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Pulse timing of the ongoing outburst of SAX J1808.4-3658 with RXTE

ATEL # 1760; A. Patruno (U. Amsterdam), J. M. Hartman (NRL), R. Wijnands (U. Amsterdam), M. van der Klis (U. Amsterdam), D. Chakrabarty (MIT), E. H. Morgan (MIT), C. B. Markwardt (U. Maryland & GSFC)
on 4 Oct 2008; 8:56 UT
Password Certification: Alessandro Patruno (

Subjects: X-ray, Binaries, Neutron Stars, Pulsars, Transients
Referred to by ATEL #: 1767

Since the start of its current X-ray outburst on 2008 Sep 21 (ATel #1728), the accretion-powered millisecond pulsar SAX J1808.4-3658 has been observed for roughly 10 ks per day with the RXTE PCA. Coherent 401 Hz pulsations are clear throughout at an amplitude of 3-6% rms (2-16 keV).

We have derived a pulse timing solution for this new outburst by phase connecting the pulsations observed during the interval MJD 54731-54739. The mean spin frequency is 400.9752100(3) Hz, consistent with the frequencies observed during the previous outbursts (Hartman et al. 2008; ApJ 675, 1468). Substantial pulse shape variability in the early part of the outburst, similar to what was observed in 2002 and 2005, makes it difficult to reliably determine whether the spin frequency has changed since the previous outburst. Additional observations should soon resolve this question.

The orbital elements for the current outburst are as follows:

    Orbital period: 7249.159(8) s
    Projected semimajor axis: 62.803(8) lt-ms
    Time of ascending node: 54733.019537(2) MJD [TDB]

(Ascending node is defined as the point at 90 degrees mean longitude. All the errors quoted are 1 sigma). This ephemeris is consistent with previous outbursts. By comparing the times of ascending node for the five outbursts observed by RXTE, we find an orbital period derivative of 3.78(6)x10-12 s/s. This value agrees with the orbital period derivative previously reported by Hartman et al. (2008) and Di Salvo et al. (2008; MNRAS 389, 185). The anomalously rapid evolution of the orbital period has continued unchanged.

The 2-16 keV flux was 40-50 mCrab during MJD 54733-54739. Around MJD 54739, the flux started slowly decreasing as the outburst apparently entered its "slow decay" stage (Hartman et al. 2008). Additional RXTE observations are planned.

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