In summary, the different components show similar evolutionary trends for the fractional rms and characteristic frequency with the hardness ratio in the LE/ME/HE energy bands. The evolutionary trend can be explained under the truncated disk/corona model (Esin et al. 1997; Done et al. 2007). Actually, there is still debate on the truncation of the accretion disk in MAXI J1820+070. Buisson et al. (2019) discovered a steady inner accretion disk measured by relativistic reflection. Kara et al. (2019) found that the reverberation time lags between the continuum-emitting corona and the irradiated accretion disk are much shorter than previously seen in truncated accretion disks, and the timescale of the reverberation lags is shortened by an order of magnitude over a period of weeks, whereas the shape of the Fe Kα emission line remains remarkably constant. Similar results are also obtained from spectral analysis. Meanwhile, there are some other studies that support the truncated accretion disk argument by either spectral analysis or timing analysis (De Marco et al. 2021; Marino et al. 2021; Zdziarski et al. 2021a, 2021b). De Marco et al. (2021) found that the frequency of the thermal reverberation lags increases steadily, and, on the other hand, the temperature of the quasi-thermal component grows as the source softens, which can be explained in terms of a decrease in the disk inner radius. Moreover, De Marco et al. (2021) measured that the values of the lag amplitude are a factor of 3 longer than those reported in Kara et al. (2019). The longer lags might not be easily reconciled with the conclusion of a disk extending close to the innermost stable circular orbit. Zdziarski et al. (2021a, 2021b) confirmed the existence of an optically thick disk of at least >10R

g
 from joint spectral analysis. To sum up, so far, all arguments in favor of the nontruncated disk model can be reasonably explained under the truncated disk model. Except for the methods mentioned above, in the present paper, we confirm the truncated accretion disk model in MAXI J1820+070 from the evolutionary trend of broadband noise components with a new perspective by means of the correspondence relation between break frequency and radiation region radius. Quantitatively, we can calculate the radiation region at a different energy band of L
2, which represents the variable emission from the outermost region (see Figure 13). We take a parameter set for a standard α-disk (α = 0.1, M
BH = 10M
⊙, scale height H/r = 0.1) to calculate the viscous frequency at a certain radius (Kato et al. 2008). As Figure 13 shows, the characteristic frequency of the L
2 component shows an energy dependence: the emitting region spans from ∼34R

g
 to ∼27R

g
 corresponding to 1–150 keV photon energy. Like Dziełak et al. (2021) and Kawamura et al. (2022) found using frequency-resolved spectral analysis that the L
2 component is supposed to come from variable disk emission. However, making use of the ME and HE data from Insight-HXMT, we actually detect high-energy emission >100 keV from the L
2 component, which cannot be attributed to a simple standard accretion disk. Considering the change of radius plotted in Figure 13, even though we can attribute the high-energy emission to the propagation of fluctuation from the disk to the hot flow, we should also expect a constant characteristic frequency for L
2 at the high-energy band according to fluctuation propagation. The characteristic frequency of L
2 remains unchanged below 20–30 keV as Kawamura et al. (2022) found the constant peak frequency for P1. However, when the photon energy is greater than 20–30 keV, the radiation radius of L
2 starts to decrease to 27R

g
. This phenomenon cannot be easily interpreted by fluctuation from a disk propagating to the hot flow. Therefore, we speculate that L
2 may originate in a warm extended variable disk region. We therefore should consider a more complicated accretion flow geometry where a standard accretion disk transits to a hot advection-dominated accretion flow geometry.
Citation: Marino et al. 2021
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