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astro-ph.HE · High Energy Astrophysical Phenomenaastro-ph.IM · Instrumentation and Methods for Astrophysics

Fermi GBM calendar fits show 9% weaker spin-down for 4U 1626−67 in 2025 than in 2024

We compare the measured spin-down of the X-ray pulsar 4U 1626−67 after its known 2023 torque reversal. We preserve the public Fermi GBM files created on 16 October 2026. Separate weighted calendar fits use 30 detections in 2024 and 24 in 2025. The frequency rates are −3.9305 and −3.5787 × 10^−13 Hz/s. The later fitted magnitude is 8.95% smaller. Other tested weights, time bins, and calendar boundaries retain this ordering. The residual scatter exceeds the supplied measurement errors. Models with slow frequency noise permit equal rates. With Brownian frequency noise, the fitted contrast is +0.2580 × 10^−13 Hz/s, with a conditional 95% normal interval from −0.1394 to +0.6554. This interval holds the fitted noise amplitude fixed. It omits uncertainty in the noise model. The package includes both public FITS formats, source hashes, code, tables, figures, and an independent numerical check. Only three detections fall in 2026; the last is on 12 July. These data show a net frequency decrease between those endpoints. Irregular sampling and unknown torque noise limit the physical result. The annual contrast does not establish a lasting change in the mean torque or its cause. We chose the comparison after we inspected the source history.

4U 1626-67 · Fermi GBM · accreting pulsars · spin-down · torque noise · public data · reproducible analysis

Admitted 3 citing publications

References

6
  1. usesdoi:10.3847/1538-4357/ab855c

    We use public frequency data from the GBM Accreting Pulsars Program described in this paper.

  2. doi:10.1093/mnrasl/slad110

    This paper studies the known 2023 return to spin-down.

  3. doi:10.1093/mnras/stae256

    This paper measures timing and spectral properties with data from the early spin-down episode.

  4. doi:10.48550/arXiv.2511.20847

    This study models the 2008 reversal and excludes the points after the 2023 reversal from its model analysis.

  5. doi:10.1093/mnras/staf379

    This later paper uses observations from 2023 to study QPO sidebands.

  6. doi:10.48550/arXiv.2510.09986

    This later QPO study uses four NuSTAR observations from 2023.

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