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binary pulsarA pulsar that is in orbit about another star, and is detected by its intrinsic emission of radiation (usually radio waves) rather than by radiation resulting from mass transfer (see X-ray pulsators). The orbital motion is inferred from apparent changes in the pulse period as the pulsar orbits its companion; the companion star, which is not generally detected directly, can be a neutron star, a white dwarf, a low-mass star, or even a supergiant. Systematic surveys have discovered large numbers of binary pulsars, in particular binary millisecond pulsars, in globular clusters, but they also occur in the galactic disk. The properties of the systems vary widely: the pulse period ranges from 0.0016 second (for PSR 1957+20) to about one second; the orbital periods range from several hours to several years.
PSR 1913+16 was the first binary pulsar to be discovered (in 1974), and is still sometimes called ‘the binary pulsar’. It has a pulse period of 59 milliseconds. Its short period and highly eccentric (e = 0.617) orbit have led to accurate determinations of the system's parameters. The masses of the pulsar and the unseen companion are identical, and at 1.4 solar masses are equal to the Chandrasekhar limit for collapse to a neutron star. The orbital period is decreasing at a rate of 75 microseconds per year, implying that the stars are spiraling together. The only mechanism for losing orbital energy in this system is gravitational waves, and the analysis of PSR 1913+16 gives a rate of energy loss that is precisely in accord with the theory of general relativity. The pulsars PSR 2127+11C in the globular cluster M15 and PSR 1534+12 in the galactic disk are the two other examples of systems like PSR 1913+16 presently known. See also black-widow pulsars; planet pulsar.