Volume 496, Number 2, March III 2009
|Page(s)||L17 - L20|
|Published online||24 February 2009|
Letter to the Editor
Timing of the 2008 outburst of SAX J1808.4–3658 with XMM-Newton: a stable orbital-period derivative over ten years
Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu, KM 0.7, Monserrato 09042, Italy e-mail: email@example.com
2 Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, via Archirafi 36, Palermo 90123, Italy
3 Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
4 INAF - Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio Catone 00040, Italy
Accepted: 16 February 2009
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-Newton 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 onboard RXTE, we find an updated value of the orbital period derivative, which turns out to be = (3.89 ± 0.15) 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.
Key words: stars: neutron / stars: magnetic fields / X-rays: binaries / X-rays: individuals: SAX J1808.4–3658
© ESO, 2009
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