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

Abstract: Energy-dependent X-ray power spectral states and Band-Limited Noise (BLN) components have been seen in the low-hard state and intermediate states of black hole X-ray binaries. Here we report our analysis of Insight-HXMT observations of the black hole transient MAXI J1820$+$070 during its 2018 outburst when the source was brightest. We found opposite trends of low-frequency ($<$ 0.1 Hz) and high-frequency ($>$ 10 Hz) BLN components, i.e., decreasing vs. increasing in frequency with increasing photon energy, respectively. This establishes an apparent two-way broadening of the power spectral plateau formed by multiple BLNs towards higher photon energies. The trend of the highest BLN component with increasing photon energy has been interpreted as that the corresponding seed photons originated from a region relatively more central in the corona previously. The decreasing trend of the characteristic frequency of the lowest frequency BLN component with increasing photon energy can then be interpreted as that the corresponding seed photons originated from further out in the disk but on the opposite side of the central corona to the observer. These opposite trends then imply that the power spectral plateau represents the radial extension of the accretion disk that contributed the seed photons producing the BLNs, and show that the higher the photon energy is, the wider the plateau and the smaller the fractional variability. The plateau shows the analogy to the flat power spectrum with a low fractional variability of the Power-Law Noise seen in the high-soft state, which corresponds to photons from the entire x-ray disk.