X-ray emission and spin-up evolution of the 6.1-ms pulsar

Abstract

Neutron stars with rotation periods in the millisecond regime have been observed both as an isolated pulsar (PSR1937 + 214)¹ and as a pulsar with a low-mass binary companion (PSR1953 + 29)². We report here an upper bound to the X-ray luminosity of the latter of L_x < 4 × 10³² erg s⁻¹, and consider the consequences of this and other observational data for theoretical models of this pulsar's formation and spindown. The two fast pulsars have features in common which suggest a similar genesis (very low magnetic field and surface temperature, low galactic latitude) and we propose that both are members of a new class of radio pulsars³. We argue that their immediate ancestors were neutron stars in galactic bulge X-ray sources, which accrete from close, low-mass (≲1M_⊙) binary companions and spin-up in the process. In this scenario, an isolated fast pulsar results from a tight binary where mass outflow from a (still) main sequence secondary is controlled by angular momentum loss from the binary motion. A wider binary, where Roche-lobe overflow is sustained by nuclear evolution of the secondary into a red giant phase, ultimately leaves behind an old spun-up pulsar with a light remnant companion. In both cases, the neutron-star progenitors would have been white dwarfs which collapsed under excess accreted mass^4–7. These neutron stars, then, are old (≫ 10⁷ yr for binary pulsars, >10⁹ yr for isolated ones) which accounts for their anomously low magnetic fields, and their consequent fast spins and low spindown rates.