Searching for perturbed black-hole ringdown signals with a network of gravitational-wave detectors

Several ground-based interferometric observatories, such as LIGO, Virgo, are taking data so that astronomers can search for gravitational-wave signals in them. One such signal is that arising from a perturbed black-hole, which can result from the coalescence of a compact binary. This signal is initially in the form of a superposition of quasi-normal modes. However, at late times the waveform, which is known as ringdown, is expected to be dominated by a single mode. The optimal method for searching such a signal buried in detector noise is to match-filter the detector's output with theoretically modeled waveforms. The coherent network statistic is optimal for detecting these signals in Gaussian noise. But in real noise, which is non-Gaussian and non-stationary, additional discriminators of noise artifacts are required for obtaining a (near-)optimal statistic. Here, we construct a multi-detector search statistic that combines the power of the coherent statistic and those discriminators. The efficiency of this new search statistic is compared with the existing ``coincidence'' search statistic, which does not check for the consistency of the phase of the signal in the separate detectors.