On the connection between accreting X-ray and radio millisecond pulsars

TL;DR

This paper explains how neutron stars in low-mass X-ray binaries can remain fast-spinning after mass transfer ends, by showing that their spin equilibrium is broken during the Roche-lobe decoupling phase, resolving a long-standing paradox.

Contribution

It introduces a model combining binary evolution and torque calculations to explain the retention of rapid spins in millisecond pulsars after accretion stops.

Findings

Spin equilibrium is broken during the Roche-lobe decoupling phase.

Neutron stars remain fast-spinning after mass transfer ends.

The model aligns with observed spin distributions of pulsars.

Abstract

For many years it has been recognized that the terminal stages of mass transfer in a low-mass X-ray binary (LMXB) should cause the magnetosphere of the accreting neutron star to expand, leading to a braking torque acting on the spinning pulsar. After the discovery of radio millisecond pulsars (MSPs) it was therefore somewhat a paradox (e.g. Ruderman et al. 1989) how these pulsars could retain their fast spins following the Roche-lobe decoupling phase, RLDP. Here I present a solution to this so-called "turn-off problem" which was recently found by combining binary stellar evolution models with torque computations (Tauris 2012). The solution is that during the RLDP the spin equilibrium of the pulsar is broken and therefore it remains a fast spinning object. I briefly discuss these findings in view of the two observed spin distributions in the populations of accreting X-ray millisecond pulsars (AXMSPs) and radio MSPs.