Vetoing techniques for binary inspirals have already been described in subsection ; these techniques are equally applicable to searches for ringdown waveforms. However, since ringdown waveforms are short lived and have a narrow frequency band, it is much more difficult to distinguish between a ringdown waveform and a purely impulsive event. Furthermore, since the ringdown waveform will be preceded by some unknown waveform corresponding to a black hole merger, one should not be too selective as to which events should be vetoed.
Nevertheless, the Caltech 40 meter interferometer data has many spurious events that will trigger a ringdown filter, and we would expect that other instruments will have similar properties. These spurious events will (hopefully) not be too common, and most will be able to be rejected if they are not reported by other detectors. At present, however, we have only the Caltech 40 meter data to analyze, so we must consider every event that is detected by the optimal filter. The single vetoing technique that we will use at present is to look for non-Gaussian events in the detector output using the routine is_gaussian(). Since the expected ringdown waveforms will be only barely discernible in the raw data, such a test has no chance of accidentally vetoing an actual ringdown, but it will veto the obvious irregularities in the data.