Kavli Affiliate: Wayne Hu
| First 5 Authors: Jose María Ezquiaga, Wayne Hu, Rico K. L. Lo, ,
| Summary:
Strongly lensed gravitational waves (GWs) from binary coalescence manifest as
repeated chirps from the original merger. At the detectors, the phase of the
lensed GWs and its arrival time differences will be consistent modulo a fixed
constant phase shift. We develop a fast and reliable method to efficiently
reject event pairs that are not-lensed copies and appropriately rank the most
interesting candidates. Our method exploits that detector phases are the best
measured GW parameter, with errors only of a fraction of a radian and
differences across the frequency band that are better measured than the chirp
mass. The arrival time phase differences also avoid the shortcomings of looking
for overlaps in highly non-Gaussian sky maps. Our basic statistic determining
the consistency with lensing is the distance between the phase posteriors of
two events and it directly provides information about the lens-source geometry
which helps inform electromagnetic followups. We demonstrate that for simulated
signals of not-lensed binaries with many shared parameters none of the pairs
have phases closer than $3sigma$, and most cases reject the lensing hypothesis
by $5sigma$. Looking at the latest catalog, GWTC3, we find that only $6%$ of
the pairs are consistent with lensing at $99%$ confidence level. Moreover, we
reject about half of the pairs that would otherwise favor lensing by their
parameter overlaps and demonstrate good correlation with detailed joint
parameter estimation results. This reduction of the false alarm rate will be of
paramount importance in the upcoming observing runs and the eventual discovery
of lensed GWs. Our code is publicly available and could be applied beyond
lensing to test possible deviations in the phase evolution from modified
theories of gravity and constrain GW birefringence.
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