  We study the absorption of scalar fields by extreme/exotic compact objects
(ECOs) -- horizonless alternatives to black holes -- via a simple model in
which dissipative mechanisms are encapsulated in a single parameter. Trapped
modes, localized between the ECO core and the potential barrier at the
photosphere, generate Breit-Wigner-type spectral lines in the absorption cross
section. Absorption is enhanced whenever the wave frequency resonates with a
trapped mode, leading to a spectral profile which differs qualitatively from
that of a black hole. We introduce a model based on Nariai spacetime, in which
properties of the spectral lines are calculated in closed form. We present
numerically calculated absorption cross sections and transmission factors for
example scenarios, and show how the Nariai model captures the essential
features. We argue that, in principle, ECOs can be distinguished from black
holes through their absorption spectra.
