Discovery of a binary millisecond pulsar in the globular cluster M4

We report the discovery of an 11-ms pulsar, PSR1620–26, in the closest globular cluster, M4 (NGC6121). It is the fifth millisecond pulsar to be found, and the second in a globular cluster. Unlike the other cluster pulsar, PSR1821 – 24 in M28 (ref. 1), PSR1620 – 26 is in a low-mass binary system. This provides strong support for formation mechanisms in which an old neutron star is spun up to millisecond periods by accretion during an X-ray binary phase. The orbit's small eccentricity is likely to have significant implica-tions for both the age of the millisecond pulsar and its surface magnetic field. We conclude that the pulsar's current companion is, in fact, the star which was responsible for the spin-up.

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