Do galactic bulge X-ray sources evolve into millisecond pulsars?

The recently discovered¹ pulsar PSR1937 + 21 in 4C21.53 is remarkable for the shortness of its period (P = 1.5 ms) and for the length² of its characteristic age, τ_P = P/2Ṗ ≳ 10⁵ yr. The simple explanation that this is an ordinary supernova-formed pulsar but with an unusually weak magnetic field and hence weak magnetic dipole radiation (ref. 3), that is, B~=2×10¹⁰(τ_P/10⁵ yr)^-1/2(P/1.5 ms) G fits in with the position of the pulsar close to the galactic plane. We suggest here an alternative origin for the pulsar as being the remnant of one of the `galactic bulge' X-ray binary stars.