The Spin Period of PSR J1455−3330: Evidence for Disk Instability in Wide Low‐Mass X‐Ray Binaries?

We consider the influence on spin evolution, in a neutron star with a low-mass giant companion, of thermal-viscous instability in the accretion disk around the neutron star. Recent investigations on disk instability predicted that most low-mass X-ray binaries (LMXBs) with orbital period Porb ≳ 1-2 days should be transient. We show that during the LMXB evolution, because of the transient behavior in mass transfer, the neutron star's spin can deviate from the conventional "equilibrium spin" determined by the stable mass transfer rate. We discuss the application of such a scenario to the prior spin evolution during the LMXB phase in the binary and millisecond radio pulsar PSR J1455 -3330. The anomalously high characteristic age of the pulsar indicates that the initial pulse period is close to its current value, 7.99 ms, which is much longer than that attained via stable mass accretion. We argue that such a spin period can be accounted for if only a small fraction of mass transferred from the secondary was accreted by the neutron star during (at least) the late stage of mass transfer, thus presenting positive evidence for disk instability in wide LMXBs. © 1998. The American Astronomical Society. All rights reserved.