On the origin of the 6.1-ms pulsar

Abstract

Boriakoff et al.^1,2 have recently reported the discovery of a pulsar, PSR1953+29, with a pulse period, P, of only 6.1 ms, and with period derivative Ṗ<6 × 10⁻¹⁶ ss⁻¹. This is only the second known pulsar with P in the millisecond range, the first being PSR1937 + 214^3,4. PSR1953 + 29 is in a binary system with an unseen companion: its orbital period, P_orb, is ∼120 days, its projected semi-major axis, a_p sin_i, is ∼9 × 10¹¹ cm, and its orbital eccentricity, e, is small. We present here a model for the origin and evolution of this binary system that quantitatively accounts for all of its salient features. Our evolutionary scenario begins with a luminous binary X-ray source (see also refs 17,18) composed of a neutron star and a lower giant-branch companion (we do not attempt to understand the preceding evolution, which may well have included a common-envelope phase^5,6), and terminates when the system has evolved into its present configuration and the companion has become a low-mass degenerate dwarf. We show that our model can explain both the large separation of the binary and the approximate circularity of the orbit despite the large separation relative to the present sizes of the component stars (which tends to render inoperative the tidal dissipation that might otherwise circularize the orbit^7,8).