Long-term variability of the 35-day cycle in Her X-1
Abstract
Her X-1 is an eclipsing X-ray binary system consisting of an accreting neutron star and an optical component HZ Her. In addition to the 1.24-s X-ray pulsations and the 1.7-d orbital period, the system also shows a 35-day X-ray variability related to the precessional motion of a tilted and twisted accretion disk around the neutron star. The long-term persistence of the 35-day X-ray variability suggests the presence of a stable clock mechanism, most likely free precession of the magnetized neutron star. We have studied the long-term behavior of the 35-day cycle of Her X-1 using all available CGRO/BATSE, RXTE/ASM, Swift/BAT, Fermi/GBM, MAXI data and archival turn-on moments of the 35-day cycle spanning more than 50 years. We have used a sliding-window averaged X-ray phase light curve and the classical O-C (observed minus calculated) diagram for individual 35-day turn-ons to inspect the long-term trends of the O-C diagram. The O-C diagram demonstrates two similar parabolic trends separated by an anomalous low state (ALS) of the source suggesting a secular decrease in the 35-d precession period with Ṗ35 =(−1.3±0.2)×10−5 s s−1 and Ṗ35 =(−4.7±0.1)×10−5 s s−1 before and after the ALS, respectively. We conclude that the decay of the 35-day period as inferred from the O-C analysis can be interpreted in terms of the direct relationship between the change of the precessional period of the accretion disk to the long-term decrease of the period of the free precessional neutron star - due to continued accumulation of accreted matter.
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BibTeX
@misc{revnivtsev2026longterm,
title = {{Long-term variability of the 35-day cycle in Her X-1}},
author = {Vladislav Revnivtsev and Konstantin Postnov and Ruediger Staubert and Sergey Antipin and Sergey Molkov and Nikolai Shakura},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.41734},
eprint = {2610.41734},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.41734},
}
RIS
TY - UNPB
TI - Long-term variability of the 35-day cycle in Her X-1
AU - Vladislav Revnivtsev
AU - Konstantin Postnov
AU - Ruediger Staubert
AU - Sergey Antipin
AU - Sergey Molkov
AU - Nikolai Shakura
PY - 2026
DA - 2026/10/30
PB - arXiv
DO - 10.48550/arxiv.2610.41734
UR - https://arxiv.org/abs/2610.41734
AB - Her X-1 is an eclipsing X-ray binary system consisting of an accreting neutron star and an optical component HZ Her. In addition to the 1.24-s X-ray pulsations and the 1.7-d orbital period, the system also shows a 35-day X-ray variability related to the precessional motion of a tilted and twisted accretion disk around the neutron star. The long-term persistence of the 35-day X-ray variability suggests the presence of a stable clock mechanism, most likely free precession of the magnetized neutron star. We have studied the long-term behavior of the 35-day cycle of Her X-1 using all available CGRO/BATSE, RXTE/ASM, Swift/BAT, Fermi/GBM, MAXI data and archival turn-on moments of the 35-day cycle spanning more than 50 years. We have used a sliding-window averaged X-ray phase light curve and the classical O-C (observed minus calculated) diagram for individual 35-day turn-ons to inspect the long-term trends of the O-C diagram. The O-C diagram demonstrates two similar parabolic trends separated by an anomalous low state (ALS) of the source suggesting a secular decrease in the 35-d precession period with Ṗ_35 =(−1.3±0.2)×10^−5 s s^−1 and Ṗ_35 =(−4.7±0.1)×10^−5 s s^−1 before and after the ALS, respectively. We conclude that the decay of the 35-day period as inferred from the O-C analysis can be interpreted in terms of the direct relationship between the change of the precessional period of the accretion disk to the long-term decrease of the period of the free precessional neutron star - due to continued accumulation of accreted matter.
ER -