Radio pulse search and X-Ray monitoring of SAX J1808.4-3658: What Causes its Orbital Evolution?

TL;DR

This study investigates the rapid orbital evolution of the accreting millisecond X-ray pulsar SAX J1808.4-3658 through radio and X-ray observations, exploring potential mechanisms like neutron star properties or donor star variations.

Contribution

It provides the first deep radio pulsation search limits for SAX J1808.4-3658 and analyzes its orbital evolution, proposing two possible underlying physical scenarios.

Findings

No radio pulsations detected, setting the strongest flux density limits.

The orbital period continues evolving rapidly, faster than models predict.

Two scenarios proposed: large neutron star moment of inertia or donor star variations.

Abstract

The accreting millisecond X-ray pulsar (AMXP) SAX J1808.4-3658, shows a peculiar orbital evolution that proceeds at a much faster pace than predicted by conservative binary evolution models. It is important to identify the underlying mechanism responsible for this behavior because it can help to understand how this system evolves. It has also been suggested that, when in quiescence, SAX J1808.4-3658 turns on as a radio pulsar, a circumstance that might provide a link between AMXPs and black-widow radio pulsars. In this work we report the results of a deep radio pulsation search at 2 GHz using the Green Bank Telescope in August 2014 and an X-ray monitoring of the 2015 outburst with Chandra, Swift, and INTEGRAL. In particular, we present the X-ray timing analysis of a 30-ks Chandra observation executed during the 2015 outburst. We detect no radio pulsations, and place the strongest limit to date on the pulsed radio flux density of any AMXP. We also find that the orbit of SAX J1808.4-3658 continues evolving at a fast pace and we compare it to the bhevior of other accreting and non-accreting binaries. We discuss two scenarios: either the neutron star has a large moment of inertia (I>1.7x10^45 g cm^2) and is ablating the donor (by using its spin-down power) thus generating mass-loss with an efficiency of 40% or the donor star is undergoing quasi-cyclic variations due to a varying mass-quadrupole induced by either a strong (1 kG) field or by some unidentified mechanism probably linked to irradiation.