When can gravitational-wave observations distinguish between black holes and neutron stars?

Gravitational-wave observations of compact binaries have the potential to uncover the distribution of masses and spins of black holes and neutron stars in the universe. The binary components physical parameters can be inferred from their effect on the phasing of the gravitational-wave signal, but a partial degeneracy between the components mass ratio and their spins limits our ability to measure the individual component masses. At the typical signal amplitudes expected by the Advanced Laser Interferometer Gravitational-wave Observatory (signal-to-noise ratios between 10 and 20),we showthat it will in many cases be difficult to distinguish whether the components are neutron stars or black holes. We identify when the masses of the binary components could be unambiguously measured outside the range of current observations: a system with a chirp massM< 0.871M<would unambiguously contain the smallest-mass neutron star observed, and a system withM <2.786M must contain a black hole. However, additional informationwould be needed to distinguish between a binary containing two 1.35M neutron stars and an exotic neutron-starblack-hole binary.We also identify those configurations that could be unambiguously identified as black hole binaries, and show how the observation of an electromagnetic counterpart to a neutron-starblack-hole binary could be used to constrain the black hole spin.