Pulsar discoveries by volunteer distributed computing: and the strongest continuous gravitational wave signal

Neutron stars are the endpoints of stellar evolution and one of the most compact forms of matter in the universe. They can be observed as radio pulsars and are promising sources for the emission of continuous gravitational waves. Discovering new radio pulsars in tight binary orbits offers the opportunity to conduct very high precision tests of General Relativity and to further our understanding of neutron star structure and matter at super-nuclear densities. The direct detection of gravitational waves would validate Einstein's theory of Relativity and open a new window to the universe by offering a novel astronomical tool. This thesis addresses both of these scientific fields: the first fully coherent search for radio pulsars in tight, circular orbits has been planned, set up and conducted in the course of this thesis. Two unusual radio pulsars, one of them in a binary system, have been discovered. The other half of this thesis is concerned with the simulation of the Galactic neutron star population to predict their emission of continuous gravitational waves. First realistic statistical upper limits on the strongest continuous gravitational-wave signal and detection predictions for realistic all-sky blind searches have been obtained. The data from a large-scale pulsar survey with the 305-m Arecibo radio telescope were searched for signals from radio pulsars in binary orbits. The massive amount of computational work was done on hundreds of thousands of computers volunteered by members of the general public through the distributed computing project Einstein@Home. The newly developed analysis pipeline searched for pulsar spin frequencies below 250 Hz and for orbital periods as short as 11 min. The structure of the search pipeline consisting of data preparation, data analysis, result post-processing, and set-up of the pipeline components is presented in detail. The first radio pulsar, discovered with this search, PSR J2007+2722, is an isolated radio pulsar, likely from a double neutron star system disrupted by the second supernova. We present discovery and initial characterisation using observations from five of the largest radio telescopes worldwide. Only a dozen similar systems were previously known. The second discovered radio pulsar, PSR J1952+2630, is in a 9.4-hr orbit with most likely a massive white dwarf of at least 0.95 M⊙. We characterise its orbit by analysis of the apparent spin period changes. This pulsar most likely belongs to the very rare class of intermediate-mass binary pulsars, from which only five systems were previously known. It is a promising target for the future measurement of relativistic effects. In the second half of this thesis, the emission of continuous gravitational waves from a Galactic population of neutron stars is studied. For the first time, realistic estimates of the statistical upper limit of the expected gravitational wave signal are obtained, improving previous estimates by about a factor of six. The simulation is used to obtain for the first time detectability predictions for these objects with ground based gravitational wave detectors and realistic blind searches. It is also shown how to improve possible searches by maximising the number of detections for a fixed amount of computation cycles.