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
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.
