Some first results on different combs at Virgo were discussed during the weekly detchar meeting on Friday 20/03/2020 (slides on TDS: https://tds.virgo-gw.eu/?content=3&r=16982)

The general conclusions in this presentation were (Fig 1 & 2):

- There is a 1 Hz and 5Hz comb @ Virgo at integer frequencies

- The 5Hz comb is relative "stable", while the 1Hz comb misses a lot of lines in Hrec

- looking at the coherence between Hrec and a magnetometer (in example ENV_CEB_MAG_W) enables us to identify combs which are not directly visible in Hrec. They are probably just below the noise floor but using efficient witness sensors we can try to identify more peaks. This can be of great importance for the stochastic search since the noise floor in this search will go down, by cross correlating detectors.

- Since they are at exact integer frequencies they are related to our own electronics and the clocks which are driving them.

During this meeting a reference was made to some recent work (10/03/2020) on reducing the 5 Hz comb (https://tds.virgo-gw.eu/?content=3&r=16982). The results in the presentation used data before this activity.

Here two more time segments - afer the activity - are reported and discussed. For both 4 figures are given. First an overview of the region ~0-700 Hz. Afterwards there are details of 3 different lines of the 5Hz comb: 125Hz, 130Hz and 695Hz. (Fig 3-6 & 7-10)

The conclusions are the same for the two different time segments.

- The overall sensitivity at low frequencies is not as good as during the first analysis - before the intervention. Choosing a different time segment seemed not a possibility without cutting in the time length (36 hours)

- In al three peaks the amplitude is higher after the activity compared to the reference of before the activity.

At first sight this might indicate that instead of improving, the 5 Hz comb might be even stronger present in Hrec. However there are several factors we should take into account:

- As mentioned the sensitivity at low frequencies is lower in the time segments after the activity. If the quality of the lock is lower this might also enhance combs.

- Maybe the number or severity of BNS range drops can play an important role in accumulating more noise in this 5Hz comb lines. (Fig 11-13)

Further investigation is ongoing to determine whether was no, a negligible, a positive or a negative effect from the performed activity on the 5Hz comb in Hrec.