A second companion of the millisecond pulsar 1620 - 26

MILLISECOND pulsars are usually found in binary systems. This is in keeping with the generally accepted model for the formation of such pulsars¹⁴ in which an old neutron star is spun up to high angular velocities by the accretion of matter from a companion star. The millisecond pulsar 1620 - 26 in the globular cluster M4 is no exception: timing measurements^1,2 reveal the presence of a 0.3-solar-mass companion star (probably a white dwarf) with an orbital period of 191 days. But subsequent measurements of this pulsar have identified a small but significant deviation from the expected behaviour^3,4, suggestive of an unusually large second derivative in the pulsar's rotation rate⁵. Here we examine several possible sources—both intrinsic and extrinsic—for this derivative, and we find that it is best explained by the presence of a second, weakly bound companion object moving in a wide orbit around the main binary system. The third body in this hierarchical system has an orbital period of ~100 years and a mass approximately ten times that of Jupiter, and may have been captured during a recent collision with another stellar system⁶.