  For the first time we analyse gravitational-wave strain data using waveforms
constructed from strong gravity simulations of cosmic string loops collapsing
to Schwarzschild black holes; a previously unconsidered source. Since the
expected signal is dominated by a black-hole ringdown, it can mimic the
observed gravitational waves from high-mass binary black hole mergers. To
illustrate this, we consider GW190521, a short duration gravitational-wave
event observed in the third LIGO--Virgo--KAGRA observing run. We show that
describing this event as a collapsing cosmic string loop is favoured over
previous cosmic string analyses by an approximate log Bayes factor of $22$. The
binary black hole hypothesis is still preferred, mostly because the cosmic
string remnant is non-spinning. It remains an open question whether a spinning
remnant could form from loops with angular momentum, but if possible, it would
likely bring into contention the binary black hole preference. Finally, we
suggest that searches for ringdown-only waveforms would be a viable approach
for identifying collapsing cosmic string events and estimating their event
rate. This work opens up an important new direction for the cosmic-string and
gravitational-wave communities.
