https://phys.org/news/2019-01-x-ray-pulse-event-horizon-black.html

The researchers believe that whatever is emitting the periodic signal must be orbiting the black hole, just outside the event horizon, near the Innermost Stable Circular Orbit, or ISCO—the smallest orbit in which a particle can safely travel around a black hole.

Given the signal's stable proximity to the black hole, and the black hole's mass, which researchers previously estimated to be about 1 million times that of the sun, the team has calculated that the black hole is spinning at about 50 percent the speed of light.