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

TL;DR

This paper analyzes the spin evolution of two accreting millisecond X-ray pulsars, constraining their torques and suggesting they are near spin equilibrium, with implications for gravitational wave emission.

Contribution

It provides new torque constraints on XTE J1814-338 and SAX J1808.4-365, and discusses their spin equilibrium status considering gravitational wave emission.

Findings

Upper limit on spin-up/down rate for XTE J1814-338: |ν̇| ≲ 1.5×10⁻¹⁴ Hz/s

Both systems are likely near spin equilibrium due to disc/magnetosphere interaction

Gravitational wave torque is less likely than spin equilibrium scenario

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.