A preliminary orbital solution for the newly discovered AMSP, IGRJ17498-2921

ATel #3563; A. Papitto (Univ. Cagliari, Italy), T. M. Belloni (INAF Brera Obs., Italy), C. Ferrigno, E. Bozzo (ISDC, Switzerland), L. Burderi (Univ. Cagliari, Italy), T. Di Salvo (Univ. Palermo, Italy), A. Riggio (INAF Cagliari Obs., Italy)
on 16 Aug 2011; 18:41 UT
Credential Certification: E. Bozzo (enrico.bozzo@unige.ch)

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By analyzing the observations performed by RXTE from 2011-08-13 to 2011-08-16 (ObsId. 96435-01-01-00; 96435-01-02-00; 96435-01-02-01, summing up an exposure of 132 ks) of the newly discovered Accreting millisecond pulsar, IGR J17498-2921 (ATel #3551, #3555, #3556, #3558, #3559, #3560, #3561), we propose the following preliminary solution for the, assumed circular, NS orbital motion:

Porb= 13834.1(2) s = 3.84281(6) hr

a*sini/c = 0.364(2) lt-s

This solution is very close to one of the two proposed by Markwardt et al. (ATel #3561), while we checked that the other (Porb=4.0834(1) hr) is inconsistent with the observed data.

Given the lack of publicly available ephemeris of the orbital motion of the spacecraft, the proposed solution was obtained by fitting the evolution of the signal frequency also with an additional orbital motion of period, Porb_XTE=5646(2) s, and projected semi-major axis, a_XTE*sin i_XTE / c = 0.0148(4) lt-s. The value of the orbital period of the RXTE orbit is compatible with the actual one, at the best of our knowledge.

The best-fitting value of the barycentric NS spin frequency is 400.9576(2) Hz, though we stress that such a value is biased with respect to the actual one by the absence of a knowledge of the position of the spacecraft in the Solar System.

Further X-ray observations to confirm and refine such a solution are strongly encouraged.

We thank the RXTE team for the prompt scheduling of this observation.