On The Formation Of Peculiar Pulsar Binaries

Four dozen years have passed since the first pulsar was discovered. Lots of interesting and unexpected discoveries flood out. Millisecond Pulsars (MSPs) have been thought to be connected with the formation and evolution of low mass x-ray binaries (LMXBs).But the classical recycling theory of MSPs has been challenged by new observations.In this thesis, we investigate the formation of the peculiar object IGR J17480-2446 harboring in the globular cluster (GC)Terzan 5 and three fully recycled MSPs in eccentric orbital binary. The thesis is organized as follow:In chapter one, we first introduce different types of neutron stars and their characteristics. Then, we describe the rotation evolution, the structure of the magnetosphere and the radiation mechanism of neutron stars. At last, the interior structure and equations of state are discussed, together with the possible neutron star-strange star phase transition.In the second chapter, we introduce theory of the formation and evolution of LMXBs and the recycling theory of MSP formation. Some problems in the LMXB formation and evolution are aslo discussed, such as the mass transfer between the binary components, formation of LMXBs in GCs, impacts supernove explosions on the binary orbit, and the relation between the orbital period and the mass of white dwarf company in binary MSPs.In chapter three, we have discussed the formation of IGR J17480-2446 which is an accret-ing X-ray pulsar in a low-mass X-ray binary harbored in the Galactic globular cluster Terzan 5.Compared with other accreting millisecond pulsars, IGR J17480.2446 is peculiar for its low spin frequency (11 Hz), which suggests that it might be a mildly recycled neutron star at the very early phase of mass transfer. However, this model seems to be in contrast with the low field strength deduced from the kilo-Hertz quasi-periodic oscillations observed in IGR J 17480.2446. Here, we suggest an alternative interpretation, assuming that the current binary system was formed during an exchange encounter either between a binary (which contains a recycled neu-tron star) and the current donor, or between a binary and an isolated, recycled neutron star. In the resulting binary, the spin axis of the neutron star could be parallel or anti-parallel with the orbital axis. In the latter case, the abnormally low frequency of IGR J17480.2446 may result from the spin-down to spin-up evolution of the neutron star. We also briefly discuss the possi-ble observational implications of the pulsar in this scenario. Two other explanations about the formation of IGR J17480.2446 are discussed also together with differences between models.In chapter four we have discussed the formation of three fully recycled MSPs in eccentric orbits. According to the recycling scenario, millisecond pulsars (MSPs) have evolved from low-mass X-ray binaries (LMXBs). Their orbits are expected to be circular due to tidal interactions during the binary evolution, as observed in most of the binary MSPs. There are some peculiar systems that do not fit this picture. Three recent examples are PSRs J2234+06, J1946+3417 and J1950+2414, all of which are MSPs in eccentric orbits but with mass functions compatible with expected He white dwarf companions. It has been suggested these MSPs may have formed from delayed accretion-induced collapse of massive white dwarfs, or the eccentricity may be derived by dynamical interaction between the binaries and a circumbinary disk. Assuming that the core density of accreting neutron stars in LMXBs may reach the density of quark deconfinement, leading to phase transition from neutron stars to strange quark stars, we show that the resultant MSPs are likely to have an eccentric orbit, due to the sudden loss of the gravitational mass of the neutron star during the transition. The eccentricities can be reproduced with a reasonable estimate of the mass loss. This scenario might also account for the formation of the youngest known X-ray binary Cir X-1, which also possesses a low-field compact star in an eccentric orbit.In chapter five, we summarize the results in this thesis, point out the observational chal-lenges to the recycling theory, and discuss future research plan.