neutronstarblackhole

Neutronstarblackhole is not a standard astronomical object but a term used informally to refer to either a binary system containing a neutron star and a black hole, or the hypothetical end state of a neutron star that accretes mass and collapses into a black hole. In the second sense, a true neutron star cannot exist above the Tolman–Oppenheimer–Volkoff limit; once the mass exceeds this limit, the star is expected to collapse into a black hole, and the resulting object would be indistinguishable from a black hole in external properties. The mass limit depends on the equation of state of dense matter and is uncertain, roughly around 2 to 3 solar masses.

Neutron star–black hole binaries form in several ways, including the evolution of massive binary stars and dynamical capture in dense stellar environments. Their mergers are potential sources of gravitational waves and, depending on the neutron star's disruption, may produce electromagnetic counterparts such as kilonovae. The gravitational wave signal carries information on the black hole mass and spin, the neutron star radius, and the equation of state of dense matter. If the neutron star is tidally disrupted outside the event horizon, brighter EM emission is possible; if not, the event may be gravitation-only with minimal light.

Observationally, LIGO/Virgo have reported at least two confirmed neutron star–black hole mergers: GW200105 and GW200115. Ongoing observations aim to improve event rates, constrain neutron star matter, and test general relativity in the strong-field regime.