  Black holes in general relativity are characterized by their trapping
horizon, a one-way membrane that can be crossed only inwards. The existence of
trapping horizons in astrophysical black holes can be tested observationally
using a reductio ad absurdum argument, replacing black holes by horizonless
configurations with a physical surface and looking for inconsistencies with
electromagnetic and gravitational wave observations. In this approach, the
radius of the horizonless object is always larger than but arbitrarily close to
the position where the horizon of a black hole of the same mass would be
located. Upper bounds on the radius of these alternatives have been provided
using electromagnetic observations (in optical/IR band) of astronomical sources
at the center of galaxies, but lower bounds were lacking, leaving unconstrained
huge regions of parameter space. We show here that lower bounds on the radius
of horizonless objects that do not develop trapping horizons can be placed
using observations of accreting systems. This result is model-independent and
relies only on the local notion of causality dictated by the spacetime geometry
around the horizonless object. These observational bounds reduce considerably
the previously allowed parameter space, boosting the prospects of establishing
the existence of trapping horizons using electromagnetic observations.
