Nutsinee, Pat Meyers, Marissa

In the recent LHO locks, we've noticed a line in DARM that wanders around between ~410-420 Hz, consistently appearing in every lock from May 13th to the present. For example, here is a spectrogram of 2 hours from May 15.

There is a peak around that frequency in the Rayleigh plot (This plot shows the standard deviation of the PSD divided by the mean in each frequency bin. For a random Poisson-distributed signal, this ratio should be one.) For example, the Rayleigh plot from May 15 shows this peak, and going back through the summary pages, this non-stationarity in the spectrum appears to have been there since May 13.

Looking further back, we saw that there was a wandering line around the same frequency in the March 27th lock, but its frequency drifted much more slowly; see this 2 hour spectrogram.  

Pat has used STAMP-PEM to look for coherence with other channels but hasn't found any obvious culprit. We've also made spectrograms of many other channels, including MICH, PRCL, and SRCL, which don't show the same line.

Nutsinee also did some follow-up to see if this was the same as the "seismic bumbling line" from ETMX (see alog 17681), but found that it is quite different. For example, compare the DARM spectrogram with this 2 hour ETMX spectrogram from May 15 (at the same time as the spectrogram shown above for DARM). At this time, the seismic line wanders between 200 and 240 Hz, with no obvious correlations with the DARM wandering line.