Black Hole Spin Values for Sagittarius A* and M87* Obtained with the Outflow Method

One key characteristic of a black hole is how rapidly it spins, which is typically parameterized by the dimensionless spin angular momentum. The black hole at the center of the Milky Way Galaxy, known as Sagittarius A* (i.e. Sgr A*), was studied to determine how rapidly it is spinning. The dimensionless spin angular momentum of Sgr A* was obtained by applying the outflow method, and was determined to be 0.90 +/- 0.06 (Daly et al. 2024). The outflow method and its application will be described to explain how this value was obtained. This method has also been applied to M87* and the results indicate that M87* is a maximally spinning black hole (Daly 2019). The spin values obtained for Sgr A* and M87* with the outflow method will be compared with dimensionless spin values obtained with independent methods, and the implications will be discussed.