Following the installation of the three accelerometers, we have checked their coherence with Hrec with respect to the corresponding sensors already present on DT, SR and IB towers. As can be seen in Figure 1, only the accelerometers on the DT (both on the turbo pump and the tower itself) show coherence at 401 Hz.
By checking the magnetometers in CEB (see Figure 2 and Figure 3), also in this case the only coherence in from the DT magnetometer, with the only exception of ENV_CEB_MAG which is placed near to the detection lab. Please remember that all towers have their own turbo vacuum pump, each one of them rotating at a frequency near 401 Hz but with a different random drift.
From a cross-check between Hrec, accelerometer and magnetometer (Figure 4) it is evident that those channels see the same signal. Another clue comes from the comparison of the time-frequency drift of the line in the channels (Figure 5).
As a final confirmation, we can thin about slightly changing the frequency of the turbo pump on DT while in LOW_NOISE_X and checking if the line in Hrec moves accordingly.
An open point is if the 401 Hz line enters Hrec through vibrational or magnetic coupling, and through which mechanism.