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Abstract

The LIGO--Virgo--KAGRA collaborations (LVK) produced a catalogue containing
gravitational-wave (GW) observations from the first half of the third GW
observing run (O3a). This catalogue, GWTC-2.1, includes for the first time a
number of \emph{exceptional} GW candidates produced from merging
black-hole-binaries with unequivocally unequal component masses. Since
subdominant multipole moments and spin-induced orbital precession are more
likely to leave measurable imprints on the emitted GW from unequal component
mass binaries, these general relativistic phenomena may now be measurable.
Indeed, both GW190412 and GW190814 have already shown conclusive evidence for
subdominant multipole moments. This provides valuable insights into the
dynamics of the binary. We calculate the evidence for subdominant multipole
moments and spin-induced orbital precession for all merging black-hole-binaries
in GWTC-2.1 that were observed during O3a and show that (a) no
gravitational-wave candidate has measurable higher order multipole content
beyond $\ell = 3$, (b) in addition to the confident subdominant multipole
measurements in GW190412 and GW190814, GW190519\_153544 and GW190929\_012149
show marginal evidence for the $(\ell, |m|) = (3, 3)$ subdominant multipole,
(c) GW190521 may have measurable subdominant multipole content and (d) GW190412
may show evidence for spin-induced orbital precession.