Radiative Column and Light Curve of X-Ray Binary Pulsars

We examine the published light curves (LCs) of 117 X-ray binary pulsars, focusing on the dependence of their light curves on the observed energy bands. It is found that the energy dependence of the LCs appears only when the X-ray luminosity is larger than ~ 5 x 10^36 erg/s. Assuming that the behavior of light curve is related to the radiative accretion column on the neutron star surface, this energy threshold can be considered as the observational proof of the accretion column formation proposed by Basko and Sunyaev. Once we can grasp the existence of radiative column, we can also obtain several useful informations on the neutron star properties. As an instance, we perform the statistical analysis of the orientation angle of the magnetic axis, and we find that the inclination angle of magnetic axis should be small in order to explain the observed statistics.Comment: 13 pages, 7 figures, accepted for publication in PAS

Dynamical instability of differentially rotating stars

We study the dynamical instability against bar-mode deformation of differentially rotating stars. We performed numerical simulation and linear perturbation analysis adopting polytropic equations of state with the polytropic index $n=1$. It is found that rotating stars of a high degree of differential rotation are dynamically unstable even for the ratio of the kinetic energy to the gravitational potential energy of $O(0.01)$. Gravitational waves from the final nonaxisymmetric quasistationary states are calculated in the quadrupole formula. For rotating stars of mass $1.4M_{\odot}$ and radius several 10 km, gravitational waves have frequency several 100 Hz and effective amplitude $\sim 5 \times 10^{-22}$ at a distance of $\sim 100$ Mpc.Comment: 5 pages, 7 figures, accepted for publication in MNRA