Light Curves for Rapidly-Rotating Neutron Stars

We present raytracing computations for light emitted from the surface of a rapidly-rotating neutron star in order to construct light curves for X-ray pulsars and bursters. These calculations are for realistic models of rapidly-rotating neutron stars which take into account both the correct exterior metric and the oblate shape of the star. We find that the most important effect arising from rotation comes from the oblate shape of the rotating star. We find that approximating a rotating neutron star as a sphere introduces serious errors in fitted values of the star's radius and mass if the rotation rate is very large. However, in most cases acceptable fits to the ratio M/R can be obtained with the spherical approximation.Comment: Accepted by the Astrophysical Journal. 13 pages & 7 figure

Correlations in the QPO Frequencies of Low Mass X-Ray Binaries and the Relativistic Precession Model

A remarkable correlation between the centroid frequencies of quasi periodic oscillations, QPOs, (or peaked noise components) from low mass X-ray binaries, has been recently discovered by Psaltis, Belloni and van der Klis (1999). This correlation extends over nearly 3 decades in frequency and encompasses both neutron star and black hole candidate systems. We discuss this result in the light of the relativistic precession model, which has been proposed to interpret the kHz QPOs as well as some of the lower frequency QPOs of neutron star low mass X-ray binaries of the Atoll and Z classes. Unlike other models the relativistic precession model does not require the compact object to be a neutron star and can be applied to black hole candidates as well. We show that the predictions of the relativistic precession model match both the value and dependence of the correlation to a very good accuracy without resorting to additional assumptions.Comment: To appear in ApJ Letters. AASTEX Latex v. 5.0, 1 figure not include

Rotational Broadening of Atomic Spectral Features from Neutron Stars

The discovery of the first gravitationally redshifted spectral line from a neutron star (NS) by Cottam, Paerels and Mendez has triggered theoretical studies of the physics of atomic line formation in NS atmospheres. Chang, Bildsten and Wasserman showed that the hydrogenic Fe H$\alpha$ line formed above the photosphere of a bursting NS is intrinsically broad. We now include rotational broadening within general relativity and compare the resulting profile to that observed during Type I bursts from EXO 0748-676. We show that the fine structure splitting of the line precludes a meaningful constraint on the radius. Our fitting of the data show that the line forming Fe column is ${\rm log}_{10} (N_{\rm Fe, n=2}/{\rm cm^{-2}})=17.9_{-0.42}^{+0.27}$ and gravitational redshift $1+z =1.345_{-0.008}^{+0.005}$ with 95% confidence. We calculate the detectability of this spectral feature for a large range of spins and inclinations assuming that the emission comes from the entire surface. We find that at 300 (600) Hz only 10-20% (5-10%) of NSs would have spectral features as deep as that seen in EXO 0748-676.Comment: 4 pages, 4 figures, to appear in ApJ Letter

Quantum Effects in Black Hole Interiors

The Weyl curvature inside a black hole formed in a generic collapse grows, classically without bound, near to the inner horizon, due to partial absorption and blueshifting of the radiative tail of the collapse. Using a spherical model, we examine how this growth is modified by quantum effects of conformally coupled massless fields.Comment: 13 pages, 1 figure (not included), RevTe

Gravitational Radiation Instability in Hot Young Neutron Stars

We show that gravitational radiation drives an instability in hot young rapidly rotating neutron stars. This instability occurs primarily in the l=2 r-mode and will carry away most of the angular momentum of a rapidly rotating star by gravitational radiation. On the timescale needed to cool a young neutron star to about T=10^9 K (about one year) this instability can reduce the rotation rate of a rapidly rotating star to about 0.076\Omega_K, where \Omega_K is the Keplerian angular velocity where mass shedding occurs. In older colder neutron stars this instability is suppressed by viscous effects, allowing older stars to be spun up by accretion to larger angular velocities.Comment: 4 Pages, 2 Figure

Nonlinear Couplings Between r-modes of Rotating Neutron Stars

The r-modes of neutron stars can be driven unstable by gravitational radiation. While linear perturbation theory predicts the existence of this instability, linear theory can't provide any information about the nonlinear development of the instability. The subject of this paper is the weakly nonlinear regime of fluid dynamics. In the weakly nonlinear regime, the nonlinear fluid equations are approximated by an infinite set of oscillators which are coupled together so that terms quadratic in the mode amplitudes are kept in the equations of motion. In this paper, the coupling coefficients between the r-modes are computed. The stellar model assumed is a polytropic model where a source of buoyancy is included so that the Schwarzschild discriminant is nonzero. The properties of these coupling coefficients and the types of resonances possible are discussed in this paper. It is shown that no exact resonance involving the unstable $l=m=2$ r-mode occur and that only a small number of modes have a dimensionless coupling constant larger than unity. However, an infinite number of resonant mode triplets exist which couple indirectly to the unstable r-mode. All couplings in this paper involve the l>|m| r-modes which only exist if the star is slowly rotating. This work is complementary to that of Schenk et al (2002) who consider rapidly rotating stars which are neutral to convection.Comment: 21 pages, 1 figure, to appear in Ap