Searches for continuous and transient gravitational waves from known neutron stars and their astrophysical implications

Pitkin, Matthew and Woan, Graham and Hough, James (2006) Searches for continuous and transient gravitational waves from known neutron stars and their astrophysical implications. PhD thesis, Univ Glasgow.

Abstract

We have used data from the third and fourth science runs of the laser interferometric gravitational wave detectors LIGO and GEO600 to produce upper limits on the emission of gravitational waves from a selection of known neutron stars. Two different emission mechanisms are looked into; i) the emission of continuous gravitational waves from triaxial neutron stars; and ii) emission of quasi-normal mode ring-downs from glitching neutron stars. We have produced upper limits on the gravitational wave amplitude and ellipticity for 93 known pulsars assuming continuous emission via triaxiality. This selection of pulsars includes the majority of currently known pulsars with frequencies > 25 Hz, with many within binary systems and globular clusters. New algorithms to take into account the motions within binary systems and possible effects of pulsar timing noise are presented. Also shown is the first analysis to combine the data sets from two distinct science runs as a method of lowering the upper limits. The results are starting to push into the range of plausible neutron star ellipticities, with the Crab pulsar closely approaching the limit that can be set through spin-down arguments. For the 32 of these pulsars in globular clusters the results provide upper limits independent of the cluster dynamics. The astrophysical significance of these results is discussed. Along with results from true pulsars we also present the extraction of simulated signals injected into the interferometers during the science runs. These provide validation checks of both the extraction software and the coherence of the detectors. Two techniques are discussed in relation to searching for quasi-normal mode ring-down signals from excited neutron stars, for example during a glitch; one based on matched filtering and the other based on Bayesian evidence. These are both applied to a search for such a signal from SGR1806 20 during a GRB on 27th December 2004, using the LIGO H1 detector and GEO600 data. This search provided upper limits on the energy released in gravitational waves via quasi-normal modes over the range of frequencies from 1-4 kHz. These are compared with results from a previous search using the bar detector AURIGA (Baggio et al, 2005) and theoretical arguments. The limitations of the search and search techniques, and possible extensions to these, are discussed. The future of these searches is discussed with regard to extensions to the analysis techniques and number of potential sources. Particular emphasis is placed on searches using data from the current LSC S5 science run.