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Preliminary Candidate Binary Orbit Solutions for IGR J17498-2921

ATel #3561; C. B. Markwardt (NASA/GSFC); T. E. Strohmayer (NASA/GSFC); E. A. Smith (Wyle IS & NASA/GSFC)
on 16 Aug 2011; 08:55 UT
Credential Certification: Craig B. Markwardt (Craig.Markwardt@nasa.gov)

Subjects: X-ray, Request for Observations, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 3562, 3563, 3568, 3601, 3622, 3643

We report on possible orbit solutions of the new accretion powered millisecond pulsar IGR J17498-2921 (ATEL #3551, #3555, #3556, #3558, #3559, #3560).

We used RXTE PCA observations from 2011-08-13 (Papitto et al., ATEL #3556) and 2011-08-15 at 20:20 for 3.8 ks exposure. The pulsation signal is strongly detected in both observations. We corrected photon arrival times to the solar system barycenter using the Chandra localization (Jonker et al., ATEL #3559), and created dynamical power spectra.

Orbital motion is apparent as a sinusoidally modulated Doppler shift between barycentric frequencies of 400.92 and 401.03 Hz. Unfortunately, we do not yet have full orbital phase coverage of the binary, so there are several possible aliases. I.e. several possible orbits can fit the data adequately.

We searched a grid of possible orbital periods from 3 hours through 30 hours. The best fitting orbital solutions are 3.8432(1) hr and 4.0834(1) hr, both of which appear to be typical for such systems (but the system is not at the ultra-compact end of the spectrum). Both solutions are nearly equal in their goodness-of-fit measures, and are acceptable representations of the current data. Only with more data at different orbital phases can we resolve the ambiguity.

Both candidate solutions are qualitatively similar. The first solution corresponds to a projected pulsar semi-major axis, a*sin(i), of 363(2) lt-ms; a mass function of 0.00202(2) solar masses, and a minimum companion mass of 0.158 solar masses. The second solution has values of 409(2) lt-ms, 0.00254(3) solar masses, and 0.17 solar masses, respectively.

The next two best solutions beyond the two presented above, at orbital periods of 3.63 and 4.36 hr, are not favored (~99.95% confidence), but cannot be dismissed entirely, especially if significant orbital eccentricity or timing noise were present.

We encourage further observations in order to resolve this ambiguity, and investigate the source properties further.