All-sky, all-frequency directional search for persistent gravitational waves from Advanced LIGO's and advanced Virgo's first three observing runs

Abstract

We present the first results from an all-sky all-frequency (ASAF) search for an anisotropic stochasticgravitational-wave background using the data from the first three observing runs of the Advanced LIGOand Advanced Virgo detectors. Upper limit maps on broadband anisotropies of a persistent stochasticbackground were published for all observing runs of the LIGO-Virgo detectors. However, a broadbandanalysis is likely to miss narrowband signals as the signal-to-noise ratio of a narrowband signal can besignificantly reduced when combined with detector output from other frequencies. Data folding and thecomputationally efficient analysis pipeline,PyStoch, enable us to perform the radiometer map-making atevery frequency bin. We perform the search at 3072HEALPixequal area pixels uniformly tiling the skyand in every frequency bin of width1=32Hz in the range 20–1726 Hz, except for bins that are likely tocontain instrumental artefacts and hence are notched. We do not find any statistically significant evidencefor the existence of narrowband gravitational-wave signals in the analyzed frequency bins. Therefore, weplace 95% confidence upper limits on the gravitational-wave strain for each pixel-frequency pair, the limitsare in the rangeð0.030−9.6Þ×10−24. In addition, we outline a method to identify candidate pixel-frequency pairs that could be followed up by a more sensitive (and potentially computationally expensive)search, e.g., a matched-filtering-based analysis, to look for fainter nearly monochromatic coherent signals.The ASAF analysis is inherently independent of models describing any spectral or spatial distribution ofpower. We demonstrate that the ASAF results can be appropriately combined over frequencies and skydirections to successfully recover the broadband directional and isotropic results.