An<i>RXTE</i>Archival Search for Coherent X‐Ray Pulsations in the Low‐Mass X‐Ray Binary 4U 1820−30
Bibliographic record
Abstract
As part of a large-scale search for coherent pulsations from low-mass X-ray binaries (LMXBs) in the Rossi X-Ray Timing Explorer ( RXTE ) archive, we have completed a detailed series of searches for coherent pulsations of 4U 1820-30, an ultracompact LMXB with a binary period of 11.4 minutes, located in the globular cluster NGC 6624. This binary's short period implies that any coherent signal would be highly accelerated, so we used phase modulation searches, orbital-parameter-fitting coherent searches, and standard acceleration searches to give significant sensitivity to millisecond pulsations. At a range of luminosities and in four energy bands, we searched a total of 34 archival RXTE observations, 32 of which had on-source integration times longer than 10 ks and some of which were made consecutively, which allowed us to combine them. We found no pulsations. Using our phase modulation search technique, which we ran on all 34 observations, we have been able to place the first stringent (95% confidence) pulsed fraction limits of ≲0.8% on all realistic spin frequencies (i.e., ≲2 kHz) and likely companion masses (0.02 M ☉ ≤ M c ≤ 0.3 M ☉ ). Using our orbital-parameter-fitting coherent search, which we ran on only 11 selected observations, we have placed a pulsed fraction limit of ≲0.3% on spin frequencies ≲1.25 kHz and companion masses M c ≤ 0.106 M ☉ . By contrast, all five LMXBs known to emit coherent pulsations have intrinsic pulsed fractions in the range 3%-7% when pulsations are observed. Our searches therefore rule out even pulsations with significantly lower pulsed fractions than those already observed.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".