  We report the detection of ten new binary black hole (BBH) mergers in the
publicly released data from the the first half of the third observing run (O3a)
of advanced LIGO and advanced Virgo. We identify candidates using an updated
version of the IAS search pipeline and compile a catalog of signals that pass a
significance threshold of astrophysical probability greater than 0.5 (following
the GWTC-2.1 and 3-OGC catalogs). The updated IAS pipeline is sensitive to a
larger region of parameter space, applies a template prior that accounts for
different search volume as a function of intrinsic parameters, and uses an
improved coherent detection statistic that optimally combines the data from the
Hanford and Livingston detectors. Among the ten new events, we observe
interesting astrophysical scenarios including sources with confidently large
effective spin parameters in both the positive and negative directions,
high-mass black holes that are difficult to form in stellar collapse models due
to (pulsational) pair instability, and low-mass mergers that bridge the gap
between neutron stars and the lightest observed black holes. We infer source
parameters in the upper and lower black hole mass gaps with both extreme and
near-unity mass ratios, and one of the possible neutron star--black hole
mergers is well localized for electromagnetic counterpart searches. We detect
all of the GWTC-2.1 BBH mergers with coincident data in Hanford and Livingston
except for three loud events that get vetoed, which is compatible with the
false-positive rate of our veto procedure, and three that fall below the
detection threshold. We also return to significance the event GW190909_114149,
which was reduced to a sub-threshold trigger after its initial appearance in
GWTC-2. This amounts to a total of 42 BBH mergers detected by our pipeline's
search of the coincident Hanford--Livingston O3a data.
