Schwarzschild ISCO orbital frequency ($f_{\mathrm{ISCO},\infty }=c^3/(2\pi 6^{3/2}GM)$)

The Schwarzschild coordinate rotation frequency at the marginally stable circular orbit, measured per unit time of a stationary clock at infinity. It scales inversely with black-hole mass. A finite-radius static clock adds a gravitational time-dilation factor; a quasi-periodic brightness signal need not directly equal this orbital frequency.