As reported in 79897, a bump has been apparent in the H1 spectrum in the vicinity of the Crab pulsar for much of O4b.

So far, it looks like:

• This problem is distinct from the 60 Hz broadening due to issues with line subtraction (this noise feature shows up in DELTAL_EXTERNAL).
• CS microphone and accelerometer channels look suspicious, particularly H1:PEM-CS_MIC_LVEA_BS_DQ, but the noise shows up earlier there than in DARM. If it is the same noise, something must have changed with the coupling during the break.

The attached plots are all generated from Fscan weekly and monthly averaged data. The feature can be seen most clearly in these long-duration spectra.

• Fig 1: Comparison of October monthly spectra, STRAIN_CLEAN vs DELTAL_EXTERNAL. The location of interest is marked with a vertical pink line (this marker is actually too narrow, just showing general location). The 60 Hz broadening, described in 80624, mostly hides the bump in STRAIN_CLEAN, but it's still evident in DELTAL_EXTERNAL.
• Fig 2: Comparison of spectrograms made from weekly averages, STRAIN_CLEAN vs DELTAL_EXTERNAL. The region of interest is circled in red. This seems consistent with the timeline Keith reported in 79897. (Note: The colorbars here are not equivalent and have been adjusted for visibility. Some small data gaps are due to broken Fscan production jobs, which may differ by channel.)
• Fig 3: Same as Fig 2, but zoomed in and with slightly different color bar adjustment.
• Fig 4: The DELTAL_EXTERNAL zoom from Fig 3, compared with data from H1:PEM-CS_MIC_LVEA_BS_DQ . The tracks appear similar, but the noise in H1:PEM-CS_MIC_LVEA_BS_DQ predates its appearance in DARM. It doesn't show up in coherence between the two channels.

There is a git issue tracking the problem here, with some additional discussion history and more plots: https://git.ligo.org/detchar/detchar-requests/-/issues/273