40 60 76 v 2 1 2 D ec 2 00 4 Detector optimisation for gravitational waves by inspiralling binaries

For future configurations, we study the relation between the abatement of the noise sources and the Signal to Noise Ratio (SNR) for coalescing binaries. Our aim is not the proposition of a new design, but an indication to where in the bandwidth or for which noise source, a noise reduction would be most efficient. We take VIRGO as reference for our considerations. It is identified that i) the reduction in the mirror thermal noise band provides the highest gain for the SNR, when the bandwidth is divided according to the the dominant noises; ii) there exists a specific frequency at which lies the potential largest increment in the SNR, and that the enlargement of the bandwidth, where the noise is reduced, produces a shift of the optimal frequency to higher values; iii) the abatement of the pendulum thermal noise provides the largest, but modest, gain, when noise sources are considered separately. Our recent astrophysical analysis on event rates leads to a detection rate of one every 148 or 125 years for VIRGO and LIGO, respectively, while an, recently proposed, improved but still conservative, VIRGO configuration would provide an increase to 1.5 events per year. Instead, a bi-monthly event rate, similar to advanced LIGO, requires a 16 times gain. We analyse the 3D (pendulum, mirror, shot noises) parameter space showing how such gain could be achieved.