Spin evolution of neutron stars in transient low-mass X-ray binaries

TL;DR

This study models the spin evolution of neutron stars in transient low-mass X-ray binaries, revealing that accretion mode significantly impacts the final spin periods, which helps explain observed pulsar properties.

Contribution

It introduces detailed calculations of neutron star spin evolution considering transient accretion and magnetic braking, providing new insights into MSP formation.

Findings

Transient accretion results in a wider range of spin periods.

Spin periods are more affected by accretion mode than magnetic braking.

The model explains longer MSP spin periods in wide binaries.

Abstract

Millisecond pulsar + helium white dwarf (MSP+He WD) binaries are thought to have descended from neutron star (NS) low-mass X-ray binaries (LMXBs). The NSs accreted from the progenitors of the WDs and their spin periods were accordingly accelerated to the equilibrium periods of order milliseconds. Thus, the initial spin periods of the ``recycled" NSs are critically determined by the mass transfer rate in the LMXB phase. However, the standard picture neglects the possible spin-down of the NSs when the donor star decouples from its Roche lobe at the end of the mass transfer, as well as the transient behavior of most LMXBs. Both imply more complicated spin evolution during the recycling process. In this work, we perform detailed calculations of the formation of MSP+He WD binaries. We take into account three magnetic braking (MB) prescriptions proposed in the literature, and examine the effects of both persistent and transient accretion. We find that the spin periods are not sensitively dependent on the efficiency of MB, but are considerably influenced by the accretion mode. In comparison with persistent accretion, transient accretion leads to shorter and longer spin periods of the NSs in narrow and wide systems, respectively. This may help account for the measured spin periods of MSPs in wide binaries, which seem to be longer than predicted by the persistent accretion model.