Abstract
Gravitational waves from binary coalescences provide one of the cleanest signatures of the nature of compact objects. It has been recently argued that the post-merger ringdown waveform of exotic ultracompact objects is initially identical to that of a black-hole, and that putative corrections at the horizon scale will appear as secondary pulses after the main burst of radiation. Here we extend this analysis in three important directions: (i) we show that this result applies to a large class of exotic compact objects with a photon sphere for generic orbits in the test-particle limit; (ii) we investigate the late-time ringdown in more detail, showing that it is universally characterized by a modulated and distorted train of "echoes" of the modes of vibration associated with the photon sphere; (iii) we study for the first time equal-mass, head-on collisions of two ultracompact boson stars and compare their gravitational-wave signal to that produced by a pair of black-holes. If the initial objects are compact enough as to mimic a binary black-hole collision up to the merger, the final object exceeds the maximum mass for boson stars and collapses to a black-hole. This suggests that - in some configurations - the coalescence of compact boson stars might be almost indistinguishable from that of black-holes. On the other hand, generic configurations display peculiar signatures that can be searched for in gravitational-wave data as smoking guns of exotic compact objects.
Highlights
The relativistic collision of two compact objects is the Rosetta Stone of the strong-gravity regime
It has been recently argued that the postmerger ringdown waveform of exotic ultracompact objects is initially identical to that of a black hole, and that putative corrections at the horizon scale will appear as secondary pulses after the main burst of radiation
We extend this analysis in three important directions: (i) we show that this result applies to a large class of exotic compact objects with a photon sphere for generic orbits in the test-particle limit; (ii) we investigate the late-time ringdown in more detail, showing that it is universally characterized by a modulated and distorted train of “echoes”of the modes of vibration associated with the photon sphere; (iii) we study for the first time equal-mass, headon collisions of two ultracompact boson stars and compare their gravitational-wave signal to that produced by a pair of black holes
Summary
The relativistic collision of two compact objects is the Rosetta Stone of the strong-gravity regime. GW spectroscopy will play an increasingly important role as more and more events at large signal-to-noise ratio are detected These observations provide novel ways to test strong gravity [7,8,9,10], black-hole (BH) no-hair results [11], the existence of event horizons [12], possible quantum effects at the horizon scale [12,13], dark matter and environmental effects [13,14], and exotic compact objects (ECOs) which might reveal themselves for the first time in the GW band [12,15,16]. Through this work we use c 1⁄4 G 1⁄4 1 units
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