Using the null-stream of GEO 600 to veto transient events in the detector output

Abstract

A network of gravitational wave detectors is currently being commissioned around the world. Each of these detectors will search for gravitational waves from various astronomical sources. One of the main searches underway is for un-modelled, transient gravitational wave events. The nature of these signals is such that it will be difficult to distinguish them from bursts of instrumental noise that originate in or around the detector and which then couple to the main detector output. One way to deal with this is to look for events that are coincident in more than one gravitational wave detector. However, with very large event lists (potentially thousands of events per day per detector), the number of events that pass this test due to random chance can still be large. At each detector site, various methods are being developed to veto instrumental bursts from lists of candidate events from that particular detector. This reduces the size of the event lists of each detector, and hopefully the final coincident event list, to a more manageable level. This paper presents one such veto method that can be used to veto certain classes of transient events detected in the output data stream of GEO 600. The method uses events detected in the null-stream output of GEO 600 (which contains, in principle, no gravitational wave signal) with a threshold to veto events detected in the main strain output. We show that, for the certain types of signals tested, the method is very robust, delivering high efficiency for a very low false-veto rate. In particular, it is shown that when applied to real detector data, the method is able to strongly veto a certain type of events which appear around 370 Hz in the detector output.PACS numbers: 95.55. Br, 95.75.Kk, 04.80.Nn, 95.55.Ym

Bilinear noise subtraction at the GEO 600 observatory

“…Advanced GW detectors apply different kinds of veto techniques to identify these events and minimize the false alarms [30]. Vetoes could be based on statistically significant temporal co-incidences between the strain and auxiliary data channels [31] or based on events seen in null-stream constructed out of the two calibrated strain quadratures [32]. For template-based searches, χ 2 timefrequency discriminator is often used to check for consistency between the trigger and expected event [33].…”

Triple Michelson interferometer for a third-generation gravitational wave detector

“…The GEO 600 detector employs a null-stream which is created from two output channels of the main interferometer, both of which carry the gravitational wave signals [34]. This type of null-stream is aimed mainly at identifying noise events that originate in the data acquisition system.…”

Detector and data characterization at GEO 600

“…Scaling the two differential displacement signals by the length of the arms yields two estimates of the detected strain of GEO 600. These two estimates are then combined in two ways: one way to produce an optimal strain data stream (with optimal SNR for GW signals) [11], and another way (difference) to produce a null-stream (a data stream containing negligible GWs) [12].…”

“…We demand time, frequency and amplitude consistency when looking for coincident events. This veto method is described in more detail in [12]. Applying this method to the events detected in September 2006 yields a significant reduction in the candidate event list of the main GW channel.…”