Continuing the investigation of the residual spectral noise after the problems of the past weeks, I came across this wandering line at about 260 Hz. This line may have appeared after the July break, as reported in this entry by Irene #64838, and whose coherence with accelerometers in the detection area has been described in this entry #64873.

Figure 1: median-normalized spectrogram of the strain data since August in the frequency range 256-268 Hz. A structure (line plus sidebands at about +/-1 Hz) seems to be present initially at about 262 Hz. This structure is visible in this plot from August. From August 9, it seems to have disappeared, to reappear in mid-September at around 260 Hz. The sideband structure seems to suggest that this is the same structure as the previous month. This line seems then to have made a jump down to 259 Hz on October 20. Some transients at larger frequencies between the end of October and early November, to then increase in intensity since mid-November.

Figures 2 to 4 show various peculiar moments in the evolution of this line until recently.

Looking at the daily results of BruCo in the 260 Hz region on days when this line appears strongest, the coherence with various accelerometers in the detection area seems to be confirmed.

Further analysis of the data would be necessary to confirm these correlations and to understand what caused the jumps, the disappearance or increase of intensity of this line. Furthermore, in situ investigations could shed more light on what the real origin of this is.