Virgo Logbook

Since the beginning of VSR4, some short drops in the horizon are usually visible. Sometimes they appear as arches in the spectrogram lasting a few minutes (but note that we haven't been able to identify single arches lasting between 0.1 and several seconds as with typical scattered light). They are probably induced by seismic noise, as seen during one earthquake which caused a bump decaying in frequency as would be caused by scattered light from an oscillating baffle decaying in amplitude (but again no individual arches were seen). Only in one incident, were we able to blame a local farmer.

To confirm rumors that the noise appears only during daytime, I plotted the
the BRMS of the dark fringe between 60 and 70 Hz versus time of day, for all the days of the run so far, see attached plot. Vertical scale is logarithmic and a median value calculated over +- 15 minutes has been subtracted for each point. The plot shows that the bumps appear mostly during work-days and between 7:30am (when the farmer wakes up?) and 6pm local time. This proves that the noise is caused by human activity.

Various people have been trying to correlate these bumps with seismic noise. Although there is usually some seismic activity visible in roughly the same period as the noise, no direct correlation has been found that can pinpoint the origin of the noise to one of the buildings.