Continuing the investigation of the spectral noise on the first week of O4b data, we found an interesting structure of "wandering sidebands" around the 50 Hz line. (S

Figure 1 shows the spectrogram in the region 42-58 Hz around the frequency of the electric mains. Two pairs of wandering sidebands are visible at 44 (Figure 2) and 56 Hz, and at 46.7 (Figure 3) and 53.3 Hz (Figure 5).

The oscillations in frequency, especially those of the 44 and 56 Hz lines, are the same as those of the 50 Hz line (Figure 4) but enlarged by many (~20) times: ~0.50 Hz vs ~0.025 Hz

Additionally, taking advantage of a couple of glitches present in the analyzed data period (the vertical lines), we notice that the 44 and 47.3 Hz lines are synchronous with the 50 Hz line. Also, the 53.3 Hz line seems to have the same phase, while the one at 56 Hz is flipped vertically: Figure 5.

The search for bilinear couplings with MONET highlighted that the 46.7 and 53.3 Hz sidebands are produced by the modulation of the mains line (ENV_CEB_UPS_VOLT_R_2000Hz) and the ASC_DIFFp_T{X,Y} angular control channels: Figure 6 and text file.

A barely negligible coherence has been found for the lines at 44 and 56 Hz; just a 0.1 with SDB1_LC_TX at 56 Hz: Figure 7. This could be a consequence of the large variations in the frequency of the line, which dilute the cohere over multiple frequency bins and, averaged gets small.

Of course, the same structure is present around the other harmonics of the 50 Hz line.