Precise orbital ephemeris for the Accreting Millisecond Pulsar IGR J17511-3057

ATel #2221; A. Riggio (INAF-OACagliari & Univ. of Cagliari), A. Papitto (INAF-OACagliari & Univ. of Cagliari), L. Burderi (Univ. of Cagliari), T. Di Salvo (Univ. of Palermo), A. D'A&igrave (Univ. of Palermo), R. Iaria (Univ. of Palermo), M. T. Menna (INAF-OAR)
on 2 Oct 2009; 09:33 UT
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Credential Certification: Alessandro Papitto (papitto@oa-roma.inaf.it)

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Taking advantage from the precise source position given by Chandra (ATEL #2215), we report here refined orbital parameters of the newly discovered Accreting Millisecond Pulsar IGR J17511-3057 (ATEL #2196, #2197, #2198, #2215, #2216).
We have analysed the RXTE(PCA) public observations (Full and pseudo-production Data, http://heasarc.gsfc.nasa.gov/docs/xte/SOF/SOF_data_hp.html ) available up to date and spanning from 2009 Sep 13 to 2009 Sep 26. The pulsation is present in all the data analysed. The pulse profile was modelled with a fundamental harmonic and a 1st overtone. The pulsed fraction (not corrected for the background) are of about 12% and 2%, respectively. The pulse phase delays of the fundamental do not show a clear spin-up/down trend, while there is evidence of pulse phase delays fluctuations with an amplitude of about 0.05 in phase units on timescales of few days. The mean spin frequency calculated on all the considered data and taking into account the positional uncertanty given in ATEL #2215 is 244.83395157(7) Hz, although local deviations up to 4x10^-7 Hz around this mean value are observed caused by the mentioned phase fluctuations.
The orbital ephemeris derived from our analysis is:

Orbital period: 12487.5126(9) s
Projected semimajor axis: 275.194(3) lt-ms
Time of passage for ascending node: 55088.0320280(5) MJD [TDB]
Eccentricity e: < 6e-5 (2 sigma c.l.)

Unless otherwise stated, errors in parentheses are quoted at the 1 sigma level. The orbital parameters given here are consistent, within the errors, with the first RXTE timing reported in Atel #2197, and with the recent XMM-Newton timing reported in Atel #2220. We appreciate the RXTE rapid follow-up scheduling and constant monitoring of the current source outburst.