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 \pm 0.0002 Hz ( 1.43546 times the stellar spin frequency of 582.14323 Hz ) and the modulation is well described by a sinusoid ( A + B \sin ( \phi - \phi _ { 0 } ) ) with fractional half-amplitude of B / A = 0.19 \pm 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 .