On the formation of the peculiar low-mass X-ray binary IGR J17480$-$2446 in Terzan 5

TL;DR

This paper proposes an alternative formation scenario for the peculiar low-mass X-ray binary IGR J17480$-$2446 in Terzan 5, suggesting it was formed through an exchange encounter involving a recycled neutron star, explaining its low spin frequency.

Contribution

It introduces a new formation model for IGR J17480$-$2446 involving exchange encounters, contrasting with previous models based on early mass transfer phases.

Findings

The low spin frequency can result from spin-down to spin-up evolution.

The binary may have formed via exchange encounters in the globular cluster.

The neutron star's spin axis may be aligned or anti-aligned with the orbital axis.

Abstract

IGR J17480$-$2446 is an accreting X-ray pulsar in a low-mass X-ray binary harbored in the Galactic globular cluster Terzan 5. Compared with other accreting millisecond pulsars, IGR J17480$-$2446 is peculiar in its low spin frequency (11 Hz), which suggests that it might be a mildly recycled neutron star at the very early phase of mass transfer. However, this model seems to be in contrast with the low field strength deduced from the kiloHertz quasi-periodic oscillations observed in IGR J17480$-$2446. Here we suggest an alternative interpretation, assuming that the current binary system was formed during an exchange encounter either between a binary (which contains a recycled neutron star) and the current donor, or between a binary and an isolated, recycled neutron star. In the resulting binary, the spin axis of the neutron star could be parallel or anti-parallel with the orbital axis. In the later case, the abnormally low frequency of IGR J17480$-$2446 may result from the spin-down to spin-up evolution of the neutron star. We also briefly discuss the possible observational implications of the pulsar in this scenario.