摘要：Recent timing observation constrained the braking index of the X-ray pulsar PSR
J1640-4631 to be n = 3.15 ±
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
M⊙, 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°. 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.
关键词：pulsars: individual: PSR J1640-4631;pulsars: general;stars: formation;gravitational waves
