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Abstract: Using the Rossi X-ray Timing Explorer (RossiXTE), astronomers have discoveredthat disk-accreting neutron stars with weak magnetic fields produce threedistinct types of high-frequency X-ray oscillations. These oscillations arepowered by release of the binding energy of matter falling into the stronggravitational field of the star or by the sudden nuclear burning of matter thathas accumulated in the outermost layers of the star. The frequencies of theoscillations reflect the orbital frequencies of gas deep in the gravitationalfield of the star and-or the spin frequency of the star. These oscillations cantherefore be used to explore fundamental physics, such as strong-field gravityand the properties of matter under extreme conditions, and importantastrophysical questions, such as the formation and evolution of millisecondpulsars. Observations using RossiXTE have shown that some two dozen neutronstars in low-mass X-ray binary systems have the spin rates and magnetic fieldsrequired to become millisecond radio-emitting pulsars when accretion ceases,but that few have spin rates above about 600 Hz. The properties of these starsshow that the paucity of spin rates greater than 600 Hz is due in part to themagnetic braking component of the accretion torque and to the limited amount ofangular momentum that can be accreted in such systems. Further study will showwhether braking by gravitational radiation is also a factor. Analysis of thekilohertz oscillations has provided the first evidence for the existence of theinnermost stable circular orbit around dense relativistic stars that ispredicted by strong-field general relativity. It has also greatly narrowed thepossible descriptions of ultradense matter.

Autor: Frederick K. Lamb, Stratos Boutloukos


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