Unveiling the spectrum of inspiralling binary black holes

The measurement of sub-leading modes of gravitational wave signals from compact binary coalescences is of central importance for astrophysics and fundamental physics with wide-ranging implications. The detection of subdominant modes is very challenging as they carry much less power compared to the dominant quadrupole mode, and are visible only for binaries consisting of unequal masses with an orbital geometry that is not face-on. In this {\emph{Letter}}, we present a novel method to combine the energy from multiple events observed in interferometric gravitational wave detectors. The events are stacked using time-frequency representations of the data, making use of the set of intrinsic parameters (mass, spin, and orbital parameters) inferred from the measurement of the dominant quadrupole mode. Stacking the events enhances the signal-to-noise ratio of the sub-dominant modes of the inspiral part of the signals, thereby increasing the chances of their detection. Our studies suggest that there is a ? 95% chance of detecting these subdominant modes at a false alarm probability of 1% from O(250) events observed simultaneously in two advanced-LIGO detectors, a target that may be easily achieved soon.