Spin equilibrium with or without gravitational wave emission: the case of XTE J1814-338 and SAX J1808.4-3658

Abstract

In this letter we present a new analysis of the torques acting on the accreting millisecond X-ray pulsars SAX J1808.4-365 and XTE J1814-338, and show how our results can be used to constrain theoretical models of the spin evolution. In particular we find upper limits on any spin-up/down phase of XTE J1814-338 of $|\dot{\nu}|\lesssim\,1.5\times\,10^{-14}$ Hz/s at 95% confidence level. We examine the possibility that a gravitational wave torque may be acting in these systems and suggest that a more likely scenario is that both systems are close to spin equilibrium, as set by the disc/magnetosphere interaction.