Search for Lensed Gravitational Waves Including Morse Phase Information:
  An Intriguing Candidate in O2
  We search for strongly lensed and multiply imaged gravitational wave signals
in the second observing run of Advanced LIGO and Advanced Virgo (O2). We
exploit a new source of information, the so-called Morse phase, which further
mitigates the search background and constrains viable lenses. The best
candidate we find is consistent with a strongly lensed signal from a massive
binary black hole (BBH) merger, with three detected images consisting of the
previously catalogued events GW170104 and GW170814, and a subthreshold trigger,
GWC170620. Given the number of BBH events detected so far, we estimate an
overall false alarm probability $\sim 10^{-4}$ for the observed high degree of
parameter coincidence between the three events. On the flip side, we measure
the Morse phase differences which suggest a complex and atypical lens system,
with at least five images including a magnified image at a local maximum of the
Fermat potential. The low prior probability for multiple lensed images and the
amount of fine tuning required in the lens model reduce the credibility of the
lensing hypothesis. The long time delays between lensed images point toward a
galaxy cluster lens with an internal velocity dispersion $\sigma \sim 650\,{\rm
km/s}$, and the observed strain amplitudes imply a likely range $0.4 < z
\lesssim 0.7$ for the source redshift. We provide an error ellipse of $\sim
16\,{\rm deg}^2$ for the sky location of the source together with additional
specific constraints on the lens-host system, and encourage follow-up efforts
to confirm or rule out any viable lens. If this is indeed a lensed event,
successfully pinpointing the system would offer a unique opportunity to
identify the host galaxy of a BBH merger, and even localize the source within
it.
