out in the disk flow but on the opposite side of the central corona as to the observer. The opposite trends then implies that the the plateau in the power spectra formed by the multiple BLNs represents the radial extension of the accretion disk that contributes seed photons which produce the observed BLNs; the higher the photon energy is, the wider the power plateau and the smaller the fractional variability are, probably approaching to a Power-Law Noise (PLN) seen in the soft state.