A new class of radio pulsars

Sufficiently low magnetic field neutron stars which accrete for long times from a surrounding keplerian disk can be spun up to millisecond periods. After accretion ceases, such stars could become isolated or binary pulsars, depending on whether their companions have subsequently disrupted the binary by either their tidal break ups or their supernova explosions. Such pulsars constitute a new class, with short periods, long apparent ages (P/Ṗ ≳ 10 8 yr) and pulsed optical, X-ray and γ -ray fluxes significantly below those expected for canonical pulsars with similar periods. Because of their long subsequent spin-down lifetimes, such pulsars would probably be observable if their birth rate exceeded even 10 −4 that of the canonical ones. We propose here that the recently discovered millisecond pulsar 4C21.53 belongs to this new class, together with the binaries PSRs 1913+16, 0820+02 and 0655+64, as well as possibly several isolated pulsars such as PSRs 1952+29 and 1804−08. The possibility of such a class of fast accretion-spun-up pulsars has also been noted by Backus et al. 1 .