Evidence for neutron star triaxial free precession in Her X-1 from Fermi/GBM pulse period measurements

Her X-1/HZ Her is one of the best studied accreting X-ray pulsars. In addition to the pulsating and orbital periods, the X-ray and optical light curves of the source exhibit an almost periodic 35-day variability caused by a precessing accretion disk. The nature of the observed long-term stability of...

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Bibliographic Details
Published in:arXiv.org 2022-04
Main Authors: Kolesnikov, Dmitry, Shakura, Nikolai, Postnov, Konstantin
Format: Article
Language:English
Subjects:
Accretion disks
Frequency variation
Intervals
Light curve
Moments of inertia
Neutron stars
Neutrons
Orbits
Precession
Pulsars
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Summary:Her X-1/HZ Her is one of the best studied accreting X-ray pulsars. In addition to the pulsating and orbital periods, the X-ray and optical light curves of the source exhibit an almost periodic 35-day variability caused by a precessing accretion disk. The nature of the observed long-term stability of the 35-day cycle has been debatable. The X-ray pulse frequency of Her X-1 measured by the Fermi/GBM demonstrates periodical variations with X-ray flux at the Main-on state of the source. We explain the observed periodic sub-microsecond pulse frequency changes by the free precession of a triaxial neutron star with parameters previously inferred from an independent analysis of the X-ray pulse evolution over the 35-day cycle. In the Fermi/GBM data, we identified several time intervals with a duration of half a year or longer where the neutron star precession period describing the pulse frequency variations does not change. We found that the NS precession period varies within one per cent in different intervals. Such variations in the free precession period on a year time scale can be explained by
ISSN:2331-8422
DOI:10.48550/arxiv.2204.06408