  Multiple gauge theories predict the presence of cosmic strings with different
mass densities $G\mu/c^2$. We derive an equation governing the perturbations of
a rotating black hole pierced by a straight, infinitely long cosmic string
along its axis of rotation and calculate the quasinormal-mode frequencies of
such a black hole. We then carry out parameter estimation on the first detected
gravitational-wave event, GW150914, by hypothesizing that there is a string
piercing through the remnant, yielding a constraint of $G\mu/c^2 <3.8\times
10^{-3}$ at the 90\% confidence interval with a comparable Bayes factor with an
analysis for a Kerr black hole without a string. In contrast to existing
studies which focus on the mutual intersection of cosmic strings, or the cosmic
string network, our work focuses on the intersection of a cosmic string with a
black hole, with characteristics which can be identified in binary coalescence
signals.
