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A binary neutron star (or BNS or double neutron star or DNS, or occasionally neutron star binary or NSB) is two neutron stars co-orbiting. These are of interest because neutron star mergers can be observed through GW detections. If both are pulsars, it is termed a binary pulsar. If at least one is a pulsar, some characteristics can be determined through timing of pulses, in particular, their orbital period. Observing the period over time can reveal orbital decay, and analysis can determine whether gravitational waves are a contributor. A BNS could be the remnant of a main-sequence binary star, or could result from a capture. Those that formed in situ have undergone considerable stellar evolution within close proximity, perhaps with some common envelope (CE) phases, or phases in which a neutron star orbited within its companion's envelope.
The term neutron star binary (NSB) is generally used to indicate a binary star that includes at least one neutron star (versus binary neutron star implying two), but occasionally the terms are used without this distinction.