Abstract:
The sources of four peaks in DARM sensitivity are identified as follows:
43 Hz: ITMY pitch,
43.26 Hz: ETMY pitch,
44.03 Hz: ITMX pitch,
44.48 Hz: ETMX pitch.

Detail:
Owing to VIS noise budget, we confirmed the low frequency noises are coming from several suspensions and MICH/PRCL controls.
However, there are still some peaks in DARM sensitivity, which do not appear in the noise budget.
So, I started to identify the sources of these peaks.

I measured the spectrum of DARM with 0.001 Hz bandwidth (fig1).
From the experiences during O3 commissioning, peaks around 44 Hz are coming from pitch resonances of Type-A suspensions, so I first try to confirm which peak is coming from which suspensions for the demonstration of peak identification.

First, I measured the spectra of MNP OpLevs without excitation (blue lines in fig2 - fig5).
Then, I injected the white noise excitation from 39 Hz to 45 Hz with 100cnts.
Red lines in fig2 - fig5 shows the spectra with injections.
Excited peaks can clearly be seen in the MNP OpLev signals.

Comparing the obtained peaks with that in DARM sensitivity, we can confirm following peaks.
43 Hz: ITMY pitch
43.26 Hz: ETMY pitch
44.03 Hz: ITMX pitch
44.48 Hz: ETMX pitch

I will continues the peak identification at other frequency bands.