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Philip Kaaret (Harvard-Smithsonian CfA)
The discovery of millisecond quasi-periodic oscillations (QPOs) from neutron-star x-ray binaries has opened a new window for the study of accretion flows in strong gravitational fields. Recently, QPO observations of 4U1820-30 have been interpreted as evidence for the existence of the marginally stable orbit, a key prediction of general relativity. I present new data on 4U1820-30 and on the relation between kHz QPOs, x-ray spectra, and low-frequency timing noise. I examine the claims for and against detection of the marginally stable orbit.
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