Noise from scattered light in Virgo’s second science run data

Virgo is one of the large, ground-based interferometers aimed at detecting gravitational waves. One of the technical problems limiting its sensitivity is caused by light in the output beams which is backscattered by seismically excited surfaces and couples back into the main beam of the interferometer. The resulting noise was thoroughly studied, measured and mitigated before Virgo's second science run (VSR2). The residual noise during VSR2, which increases in periods with a large microseismactivity, is accurately predicted by the theoretical model. The scattered light has been associated with transient events in the gravitational-wavesignal of the interferometer.