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Orbital parameters of HETE J1900.1-2455

ATel #538; Philip Kaaret (U. Iowa), Ed Morgan, and Roland Vanderspek (MIT)
on 28 Jun 2005; 18:58 UT
Credential Certification: Philip Kaaret (philip-kaaret@uiowa.edu)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 543

We have measured the binary orbit of the millisecond X-ray pulsar HETE J1900.1-2455 (ATEL #523) in RXTE PCA observations made during June 17 to 25. Using the position of the tentative optical counterpart (ATEL #526), we find an orbital period of 4995.24(3) s, a projected semimajor axis of 18.39(2) lt-ms, and an epoch of 90 degrees mean longitude of 53539.02765(3) MJD (TT). Addition of eccentricity does not improve the fit and we place an upper bound on the eccentricity of 0.005. The source flux was 7.5E-10 erg cm^-2 s^-1 in the 2-20 keV band on June 25 and has continued to brighten. Observations to refine the X-ray position are encouraged.