Preliminary RXTE timing solution confirms the long term spin down of SAX J1808.4-3658 and the expansion of its orbit.

ATel #3757; A. Papitto (Univ. Cagliari, Italy), A. Riggio (INAF OAC, Italy), L. Burderi (Univ. Cagliari, Italy), T. Di Salvo, A. D'Ai', R. Iaria (Univ. Palermo, Italy), E. Bozzo, C. Ferrigno (ISDC, Switzerland), T. M. Belloni (INAF OAB, Italy)
on 11 Nov 2011; 16:41 UT
Credential Certification: Alessandro Riggio (riggio@dsf.unica.it)

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We report a preliminary solution for the accreting millisecond pulsar based on RXTE PCA observations performed between 2011 Nov 4.9 and Nov 9.2, during its ongoing outburst (ATel #3733, #3736), with a total exposure of 66.5 ks. The times of arrival of photons detected by the PCA on-board RXTE were reported to the Solar system barycentre using the preliminary RXTE orbit and the position of the optical counterpart given by Hartman et al. (2008, ApJ, 675, 1468), which has an uncertainty of 0.15''. The pulse profile obtained by folding PCA light curves are modelled with two harmonic components. The RMS amplitude of the first is between 3 and 5 per cent, while the amplitude of the second is roughly 1 per cent, not background corrected. Since RXTE fine clock corrections are not yet available, a systematic error of 60 μs was summed in quadrature to the statistical error affecting the pulse phases. The preliminary best-fit solution based on the analysis of the temporal evolution of the pulse phase delays has an average frequency of 400.97520994(3) Hz, an orbital period of 7249.154(4) s, a projected semi-major axis of the NS orbit equal to 0.062803(6) lt-s, and an epoch of zero mean longitude of MJD(TDB) 55869.895634(2). No eccentricity or evolution of the pulse frequency is detected over the considered time interval. Errors are given at the 1 sigma confidence level and, for the frequency, include the uncertainty caused by the error in the position (11 nHz), which was summed in quadrature to the statistical error. By comparing the value measured for the pulse frequency with those measured during the past outbursts shown in 1998, 2000, 2002, 2005 and 2008 (Hartman et al. 2008, 2009) we give a preliminary estimate of the long term spin-down rate of (-7.0+/-1.4)E-16 Hz/s. Similarly, a comparison of the epoch of the zero mean longitude with the values previously determined (Di Salvo et al. 2008, MNRAS, 389, 1851; Burderi et al. 2009, A&A, 496L, 17B) indicates an average secular derivative of the orbital period of 4.5(3)E-12 s/s. These values confirm the earlier estimates of the long term behaviour of the source made by the cited authors. We thank the RXTE team for promptly scheduling the observations.