Evolutionary paths of binaries with a neutron star. I. The case of SAX J1808.4-3658

TL;DR

This study models the evolution of the binary system SAX J1808.4-3658, highlighting the importance of pulsar irradiation in reaching observed orbital parameters and affecting the donor star's internal structure and long-term evolution.

Contribution

It introduces a detailed evolutionary model including irradiation effects, magnetic braking, and gravitational radiation to explain the properties of SAX J1808.4-3658.

Findings

Pulsar irradiation is essential for matching the observed orbital period.

The model reproduces the orbital period derivative consistent with observations.

Irradiation significantly alters the donor star's internal structure and evolution.

Abstract

The evolutionary status of the low mass X-ray binary SAX J1808.4-3658 is simulated by following the binary evolution of its possible progenitor system through mass transfer, starting at a period of $\sim$6.6 hr. The evolution includes angular momentum losses via magnetic braking and gravitational radiation. It also takes into account the effects of illumination of the donor by both the X-ray emission and the spin down luminosity of the pulsar. The system goes through stages of mass transfer and stages during which it is detached, where only the rotationally powered pulsar irradiates the donor. We show that the pulsar irradiation is a necessary ingredient to reach SAX J1808.4-3658 orbital period when the donor mass is reduced to 0.04 - 0.06 M$_\odot$. We also show that the models reproduce important properties of the system, including the orbital period derivative, which is shown to be directly linked to the evolution through mass transfer cycles. Moreover we find that the effects of the irradiation on the internal structure of the donor are non negligible, causing the companion star to be non completely convective at the values of mass observed for the system and significantly altering its long term evolution, as the magnetic braking remains active along the whole evolution.