High braking index pulsar PSR J1640-4631: low-mass neutron star with a large inclination angle?

Abstract

Recent timing observation constrained the braking index of the X-ray pulsar PSR J1640-4631 to be $n=3.15\pm 0.03$, which is the highest value of all pulsars with measured braking indices so far. In this Letter, we investigate whether pulsar braking by combined between the magnetic dipole emission and the gravitational radiation might have a braking index greater than three. For conventional neutron star and low mass quark star candidates, the inferred ellipticities derived by the observed braking index are obviously much larger than the theoretical estimated maximum value. If PSR J1640-4631 is a low-mass neutron star with a mass of $0.1{\mathrm{M}}_{\odot }$, the inferred ellipticity can be approximately equal to the theoretical estimated maximum value. Because of the radio-quiet nature of this source, we employ the vacuum gap model developed by Ruderman and Sutherland to constrain the inclination angle to be $87.2-90^{\circ }$. Based on this, we propose that a low-mass neutron star with a large inclination angle can interpret the high braking index and the radio-quiet nature of this source. Future observations such as gravitational wave detection and long-term timing for this source are required to confirm or confute our scenario.

4 pages, 2 figures, A&A letters in press

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• Constraining the ellipticity of millisecond pulsars with observed spin-down rates
• Ultra-compact binary pulsars as continuous dual-line gravitational wave sources
• Constraints on the internal physics of neutron stars from the observational data of several young pulsars: the role of a power-law decaying dipole magnetic field