Refined Orbital Timing Solution for IGR J17498-2921

ATel #3601; C. B. Markwardt (NASA/GSFC), T. E. Strohmayer (NASA/GSFC)
on 27 Aug 2011; 22:39 UT
Credential Certification: Craig B. Markwardt (Craig.Markwardt@nasa.gov)

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We report a refined solution for the accreting millisecond X-ray pulsar IGR J17498-2921 (ATels #3551, #3555, #3556, #3558, #3559, #3560, #3561, #3562, #3563, #3568).

Analysis is based on RXTE PCA data in the energy band 2-30 keV between 2011-08-13 and 2011-08-23, for total exposure of 121 ksec. A coherent orbit solution was found using the Z^2 technique (Bucheri et al 1983). We caution that this solution is preliminary in the sense that definitive RXTE orbit and fine clock correction data are not yet used.

The best-fit orbital solution has a barycentered pulsar frequency of 400.990187287(4) Hz, a projected pulsar semi-major axis of 0.365108(3) lt-sec, and an orbital period of 13835.615(1) s. There is no detectable orbital eccentricity or long-term pulse frequency change. The mass function is 0.002042(1) solar masses, implying a minimum companion mass of approximately 0.17-0.22 solar masses, depending on the neutron star mass. The epoch of the ascending node is JD 2455786.5608963(4) referred to the TDB time system. Numbers in parentheses are formal 1-sigma errors in the final digits. These quantities are substantially consistent with the earlier preliminary results reported by Papitto et al. (ATEL #3563).

The pulse profile semi-amplitude is between 10% and 30%, depending on the amount of contamination due to other point-like and diffuse sources in the PCA collimator field of view.

The X-ray light curve has varied somewhat in the 30-40 mCrab range, but does not show clear signs of rapid decline. This increases the chances that it will be detectable by other observatories.