Search for gravitational waves associated with gamma-ray bursts detected by Fermi and Swift during the LIGO–Virgo run O3b

We search for gravitational-wave signals associated with gamma-ray bursts (GRBs) detected by the Fermi and Swift satellites during the second half of the third observing run of Advanced LIGO and Advanced Virgo (2019 November 1 15:00 UTC–2020 March 27 17:00 UTC). We conduct two independent searches: a generic gravitational-wave transients search to analyze 86 GRBs and an analysis to target binary mergers with at least one neutron star as short GRB progenitors for 17 events. We find no significant evidence for gravitational-wave signals associated with any of these GRBs. A weighted binomial test of the combined results finds no evidence for subthreshold gravitational-wave signals associated with this GRB ensemble either. We use several source types and signal morphologies during the searches, resulting in lower bounds on the estimated distance to each GRB. Finally, we constrain the population of low-luminosity short GRBs using results from the first to the third observing runs of Advanced LIGO and Advanced Virgo. The resulting population is in accordance with the local binary neutron star merger rate

Jet speed variability obscures Euro‐Atlantic regime structure

Euro‐Atlantic regimes are typically identified using either the latitude of the North Atlantic jet or clustering algorithms in the phase space of 500‐hPa geopotential (Z500). However, while robust trimodality is visibly apparent in jet latitude indices, Z500 clusters require highly sensitive significance tests to distinguish them from autocorrelated noise. This leads to considerable decadal variability in regime patterns, confounding many potential applications. A clear‐cut choice of the optimal number of regimes is also hard to justify. We argue that the jet speed, a near‐Gaussian distribution projecting strongly onto the Z500 field, is the source of these difficulties. Once its influence is removed, the phase space becomes visibly non‐Gaussian, and clustering algorithms easily recover three regimes, closely corresponding to the jet latitude modes. Further analysis supports the existence of two additional blocking regimes, corresponding to a tilted and split jet. All five regimes are approximately stationary across the twentieth century