Abstract
Continuous gravitational waves are long-lasting forms of gravitational radiation produced by persistent quadrupolar variations of matter. Standard expected sources for ground-based interferometric detectors are neutron stars presenting non-axisymmetries such as crustal deformations, r-modes or free precession. More exotic sources could include decaying ultralight boson clouds around spinning black holes. A rich suite of data-analysis methods spanning a wide bracket of thresholds between sensitivity and computational efficiency has been developed during the last decades to search for these signals. In this work, we review the current state of searches for continuous gravitational waves using ground-based interferometer data, focusing on searches for unknown sources. These searches typically consist of a main stage followed by several post-processing steps to rule out outliers produced by detector noise. So far, no continuous gravitational wave signal has been confidently detected, although tighter upper limits are placed as detectors and search methods are further developed.
Highlights
We consider three kinds of searches for unknown sources: (i) blind searches, with weak prior assumptions about possible source parameters [40,41,42,43,44,45,46,47,48,49,50,51,52,53,54]; (ii) spot-light searches, focused at sky regions harbouring an interesting population of objects whose exact frequency is unknown, such as globular clusters or the Galactic Center (GC) [55,56]; and (iii) directed searches, targeting specific celestial objects compatible with a continuous gravitational-wave signals (CWs) source, such as supernova remnants (SNRs) or low-mass X-ray binaries (LMXBs) [57,58,59,60,61,62,63,64,65,66,67,68,69]
Instead of computing semicoherent quantities, it focuses on significant parameter-space points at coherent-segment level, looking for coincident candidates across different time segments and detectors. Using this coincidence criterion, the pipeline automatically becomes robust to strong instrumental features, since they tend to overlap with different parameter-space regions as an observing run progresses. This particular implementation of the F -statistic uses its own template bank setup in order to optimize the number of fast Fourier transform (FFT) computations [103,104], which normally takes a significant part of the overall computing cost of the search
We review two families of searches stemming from Equation (5), depending on whether their primary target is to increase the robustness against deviations from the intended CW model (PowerFlux & Falcon) or to improve sensitivity by increasing the effective amount of data used (Cross-Correlation)
Summary
The search for continuous gravitational-wave signals (CWs), long-duration forms of gravitational radiation, is one of the endeavours of gravitational-wave astronomy. The standard strategy, in a broad sense, is to reduce the effective length of the datastream by performing matched filtering over shorter segments; the segment-wise results can be combined into a final statistic These kind of schemes are usually referred to as semicoherent searches [19]: the segment-wise analysis is typically referred to as coherent, as it compares the phase evolution of a signal with the datastream throughout a coherence time Tcoh. The characteristic width of detection-statistic peaks widens, reducing the required number of templates to ensure a good covering of the parameter space
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