Preparing for gravitational wave astronomy: A verification of the GEO 600 detection chain by generation, injection, and extraction of continuous signals

Promising sources of gravitational waves are spinning neutron stars with a non-zero quadrupole moment. If the position and spin-down parameters of the neutron stars are known (such as for pulsars), the signal waveform of the emitted gravitational wave is well understood for a certain emission mechanism except for four remaining parameters. To search for gravitational waves of such sources, a time-domain search algorithm has been developed at the University of Glasgow. The algorithm can identify a continuous gravitational wave signal emitted by a known pulsar in the output of a gravitational wave detector which contains the signal and detector noise. The British-German laser-interferometric gravitational wave detector GEO 600 is one of a worldwide network of earth-bound gravitational wave detectors. To test the detection chain for continuous gravitational waves, a simulated, continuous gravitational wave signal has been injected into the detector hardware of GEO 600 and successfully recovered from the data using the time-domain search algorithm. In particular, the amplitude and the phase were recovered with values consistent with the parameters of the injected signal, thus proving the full detection chain for continuous gravitational waves at GEO 600. For a reliable test, the injected signal needs high phase and amplitude accuracy. An instrument has been developed that generates a simulated, continuous gravitational wave signal with a phase error of less than 1% of 2pi. The signal has been measured to be stable over several months and autonomously recovers from interruptions, such as data-transfer failures. The key component of the instrument is a microcontroller operated as a direct digital frequency synthesiser to generate the signal in an analog electronic form. A digital phase-locked loop running on a control computer controls the phase accumulator of the microcontroller via its phase increment register. The absolute timing of the injected signal is controlled by locking the microcontroller to GPS time. The signal to be injected is calculated by means of the LIGO/LSC Algorithm Library on the control computer. After the first detections of gravitational waves, the network of detectors will start a new type of astronomy: gravitational wave astronomy. For this purpose the best possible calibration accuracy is desirable. The theoretical accuracy of a photon pressure actuator, that excites a main interferometer mirror by the radiation pressure of laser light, is investigated. First measurements with a photon pressure actuator installed at the GEO 600 interferometer are presented. An advanced setup is proposed along with the necessary steps to obtain an accuracy of a few percent