Using Neutron Stars and Black Holes in X-ray Binaries to Probe Strong Gravitational Fields

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Abstract
Neutron stars and black holes can be studied by observation of the radiation produced as matter falls into their gravitational fields. X-ray binaries, which are systems consisting of a neutron star or black hole and a companion gaseous star, produce radiation in this manner. Recently, oscillations at frequencies near 1000 cycles per second have been detected from x-ray binaries. These oscillations are likely produced in regions of very strong gravitational fields within a few tens of kilometers of the compact star. The oscillations have been interpreted as evidence for the existence of an innermost stable orbit near a compact star, a key prediction of general relativity theory. The study of x-ray binaries has also advanced the search for definitive evidence of black holes. Recent developments in our understanding of accretion flows in x-ray binaries have provided evidence for the existence of event horizons in x-ray binaries thought to contain black holes.