# Is this approximately 28 Hz ringing transient in L1 at GPS 1382955253.17 a known O4a glitch morphology? **URL:** \ **Category:** Data Analysis\ **Created:** [September 3, 2026, 5:12pm UTC](https://ask.igwn.org/t/is-this-approximately-28-hz-ringing-transient-in-l1-at-gps-1382955253-17-a-known-o4a-glitch-morphology/1579 "2026-09-03T17:12:12Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![lucacirfeta](https://yyz1.discourse-cdn.com/flex031/user_avatar/ask.igwn.org/lucacirfeta/32/645_2.png) [@lucacirfeta](https://ask.igwn.org/u/lucacirfeta)\ **Post date:** [September 3, 2026, 5:12pm UTC](https://ask.igwn.org/t/is-this-approximately-28-hz-ringing-transient-in-l1-at-gps-1382955253-17-a-known-o4a-glitch-morphology/1579/1 "2026-09-03T17:12:12Z")
Hi all, I am posting a self-contained summary of an unusual transient found in public L1 O4a strain. I am an independent researcher without LVK-side access, and I would appreciate help determining whether this is a known instrumental morphology. **This is not a detection claim, a claim of a new glitch class, or a globally calibrated significance claim.** The purpose of this post is to ask what instrumental information may be missing from the public data. ### Event summary | Property | Value | | --- | --- | | Detector | **L1** | | Observing run | O4a | | Analysis window | GPS **1382955232–1382955264** (32 s) | | Historical catalogue label | GPS 1382955228, corresponding to the start of the padded crop rather than the analysis window | | Localized feature | GPS **1382955253.17** , or +21.17 s from the analysis-window start | | Frequency | Approximately **28–30 Hz** , primarily within 26–42 Hz | | Morphology | Sharp impulsive onset followed by several seconds of narrowband ringing, with weaker structure near 50–60 Hz | ![forum_L1_1382955253_corrected](https://canada1.discourse-cdn.com/flex031/uploads/gwosc/original/1X/16b6856502abcecfc176a07b7a79b3bbae1bbcb9.png) The public-strain characterization places the in-band peak at 28.0 Hz. A separate measurement gave 28.4 Hz, so the two determinations agree within 0.4 Hz. The in-band energy is approximately 304 times the mean of 16 adjacent 32 s windows. Replacing the mean with the median as a sensitivity check gives approximately 315 times. These values are only loudness descriptors; they are not calibrated significances because whitening can react strongly to a transient of this amplitude. ### What the anomaly detector selected The anomaly score is produced by Top-_k_ pooling over spectrogram patches. After correcting the diagnostic described below, the selected patches were found to concentrate on the feature itself rather than diffusely across the window. The selected region spans approximately GPS 1382955253–1382955264 in the 26–42 Hz band. The feature at GPS 1382955253.17 lies at the beginning of this region, and 20 of the 68 selected patches fall within ±1 s of it. The corrected detector-aware reconstruction classifies this window as **ROBUST** , with a native anomaly score of **0.6122661233**. In this context, ROBUST is a pipeline classification relative to the detector-specific reference distribution; it is not an astrophysical classification. ### Corrections made during the investigation For transparency, three errors affected an earlier diagnostic interpretation: 1. A candidate lookup used `np.isclose(gps, target, atol=0.5)` without setting `rtol=0`. At GPS values near 1.38 × 109, NumPy's default relative tolerance produced an effective tolerance of approximately 13,800 s and silently selected a different event about 3.8 hours away. 2. The Q-transform axes were transposed. The array is ordered as time by frequency, but it had been interpreted as frequency by time. Both dimensions were 256 pixels after resizing, so this produced no shape error. 3. The quoted GPS 1382955228 identified the start of the padded crop. The 32 s analysis window actually begins four seconds later, at GPS 1382955232. These errors produced the incorrect claim that the selected patches were diffuse and that the approximately 30 Hz feature had not driven the anomaly score. That claim is withdrawn. With the correct event, axes, and time convention, the selected patches concentrate on the ringing feature. I also tested whether a change in the major version of the analysis library could explain numerical differences. Isolated runs using versions 3.0.13 and 4.0.1 produced spectrogram agreement at approximately 5 × 10−10 and identical scores to six decimal places. The library version was therefore not the source of the discrepancy. ### H1 cross-detector checks A raw public-data H1–L1 correlation calculation gave a maximum of 0.0585 at −11.96 ms. Repeated calculations showed that the maximizing lag can move to the edge of the search interval. The value and lag should therefore not be treated as a stable measurement; the result is consistent with no resolved H1 counterpart. An earlier stored production coincidence check gave an on-source statistic of 0.0716, compared with a time-shift null mean of 0.1970 and a null maximum of 0.2864 over seven admissible shifts. With so few shifts, I do not assign a tail probability to this comparison. It supports only the limited conclusion that no robust H1 counterpart was resolved. In the subsequent corrected reconstruction, the opposite-detector data were evaluated regardless of the partner window's class or presence in the candidate catalogue. This event still did not exceed the corrected pooled-null threshold used to select cross-detector follow-ups. There is therefore no new corrected cross-detector support for it. The pooled-null threshold is a diagnostic follow-up rule. It is not a formal globally trial-corrected significance threshold. ### Auxiliary-channel limitations An earlier test on the public L1 auxiliary subset found a maximum coherence of 0.478, below a family-wise threshold of 0.663 derived from time-shift surrogates. The result reproduced exactly when rerun. However, the public subset available for this check contains only nine L1 channels and does not include a seismometer or accelerometer. It also does not provide the full seismic, suspension, and ASC witness set. Consequently, “aux-clean” means only that no coupling was resolved in the tested public channels. It does **not** exclude scattered-light coupling, mechanical excitation, or another instrumental mechanism visible only in unavailable witness channels. The corrected follow-up used a 65-event PEM shortlist consisting of 9 ROBUST primary targets and 56 AMBIGUOUS diagnostic targets. This event did not enter that shortlist, so there is no new corrected PEM result to report for it. ### Catalogue reconstruction and boundary audit A later provenance audit identified a boundary-condition defect in the historical frozen catalogue. Each 32 s analysis window should receive 40 s of strain: 4 s of context on both sides, followed by whitening and cropping. For 169 of 10,429 frozen detector–GPS windows, the historical input crossed an HDF5 boundary and supplied only 36 s, omitting the right-hand context. In a controlled replay using the historical representation and historical detector-specific thresholds, restoring the complete context changed 120 class assignments and reversed 97 routing decisions, all from ESCALATE to ROUTINE. The subsequent end-to-end reconstruction rebuilt both the detector-aware reference representation and the detector-specific calibration. It produced 10,942 corrected candidates. The comparison contains 10,022 shared identities, 407 historical-only identities, and 920 corrected-only identities. Among the shared identities, 1,626 class assignments differ. Those broader differences cannot be attributed solely to the padding defect because the reference representation and thresholds changed together. The controlled 169-window replay is the direct evidence for the padding effect; the full-catalogue comparison is descriptive. For the specific L1 window discussed here, the corrected classification remains ROBUST. It does not exceed the corrected pooled coincidence threshold and is not part of the corrected PEM shortlist. ### Other limitations - The publicly released morphology classifications available to me do not cover O4a. A failed catalogue match is therefore a coverage gap, not evidence of novelty. - I have not been able to check the complete hardware-injection and detector-state information for this time. - This is a single observed instance in the search. It cannot establish a new glitch class. - Its instrumental origin is consistent with the available evidence, but remains an inference rather than a demonstrated identification. ### Questions for people with detector-side access 1. Is the impulsive-onset-plus-approximately-28 Hz ringing shape a known O4a morphology in L1? It appears qualitatively similar to low-frequency scattered light, but I do not have enough witness information to classify it. 2. Is there an Omicron trigger associated with GPS 1382955253.17 in L1, particularly in the 26–42 Hz band? 3. Does the full seismic, suspension, or ASC channel set show a witness around this time? 4. Was L1 in a normal operating state, or was there any known commissioning activity, hardware injection, or instrumental excursion? The most defensible description I can currently give is: **a loud, L1-local, approximately 28 Hz ringing transient, classified as a strong single-detector anomaly, with no resolved H1 counterpart, but still instrumentally unclassified.** If this is a familiar instrumental artifact, identifying it would be extremely useful. It would clarify both this event and the limits of anomaly searches based only on public strain and a restricted auxiliary-channel subset. Thanks, Luca Cirfeta Independent researcher, Rome ---
**Author:** ![jglanzer28](https://avatars.discourse-cdn.com/v4/letter/j/5daacb/32.png) [@jglanzer28](https://ask.igwn.org/u/jglanzer28)\ **Post date:** [September 16, 2026, 6:22pm UTC](https://ask.igwn.org/t/is-this-approximately-28-hz-ringing-transient-in-l1-at-gps-1382955253-17-a-known-o4a-glitch-morphology/1579/2 "2026-09-16T18:22:46Z")
Hi Luca, My name is Jane Glanzer and I am a postdoc for the LIGO Lab at Caltech. Myself and another detector characterization expert took a look at this noise. There is indeed an Omicron trigger in this time segment in the L1 data. The gpstime of the trigger is 1382955257.875 and the associated frequency is 28.62 Hz. The transient is also visible in the Omicron glitchgram. The morphology does not look like scattered light, scattered light transients usually have an arch shape, it may belong to closely related low frequency lines category, the rate of which is also highly correlated with microseismic ground motion. And this day did have high ground motion in the microseismic band. Having said that, GravitySpy does not classify this transient. An internal tool used to look at linear coherence between the gravitational wave data channel and thousands of auxiliary channels was used to look into this noise. No interesting coherences were observed in the auxiliary channels during the period of this noise around ~11 hz, ~20hz, or ~30 hz. The public alogs ( [aLIGO LLO Logbook](https://alog.ligo-la.caltech.edu/aLOG/) ) do not have any mention of anything interesting related to this noise, or of any site activities that may have been the cause. The detector was in normal Observing mode during this time. We have not noticed this noise as a recurring disturbance in the data, at least not at a SNR ratio where it would be caught by Detector Characterization tools. Given that LIGO is a very sensitive detector, it is highly possible that multiple such transients exist in the data which are caused by very short duration, local disturbances related to the environment. More generally, we continually work to study, identify, model, and mitigate significant sources of transient noise. The severity of such noise depends on several factors, particularly its occurrence rate and signal-to-noise ratio. For this particular example, neither the rate nor the SNR appears concerning. Thanks, Jane Glanzer ---
**Author:** ![lucacirfeta](https://yyz1.discourse-cdn.com/flex031/user_avatar/ask.igwn.org/lucacirfeta/32/645_2.png) [@lucacirfeta](https://ask.igwn.org/u/lucacirfeta)\ **Post date:** [September 16, 2026, 9:15pm UTC](https://ask.igwn.org/t/is-this-approximately-28-hz-ringing-transient-in-l1-at-gps-1382955253-17-a-known-o4a-glitch-morphology/1579/3 "2026-09-16T21:15:56Z")
Hi Jane, Thank you very much to you and your colleague for taking the time to investigate this. This resolves the detector-side questions that I could not answer from public data. The Omicron result is particularly helpful. Its 28.62 Hz frequency agrees closely with the approximately 28.0–28.4 Hz feature measured independently from the public strain. The trigger time, GPS 1382955257.875, falls within the several-second ringing structure that begins near GPS 1382955253.17, so these observations appear to describe the same disturbance. It is also important to know that the morphology does not resemble scattered light, GravitySpy does not classify it, high microseismic ground motion was present that day, no interesting linear coherence was found in the internal auxiliary-channel survey, and the detector was in normal Observing mode with no relevant site activity recorded. I will therefore describe it conservatively as an Omicron-confirmed, L1-local transient near 28.6 Hz, possibly related to the low-frequency-lines family in the context of elevated microseismic motion, but still unclassified and without an identified auxiliary witness. This also clarifies the meaning of the anomaly score: the pipeline identifies the window as morphologically unusual relative to its reference population, but that does not imply a concerning SNR, occurrence rate, or operational significance. Thank you again for the careful and informative investigation. This fully resolves the question I raised. Best, Luca