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; 08:56 UT
Credential Certification: Alessandro Patruno (firstname.lastname@example.org)
Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar
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:
Â Â Â Orbital period: 7249.159(8) s
Â Â Â Projected semimajor axis: 62.803(8) lt-ms
Â Â Â 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