The irreducible mass and the horizon area of LIGO's black holes

The mass of a Kerr black hole (BH) can be separated into irreducible and rotational components-the former is a lower limit to the energy that cannot be possibly extracted from the event horizon and is related to its area. Here we compute the irreducible masses of the stellar-mass BHs observed by gravitational-wave interferometers LIGO and Virgo. Using single-event data, we present a re-parameterization of the posterior distribution that explicitly highlights the irreducible and rotational contributions to the total energy. We exploit the area law to rank the black-hole mergers observed to date according to their irreversibility, thus providing a guide to selecting events for targeted tests of general relativity. Using population fits, we compute the rate by which the total area of black-hole horizons increases due to the observable mergers.