Related detchar request: #172

Key points
11.9 Hz comb is present in some ASC, IMC and ISI-HAM5 channels:

• H1_ASC-DHARD_P_OUT_DQ
• H1_IMC-DOF_4_Y_IN1_DQ
• H1_IMC-REFL_DC_OUT_DQ
• H1_ISI-HAM5_BLND_GS13RZ_IN1_DQ
• H1_ISI-HAM5_BLND_GS13X_IN1_DQ
• H1_ISI-HAM5_BLND_GS13Z_IN1_DQ

Fscan data on 2024-04-08 was analyzed with a 2-week duration. 11.9 Hz comb is common at both observatories. The 1st and 3rd harmonics (11.9hz and 35.7hz) are the strongest peaks to study.

Details
*The method is adapted from #77990.

The stamp-pem-comparison tool was used first to pick out possible witnesses from low-resolution STAMP-PEM data. The result of 2024-06 -- 2024-08 is shown in Figure 1. I then followed up the analysis with week-long Fscan data on 2024-04-08. This date was chosen because the comb appeared strong then, according to alog 72866. Figures 2-7 are the coherence plots for the witness channels listed above. The interactive plots for some more auxiliary channels, plots of data from June, speclong data and data spanning 2 weeks can be found under https://ldas-jobs.ligo-wa.caltech.edu/~zhiyu.zhang/11.9Hz_appearance/. These other plots confirm the same channels. The comb is not visible in one day in these auxiliary channels.

The results of this analysis only apply to LHO for now, but the same method can be applied to LLO data.