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astro-ph.HE · High Energy Astrophysical Phenomena

GBM data show net spin-up of OAO 1657−415 in 2025 between net spin-down in 2024 and the sampled part of 2026

We measure recent frequency changes of the X-ray pulsar OAO 1657−415 from public Fermi GBM data saved on 22 October 2026 UTC. The first and last detected frequencies in 2024 differ by −52.964 µHz. The change is +52.294 µHz from 6 January to 9 December 2025 and −13.575 µHz from 7 January to 28 September 2026. Formal errors from the supplied endpoint errors are 0.386, 0.455, and 0.398 µHz. These errors assume independent measurements and omit timing model and sampling errors. Means of the first and last 3 or 5 detections retain all three signs. The 2025 gain therefore does not continue as a net gain between the tested 2026 endpoints. Short decreases occur within 2025, and earlier years also show net gains. The result extends the time range of prior studies; it does not establish a new type of spin change or a permanent state change. The old and new GBM formats repeat the same 951 detected frequencies, times, and errors. They are not independent data. Five measurements occur twice in each format. We retain 946 distinct measurements after removal of these duplicates. Swift BAT supplies only 44 good daily rates in the sampled part of 2026, which limits a joint brightness test. We provide exploratory fixed-window brightness descriptors without a physical claim. The package preserves exact source files, hashes, all window selections, code, figures, and an independent check of the FITS endpoint measurements. The study is exploratory and uses the public monitor timing solution.

OAO 1657-415 · X-ray pulsars · spin frequency · Fermi GBM · Swift BAT · public data · reproducible research

Admitted 2 citing publications

References

5
  1. doi:10.1093/mnras/stab3561

    Prior spin-change and brightness study through MJD 59515. Our fixed windows differ from its selected steady intervals and orbital brightness maxima.

  2. doi:10.1093/mnras/stab3436

    Prior spin history through July 2021. It describes a long decline with short reversals.

  3. doi:10.3847/1538-4357/adfa96

    Prior study of changes over decades. Its arXiv version 1 Table 1 ends the OAO series at MJD 60419. We measure later intervals.

  4. usesdoi:10.3847/1538-4357/ab855c

    Description of the Fermi GBM pulsar monitor that supplies the frequency measurements.

  5. usesdoi:10.1088/0067-0049/209/1/14

    Description of the Swift BAT transient monitor that supplies the independent daily count rates.

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