Timing of the 2008 Outburst of SAX J1808.4-3658 with XMM-Newton: A Stable Orbital Period Derivative over Ten Years

TL;DR

This study analyzes XMM-Newton observations of SAX J1808.4-3658's 2008 outburst, confirming a stable orbital period derivative over ten years, supporting models of non-conservative mass transfer in the system.

Contribution

First precise measurement of the orbital period derivative during the 2008 outburst, confirming its stability over a decade and supporting non-conservative mass transfer models.

Findings

Orbital period derivative is (3.89 ± 0.15) × 10^{-12} s/s.

Orbital period derivative has remained stable over ten years.

Mass transfer is highly non-conservative, with only 1% of the lost mass accreted.

Abstract

We report on a timing analysis performed on a 62-ks long XMM-Newton observation of the accreting millisecond pulsar SAX J1808.4-3658 during the latest X-ray outburst that started on September 21, 2008. By connecting the time of arrivals of the pulses observed during the XMM observation, we derived the best-fit orbital solution and a best-fit value of the spin period for the 2008 outburst. Comparing this new set of orbital parameters and, in particular, the value of the time of ascending-node passage with the orbital parameters derived for the previous four X-ray outbursts of SAX J1808.4-3658 observed by the PCA on board RXTE, we find an updated value of the orbital period derivative, which turns out to be $\dot P_{\rm orb} = (3.89 \pm 0.15) \times 10^{-12}$ s/s. This new value of the orbital period derivative agrees with the previously reported value, demonstrating that the orbital period derivative in this source has remained stable over the past ten years. Although this timespan is not sufficient yet for confirming the secular evolution of the system, we again propose an explanation of this behavior in terms of a highly non-conservative mass transfer in this system, where the accreted mass (as derived from the X-ray luminosity during outbursts) accounts for a mere 1% of the mass lost by the companion.