Light curves of oscillating neutron stars

Abstract

We calculate light curves produced by $r$-modes with small azimuthal wavenumbers, $m$, propagating in the surface fluid ocean of rotating neutron stars. We include relativistic effects due to rapid rotation, and propagate photons from the stellar surface to a distant observer using the Schwarzschild metric. The wave motions of the surface $r$-modes are confined to the equatorial region of the star, and the surface pattern of the temperature variation can be either symmetric (for even modes) or anti-symmetric (for odd modes) with respect to the equator. Since for the surface $r$-modes the oscillation frequency in the corotating frame of the star is much smaller than the rotation frequency, $\Omega$, we employ the approximation in which the oscillation frequency in the inertial frame, $\sigma$, is given by $\sigma= -m\Omega$. We find that the $even$, $m = 1$ $r$-mode produces the largest light variations. The dominant Fourier component in the light curves of these modes is the fundamental having $\sigma=-\Omega$, and the first harmonic component having $\sigma=-2\Omega$ is always negligible in comparison. The dominant Fourier component of the even, $m=2$ $r$-modes is the first harmonic. Although the $odd$ $r$-modes produce smaller amplitude light variations compared with the $even$ modes, the light curves of the former have a stronger first harmonic component. If both $m=1$ and 2 $r$-modes are excited simultaneously, a rich variety of light curves is possible, including those having an appreciable first harmonic component. We show that the phase difference, $\delta-\delta_E$, between the bolometric light curve and that at a particular photon energy can possibly be used as a probe of the stellar compactness, $R/M$, where $R$ and $M$ are the radius and mass of the star.