  The LIGO Scientific Collaboration and Virgo Collaboration (LVC) have recently
reported in GWTC-2.1 eight additional candidate events with a probability of
astrophysical origin greater than 0.5 in the LVC deeper search on O3a running.
In GWTC-2.1, the majority of the effective inspiral spins ($\chi_{\rm eff}$)
show magnitudes consistent with zero, while two (GW190403$_{-}$051519 and
GW190805$_{-}$211137) of the eight new events have $\chi_{\rm eff}$ $> 0$ (at
90% credibility). We note that GW190403$_{-}$051519 was reported with
$\chi_{\rm eff}$ = $0.70^{+0.15}_{-0.27}$ and mass ratio $q$ =
$0.25^{+0.54}_{-0.11}$, respectively. Assuming a uniform prior probability
between 0 and 1 for each black hole's dimensionless spin magnitude,
GW190403$_{-}$051519 was reported with the dimensionless spin of the more
massive black hole, $\chi_1$ = $0.92^{+0.07}_{-0.22}$. This is the fastest
first-born black hole ever measured in all current gravitational-wave events.
If GW190403$_{-}$051519 is formed through isolated binary evolution channel,
this extremely high spin challenges, at least in that case, the existence of
efficient angular momentum transport mechanism between the stellar core and the
radiative envelope of massive stars, as for instance predicted by the
Tayler-Spruit dynamo (Spruit 2002) or its revised version by Fuller et al.
2019.
