One of the most important questions in the study of compact objects is the nature of pulsars, including whether they are composed of β-stable nuclear matter or strange quark matter. Observations of the newly discovered millisecond X-ray pulsar SAX J1808.4-3658 with the Rossi X-Ray Timing Explorer place firm constraint on the radius of the compact star. Comparing the massradius relation of SAX J1808.4-3658 with the theoretical mass -radius relation for neutron stars and for strange stars, we find that a strange star model is more consistent with SAX J1808.4-3658, and suggest that it is a likely strange star candidate.

Be stars are rapidly rotating B type stars. The origin of their rapid rotation is not certain, but binary interaction remains to be a possibility. In this work we investigate the formation of Be stars resulting from mass transfer in binaries in the Galaxy. We calculate the binary evolution with both stars evolving simultaneously and consider different possible mass accretion histories for the accretor. From the calculated results we obtain the critical mass ratios q cr that determine the stability of mass transfer. We also numerically calculate the parameter λ in common envelope evolution, and then incorporate both q cr and λ into the population synthesis calculations. We present the predicted numbers and characteristics of Be stars in binary systems with different types of companions, including helium stars, white dwarfs, neutron stars, and black holes. We find that in Be/neutron star binaries the Be stars can have a lower limit of mass ∼ 8M ⊙ if they are formed by stable (i.e., without the occurrence of common envelope evolution) and nonconservative mass transfer. We demonstrate that isolated Be stars may originate from both mergers of two main-sequence stars and disrupted Be binaries during the supernova explosions of the primary stars, but mergers seem to play a much more important role. Finally the fraction of Be stars which have involved binary interactions in all B type stars can be as high as ∼ 13% − 30%, implying that most of Be stars may result from binary interaction.

In this work, we interpreted the high braking index of PSR J1640−4631 with a combination of the magnetodipole radiation and dipole magnetic field decay models. By introducing a mean rotation energy conversion coefficient z , the ratio of the total high-energy photon energy to the total rotation energy loss in the whole life of the pulsar, and combining the pulsar's high-energy and timing observations with a reliable nuclear equation of state, we estimate the pulsar's initial spin period, P 17 44 0~( -) ms, corresponding to the moment of inertia I 0.8 2.1 10 45( -) g cm 2 .Assuming that PSR J1640−4631 has experienced a long-term exponential decay of the dipole magnetic field, we calculate the true age t age , the effective magnetic field decay timescale D t , and the initial surface dipole magnetic field at the pole B 0 p ( ) of the pulsar to be 2900−3100 yr, 1.07 2 10 5 ( ) yr, and 1.84 4.20 10 13 ( -) G, respectively. The measured braking index of n 3.15 3 = ( ) for PSR J1640−4631 is attributed to its long-term dipole magnetic field decay and a low magnetic field decay rate, dB dt 1.66 3.85 10 p 8-( -) G yr −1 . Our model can be applied to both the high braking index (n 3 > ) and low braking index (n 3 < ) pulsars, tested by the future polarization, timing, and high-energy observations of PSR J1640−4631.

We investigate the spin evolution of the binary X-ray pulsar 2S 0114+650, which possesses the slowest known spin period of ∼ 2.7 hours. We argue that, to interpret such long spin period, the magnetic field strength of this pulsar must be initially > ∼ 10 14 G, that is, it was born as a magnetar. Since the pulsar currently has a normal magnetic field ∼ 10 12 G, our results present support for magnetic field decay predicted by the magnetar model.

There is a remarkable correlation between the spin periods of the accreting neutron stars (NSs) in Be/X-ray binaries (BeXBs) and their orbital periods. Recently, Knigge et al. showed that the distribution of the spin periods contains two distinct subpopulations peaked at ∼10 s and ∼200 s, respectively, and suggested that they may be related to two types of supernovae for the formation of the NSs, i.e., core-collapse and electron-capture supernovae. Here we propose that the bimodal spin period distribution is likely to be ascribed to different accretion modes of the NSs in BeXBs. When the NS tends to capture material from the warped, outer part of the Be star disk and experiences giant outbursts, a radiatively cooling dominated disk is formed around the NS, which spins up the NS and is responsible for the short-period subpopulation. In BeXBs that are dominated by normal outbursts or are persistent, the accretion flow is advection-dominated or quasi-spherical. The spin-up process is accordingly inefficient, leading to longer periods of the neuron stars. The potential relation between the subpopulations and the supernova mechanism is also discussed.

Most ultraluminous X-ray sources (ULXs) are believed to be X-ray binary systems, but previous observational and theoretical studies tend to prefer a black hole rather than a neutron star accretor. The recent discovery of 1.37 s pulsations from the ULX M82 X-2 has established its nature as a magnetized neutron star.In this work we model the formation history of neutron star ULXs in an M82-or Milky Way-like galaxy, by use of both binary population synthesis and detailed binary evolution calculations. We find that the birthrate is around 10 −4 yr −1 for the incipient X-ray binaries in both cases. We demonstrate the distribution of the ULX population in the donor mass -orbital period plane. Our results suggest that, compared with black hole X-ray binaries, neutron star X-ray binaries may significantly contribute to the ULX population, and high-mass and intermediatemass X-ray binaries dominate the neutron star ULX population in M82-and Milky Way-like galaxies, respectively.

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 induced by dynamical interaction between the binary and a circumbinary disk. Assuming that the core density of accreting neutron stars in LMXBs may reach the density of quark deconfinement, which can lead 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.

We apply the tidal truncation model proposed by to arbitrary Be/compact star binaries to study the truncation efficiency dependance on the binary parameters. We find that the viscous decretion disks around the Be stars could be truncated very effectively in narrow systems. Combining this with the population synthesis results of Podsiadlowski, Rappaport and Han (2003) that binary black holes are most likely to be born in systems with orbital periods less than about 30 days, we suggest that most of the Be/black-hole binaries may be transient systems with very long quiescent states. This could explain the lack of observed Be/black-hole X-ray binaries. We also discuss the evolution of the Be/black-hole binaries and their possible observational features.