Is GX5 – 1 a millisecond pulsar?

Abstract

The bright galactic bulge source GX5–1 (4U1758–250) has recently been observed¹ to exhibit quasi-periods between 25 and 50 ms, with a correlation between the corresponding frequencies f and the count rate I given by dlog f/dlog I ∼ 2.5. This is the first source observed to exhibit such a phenomenon. We propose here that this behaviour results from fluctuations in accretion rate onto a millisecond pulsar, which has been spun up by accretion to its steady-state spin rate. According to models that use spin-up by accretion to bring the neutron stars to steady-state fast short periods, galactic bulge sources are likely progenitors of millisecond pulsars^2–10. After steady state is achieved in these systems, accretion may continue without further spin-up, so that for a substantial fraction of the lifetime of a galactic bulge X-ray source, the neutron star may already be spinning rapidly at the equilibrium spin. The behaviour of GX5–1 can be explained in terms of deviations from this equilibrium state resulting from changes in the accretion rate M; GX5–1 may, therefore, be a new millisecond pulsar.