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M. van der Klis (Astronomical Institute ``Anton Pannekoek", University of Amsterdam)
The kilohertz quasi-periodic oscillations in the X-ray flux from accreting low-magnetic field neutron stars discovered with RXTE probe the motion of matter in strong gravity, within a few kilometers from the neutron star surface. There are indications that we are witnessing orbits limited by the innermost stable orbit predicted by general relativity, and Lense-Thirring precession of such orbits caused by the frame dragging associated with the neutron star's spin. Constraints on the neutron star mass-radius relation begin to exclude the hardest equations of state. Similar, possibly related phenomena are seen in accreting stellar-mass black holes.
A next generation X-ray timing mission will exploit this unique window on the physics of strong gravity and dense matter opened by RXTE. It will determine the side band system that all detailed models predict to be associated with the two main peaks in the kHz QPO spectrum and of which RXTE has recently been able to obtain the first glimpse, and thereby break ambiguities which exist in current interpretations. Also, for the first time, it will measure the kHz oscillations on time scales shorter than their coherence time, thereby opening up the possibility to map out, in 4-D, the motion of matter in the strong gravity regime.
This work was supported in part by the Netherlands Organization for Scientific Research NWO and the Netherlands Research School for Astronomy NOVA.