Tidal heating as a discriminator for horizons in extreme mass ratio inspirals

TL;DR

This paper investigates how tidal heating effects in extreme mass ratio inspirals can be used to distinguish classical black holes from exotic compact objects by analyzing gravitational-wave signals, potentially ruling out echoes in ringdowns.

Contribution

It provides a model-independent method to use gravitational-wave data to bound the reflectivity of exotic objects, improving tests of black hole horizons.

Findings

Absence of absorption significantly alters gravitational-wave signals at high spins.

Potential to set upper bounds on exotic compact object reflectivity at 0.01%.

Could exclude the detection of echoes in supermassive binary mergers.

Abstract

The defining feature of a classical black hole is being a perfect absorber. Any evidence showing otherwise would indicate a departure from the standard black-hole picture. Energy and angular momentum absorption by the horizon of a black hole is responsible for tidal heating in a binary. This effect is particularly important in the latest stages of an extreme mass ratio inspiral around a spinning supermassive object, one of the main targets of the future LISA mission. We study how this effect can be used to probe the nature of supermassive objects in a model independent way. We compute the orbital dephasing and the gravitational-wave signal emitted by a point particle in circular, equatorial motion around a spinning supermassive object to the leading order in the mass ratio. Absence of absorption by the central object can affect the gravitational-wave signal dramatically, especially at high spin. This effect will make it possible to put an unparalleled upper bound on the reflectivity of exotic compact objects, at the level of ${\cal O}(0.01)\%$. This stringent bound would exclude the possibility of observing echoes in the ringdown of a supermassive binary merger.