Search methods for continuous gravitational-wave signals from unknown sources in the advanced-detector era

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.

Empirically estimating the distribution of the loudest candidate from a gravitational-wave search

“…Due to their crucial role in CW searches, we discuss in Appendix B a simple proposal to reduce the effect of narrow-band disturbances so that distromax results can still provide a cogent answer. Whenever possible, we recommend the application of informed veto strategies against instrumental artifacts (see [16] and references therein) before attempting to process the results using distromax. The method discussed in Appendix B is just a complementary algorithm to prevent a specific type of strong disturbances from invalidating an analysis.…”

“…IV, this approach is conceived to deal with a specific set of common noise disturbances so that distromax can be applied on a larger range of real-data results. However, users are encouraged to understand and curate their search results using standard CW vetoes (see [16] and references therein) before falling back to this specific notching algorithm.…”

“…not only the standard F-statistic, but also "line-robust" statistics [35,36], generalizations of the F-statistic to look for tCW [37]. Other detection statistics used in the CW literature [16], such as Hough number-count [78][79][80][81][82], cross-correlation [83], or powerbased statistics [84][85][86], could potentially benefit from distromax as well.…”

“…While much of the statistical framework used in this work is generally applicable, we mainly focus on the search for continuous gravitational-wave signals (CWs) [16], produced by long-standing quadrupolar deformations, such as in the case of non-axisymmetric spinning neutron stars (NS) [17]. From the point of view of the current generation of advanced detectors (Advanced LIGO [18], Advanced Virgo [19], and KAGRA [20]), they belong in the weak-signal regime, meaning they are expected to blend into the background distribution.…”