Quasi-periodic X-ray brightness oscillations (QPOs) with frequencies ∼ 1 kHz have now been discovered in more than a dozen neutron stars in low-mass X-ray binary systems using the Rossi X-ray Timing Explorer. There is strong evidence that the frequencies of the kilohertz oscillations are the orbital frequencies of accreting gas in nearly circular orbits around these stars. Some stars that produ...

We study the spin evolution of X-ray neutron stars in binary systems, which are being spun up by mass transfer from accretion disks. Our investigation reveals that a quark phase transition resulting from the changing central density induced by the changing spin, can lead to a pronounced peak in the frequency distribution of X-ray neutron stars. This finding provides one of several possible expl...

The gravitational waves that bathe the Earth presumably do not vary wildly in strength from year to year. However, our very imperfect understanding of their strengths occasionally does change abruptly, prompted either by new astrophysical observations or by new theoretical predictions and discoveries. Such has been the case in the last year for our expectations for periodic gravitational waves ...

Neutron stars are excellent emitters of gravitational waves. Squeezing matter beyond nuclear densities invites exotic physical processes, many of which violently transfer large amounts of mass at relativistic velocities, disrupting spacetime and generating copious quantities of gravitational radiation. I review mechanisms for generating gravitational waves with neutron stars. This includes grav...

We report on numerical results from an independent formalism to describe the quasi-equilibrium structure of nonsynchronous binary neutron stars in general relativity. This is an important independent test of controversial numerical hydrodynamic simulations which suggested that nonsynchronous neutron stars in a close binary can experience compression and even collapse prior to the last stable ci...

Strong magnetic field of accreting neutron stars (10 G) is hard to probe by Xray spectroscopy but can be indirectly inferred from spin-up/spin-down measurement in X-ray pulsars. The existing observations of slowly rotating X-ray pulsars are discussed. It is shown that magnetic fields of neutron stars derived from these observations (or lower limits in some cases) fall within the standard 10-10 ...

According to the observational limits on the radius and mass, PSR 1937+21 is probably a strange star, or some neutron star equations of state should be ruled out, if we alternatively suggest that a dipole magnetic field (rather than a quadrupole one) is relevant to its radio emission. In order to find solid evidence to shown that PSR 1937+21 is a strange star, it is strongly suggested to measur...

We analyzed the power density spectra of a sample of 9 neutron star and 9 black hole binaries in the low/hard spectral state. In the power density spectra of accreting neutron stars with a weak magnetic field a significant power is contained at frequencies close to one kHz. At the same time, most Galactic accreting black holes demonstrate a strong decline in the power spectra at the frequencies...

A remarkably precise observational relation for pulse core component widths of radio pulsars is used to derive stringent limits on pulsar radii, strongly indicating that pulsars are strange stars rather than neutron stars. This is achieved by inclusion of general relativistic effects due to the pulsar mass on the size of the emission region needed to explain the observed pulse widths, which con...

We develop a Lagrangian perturbation framework for rotating non-relativistic superfluid neutron stars. This leads to the first generalization of classic work on the stability properties of rotating stars to models which account for the presence of potentially weakly coupled superfluid components. Our analysis is based on the standard two-fluid model expected to be relevant for the conditions th...