GX 1+4 exhibits a period of nearly constant pulse frequency
ATel #17809; T. Takagi, M. Shidatsu (Ehime U.), T. Mihara (RIKEN), M. Nakajima (Nihon U.), M. Sugizaki (Kanazawa U.)
on 22 May 2026; 17:48 UT
Credential Certification: Megumi Shidatsu (shidatsu.megumi.wr@ehime-u.ac.jp)
Subjects: X-ray, Binary, Pulsar
The X-ray binary pulsar GX 1+4 has been in a long-term spin-down phase, characterized by a gradual decrease in pulse frequency, from the 1980s until recently.
However, since around MJD 61000, the pulse frequency monitored by Fermi/GBM (https://gammaray.nsstc.nasa.gov/gbm/science/pulsars/lightcurves/gx1p4.html) has remained nearly constant at ∼4.37 mHz, and this state has persisted for approximately 200 days up to the present.
Similar episodes have previously been observed, including during the CGRO/BATSE monitoring era (e.g., González-Galán et al. 2012), but the present event is the longest-lasting one reported so far (History of pulse frequency: https://maxi.riken.jp/plot/gx1p4_mjd_f.png).
In addition, a spin-up episode was observed for the first time in ∼28 years since ∼MJD 51000 (History of pulse frequency derivative: https://maxi.riken.jp/plot/gx1p4_mjd_fdot.png).
Although it remains unclear whether the source will subsequently enter a spin-up phase, in which the pulse frequency continuously increases, continued monitoring will be important.
