Sidereal filtering: A novel robust method to search for continuous gravitational waves

In this paper we introduce a novel filtering method for the search of continuous gravitational wave signals in the data of interferometric detectors, based on the estimation of the harmonic content of the detector sidereal pattern. Thanks to the very specific properties of the sidereal modulation, the method is powerful in discriminating among continuous wave signals of astrophysical origin and detector artefacts or noise outliers. We describe here the statistical properties of the filter by using both software and hardware simulated signals injected in the data of the LIGO second science run O2 and we give an estimate of the method sensitivity. A study of the method robustness with respect to a nonoptimal correction of the signal frequency variation, referring in particular to the spin-down, is also presented. This novel filtering procedure is well suited for directed searches of sources with known - or loosely known - position and largely unknown frequency and spin-down, and for the follow-up of all-sky candidates. Used as an initial processing step it has, moreover, the potential to improve the sensitivity of wide-band hierarchical searches.