2022ApJ...924..129Q

The LIGO Scientific Collaboration and Virgo Collaboration (LIGO/Virgo) have recently reported in GWTC-2.1 eight additional candidate events with a probability of astrophysical origin greater than 0.5 in the LIGO/Virgo's deeper search on O3a running. In GWTC-2.1, the majority of the effective inspiral spins (χ_eff) show magnitudes consistent with zero, while two (GW190403_–051519 and GW190805_–211137) of the eight new events have χ_eff > 0 (at 90% credibility). We note that GW190403_–051519 was reported with χ_eff = 0.70_–0.27^+0.15 and mass ratio q = 0.25_–0.11^+0.54. 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, χ₁ = 0.92_–0.22^+0.07. This is the fastest black hole ever measured in all current gravitational-wave events. If the immediate progenitor of GW190403_–051519 is a close binary system composed of a black hole and a helium star, which can be the natural outcome of the classical isolated binary evolution through the common envelope phase, this extremely high spin challenges, at least in that case, the existence of an 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 or its revised version by Fuller et al.