Discovery of a Neutron Star Oscillation Mode During a Superburst

Neutron stars are among the most compact objects in the universe and provide a unique laboratory for the study of cold ultra-dense matter. While asteroseismology can provide a powerful probe of the interiors of stars, for example, helioseismology has provided unprecedented insights about the interior of the sun, comparable capabilities for neutron star seismology have not yet been achieved. Here we report the discovery of a coherent X-ray modulation from the neutron star 4U 1636-536 during the February 22, 2001 thermonuclear superburst seen with NASA's Rossi X-ray Timing Explorer (RXTE) that is very likely produced by a global oscillation mode. The observed frequency is 835.6440 +- 0.0002 Hz (1.43546 times the stellar spin frequency of 582.14323 Hz) and the modulation is well described by a sinusoid ( A + Bsin(p - p0) ) with fractional half-amplitude of B/A = 0.19 +- 0.04% (4-15 keV). The observed frequency is consistent with the expected inertial frame frequency of a rotationally-modified surface g-mode, an interfacial mode in the ocean-crust interface or perhaps an r-mode. Observing an inertial frame frequency--as opposed to a co-rotating frame frequency--appears consistent with the superburst's thermal emission arising from the entire surface of the neutron star, and the mode may become visible by perturbing the local surface temperature. We briefly discuss the implications of the mode detection for the neutron star's projected velocity and mass. Our results provide further strong evidence that global oscillation modes can produce observable modulations in the X-ray flux from neutron stars.
Comment: 6 pages, 4 figures, accepted for publication in the Astrophysical Journal Letters