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arxiv.org/abs/1901.031=
90

We identify a binary black hole (BBH) merger that appears to be multiply=
lensed by an intervening galaxy. The LIGO/Virgo events GW170809 and GW1708=
14 have indistinguishable waveforms separated by 5 days, and overlap on the=
sky within the 90\% credible region. Their strain amplitudes are also simi=
lar, implying a modest relative magnification ratio, as expected for a pair=
of lensed gravitational waves. The phase of the two events is also consist=
ent with being the same, adding more evidence in support of both events ori=
ginating from the same BBH merger. The difference in the published inferred=
distances of each event can then be interpreted as following from their di=
fferent magnifications. The observed chirp masses of both events are also s=
imilar, as expected for a pair of lensed events, with a common detected val=
ue of $29.1^{+1.3}_{-1.0}M_{\odot}$, lying at the peak of the observed dist=
ribution of chirp masses. We infer this case is a prototypical example of a=
lensed event that supports our lensing prediction \cite{Broadhurst2018} ac=
cording to which, cosmologically distant, magnified BBH comprise most of th=
e LIGO/Virgo events with chirp masses enhanced above $\simeq 15M_{\odot}$ b=
y the cosmological expansion. From our predictions we estimate an intrinsic=
, unlensed, chirp mass of $\simeq 10-12 M_\odot$, with a source redshift in=
the range $0.9<z<2.5$. We also outline a joint analysis over all bas=
eline permutations that can stringently test our lensing interpretation of =
these two events. More generally, lensed events effectively multiply the nu=
mber of baseline permutations and motivates the use of more interferometers=
for round the clock coverage of all repeat events of a given source, in or=
der to maximise the orbital details and sky localization of lensed BBH sour=
ces.

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