A faint but large spectral artifact (bump) can be seen wandering around the 50 Hz mains spectral lines as one can see on figure 1. This artifact can go down to around 40 Hz and seems to have appeared around march 17th/18th. It was not present at the beginning of february for instance.
Recent BruCo reports do not seem to find any coherent channels with it.
This wandering line was likely already present in the March 13 data, but not in the March 12 data, i.e., during the first recovery after the electrical shutdown. The onset of the source may therefore be uncorrelated with the shutdown.
I am attaching two plots showing the median-normalized spectrogram of the strain: one covering March 12–13, and another starting from March 18, where the spectral line is particularly visible with the better sensitivity in the detuned SR configuration achieved in recent days.
Could be related to this issue found in the past: #61815?
I expect this bump to be due to the squeezer shutter being opened but the squeezing not being injected. From what I remember the shutter on SDB1 towards squeezing was openend on March 12-13, then closed for the weekend and reopned in the middle of the week. And it has remained opened since then.
This line's amplitude continuously decreased and cannot be clearly seen after midnight today.
