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Published February 15, 2005 | Published
Journal Article Open

Gravitational-wave signature of an inspiral into a supermassive horizonless object


Event horizons are among the most intriguing of general relativity's predictions. Although on firm theoretical footing, direct indications of their existence have yet to be observed. With this motivation in mind, we explore here the possibility of finding a signature for event horizons in the gravitational waves (GWs) produced during the inspiral of stellar-mass compact objects (COs) into the supermassive (~10 (to the 6th) M[sun]) objects that lie at the center of most galaxies. Such inspirals will be a major source for LISA, the future space-based GW observatory. We contrast supermassive black holes with models in which the central object is a supermassive boson star (SMBS). Provided the COs interact only gravitationally with the SMBS, stable orbits exist not just outside the Schwarzschild radius but also inside the surface of the SMBS as well. The absence of an event horizon allows GWs from these orbits to be observed. Here we solve for the metric in the interior of a fairly generic class of SMBS and evolve the trajectory of an inspiraling CO from the Schwarzschild exterior through the plunge into the exotic SMBS interior. We calculate the approximate waveforms for GWs emitted during this inspiral. Geodesics within the SMBS surface will exhibit extreme pericenter precession and other features making the emitted GWs readily distinguishable from those emitted during an inspiral into a black hole.

Additional Information

© 2005 The American Physical Society. Received 16 November 2004; published 10 February 2005. We wish to thank L. Lindblom and E.S. Phinney for useful conversations. Micah Solomon also provided help in preparing the appendix. Kesden was supported by the NASA Graduate Student Research Program. J.G.'s work was supported by NASA Grants No. NAG5-12384 and No. NAG5-10707. This work was supported in part by DoE No. DE-FG03-92-ER40701 and NASA No. NAG5-9821.

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