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

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 which started on September 21st 2008. By connecting the time of arrivals of the pulses observed during the XMM-Newton observation we derived the best fit orbital solution and a best fit value of the spin period for the 2008 outburst. Comparing these 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 the RXTE, we find an updated value of the orbital period derivative, which results to be Ṗorb = (3.89 ± 0.15) × 10 −12 s/s. This new value of the orbital period derivative is in agreement with the value previously reported, demonstrating that the orbital period derivative in this source has remained stable over the last ten years. Although this timespan is not sufficient yet to confirm 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.