Virgo Logbook

Looking back at some O2 data, it seems that some of the big drops in the horizon are in coincidence with a big increase in the RMS of Hrec, see fig 1. After such glitches, an excitation of several resonances of the payloads can be observed: the 8 Hz mode, which is the vertical bounce mode, typically only 1 out of the 16 violin modes, and one of the drum modes. Using the last one, the guilty mirror can be identified. I found 3 cases (see fig 2-4), the first two belong to the WI (drum at 7810 Hz) and the latter to the WE payload (drum at 7812 Hz). Looking in the time domain, a distinct glitch can be seen in DARM, corresponding to a minuscule step in hrec, see fig 5 and 6. This shape of the glitch in DARM is likely the impulse or step response of the loop.

This reminds a bit of the old WI glitches, which were attributed to a relaxation process in the glue of the mirror magnets, but in that case only the drum-mode was excited. Since the current glitches are also seen in the bounce mode, and in a single violin mode only, it might be something like a metal wire slipping a bit over the stand-off prism. According to Paolo, there are some known issues with the WI payload, this might be related.

A more systematic study of all big drops in the horizon should be made. Some of these might be related to the payloads, but there might be other issues. If there are enough of these events, they should be vetoed.