Application of a two-dipole model to PSR J1640–4631, a pulsar with an anomalous braking index. (30th October 2019)

• Record Type: Journal Article
• Authors: Shi, Hui; Hu, Hong-Wei; Chen, Wen-Cong
• Is Part Of: Publications of the Astronomical Society of Japan. Volume 71:Number 6(2019:Dec.)
• Publication Date: 2019-10-30
• Subjects: pulsars: general -- pulsars: individual (PSR J1640–4631) -- stars: magnetic field -- stars: neutron -- stars: rotation; Astronomy -- Periodicals; 520.5
• DOI: 10.1093/pasj/psz109
• Languages: English
• ISSNs: 0004-6264

Abstract

Abstract: Recent timing observation provides an intriguing result for the braking index of the X-ray pulsar PSR$\:$ J1640$-$ 4631, which has a measured braking index $n=3.15\pm 0.03$ . The decrease of the inclination angle between the spin axis and the magnetic axis can be responsible for such a high braking index. However, the physical mechanisms causing the change of the magnetic inclination angle have not been fully understood. In this Letter, we apply a two-dipole model given by Hamil, Stone, and Stone (2016, Phys. Rev. D., 94, 063012) to explain the decrease of the magnetic inclination angle of PSR$\:$ J1640$-$ 4631. The rotation effect of a charged sphere and the magnetization of ferromagnetically ordered material produce magnetic moments $M_{\, 1}$ and $M_{\, 2}$, respectively. There exists a minimum of the potential energy for the magnetic moment $M_{\, 2}$ in the magnetic field of $M_{\, 1}$, hence the $M_{\, 2}$ will freely rotate around the minimum energy position (i.e., equilibrium position), similar to a simple pendulum. Our calculation indicates that the magnetic moment $M_{\, 2}$ would evolve towards alignment with the spin axis for PSR$\:$ J1640$-$ 4631, and cause the magnetic inclination angle to decrease. The single peak in the pulse profile favors a relatively low change rate for the magnetic inclination angle.