All-sky search for periodic gravitational waves in LIGO S4 data

TL;DR

This paper presents an all-sky search for periodic gravitational waves in LIGO S4 data using three semi-coherent methods, setting upper limits on gravitational wave strain from neutron stars without detecting any signals.

Contribution

It introduces and compares three semi-coherent analysis methods for detecting continuous gravitational waves in LIGO data, and provides the first all-sky upper limits in the 50-1000 Hz range.

Findings

No evidence of periodic gravitational waves was found.

The best upper limit on strain amplitude is 4.28E-24 near 140 Hz.

Upper limits vary across the sky and for different source orientations.

Abstract

We report on an all-sky search with the LIGO detectors for periodic gravitational waves in the frequency range 50-1000 Hz and with the frequency's time derivative in the range -1.0E-8 Hz/s to zero. Data from the fourth LIGO science run (S4) have been used in this search. Three different semi-coherent methods of transforming and summing strain power from Short Fourier Transforms (SFTs) of the calibrated data have been used. The first, known as "StackSlide", averages normalized power from each SFT. A "weighted Hough" scheme is also developed and used, and which also allows for a multi-interferometer search. The third method, known as "PowerFlux", is a variant of the StackSlide method in which the power is weighted before summing. In both the weighted Hough and PowerFlux methods, the weights are chosen according to the noise and detector antenna-pattern to maximize the signal-to-noise ratio. The respective advantages and disadvantages of these methods are discussed. Observing no evidence of periodic gravitational radiation, we report upper limits; we interpret these as limits on this radiation from isolated rotating neutron stars. The best population-based upper limit with 95% confidence on the gravitational-wave strain amplitude, found for simulated sources distributed isotropically across the sky and with isotropically distributed spin-axes, is 4.28E-24 (near 140 Hz). Strict upper limits are also obtained for small patches on the sky for best-case and worst-case inclinations of the spin axes.