  The LIGO/Virgo collaboration has reported 50 black hole-black hole (BH-BH)
mergers and 8 candidates recovered from digging deeper into the detectors
noise. The majority of these mergers have low effective spins pointing toward
low BH spins and efficient angular momentum transport in massive stars as
proposed by several models (e.g., the Tayler-Spruit dynamo). However, out of
these 58 mergers, 7 are consistent with having high effective spin parameter
(chi_eff>0.3). Additionally, 2 events seem to have high effective spins sourced
from the spin of the primary (more massive) BH. These particular observations
could be used to discriminate between the isolated binary and dynamical
formation channels. It might seem that high BH spins point to a dynamical
origin if angular momentum transport in stars is efficient and forms
low-spinning BHs. In such a case dynamical formation is required to produce
second and third generations of BH-BH mergers with typically high-spinning BHs.
Here we show, however, that isolated binary BH-BH formation also naturally
reproduces such highly spinning BHs. Our models start with efficient angular
momentum transport in massive stars that is needed to reproduce the majority of
BH-BH mergers with low effective spins. Later, some of the binaries are subject
to a tidal spin-up allowing the formation of a moderate fraction (~10%) of
BH-BH mergers with high effective spins (chi_eff>=0.4-0.5). In addition,
isolated binary evolution can produce a small fraction of BH-BH mergers with
almost maximally spinning primary BHs. Therefore, the formation scenario of
these atypical BH-BH mergers remains to be found.
