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Orbital Parameters of SWIFT J1756.9-2508

ATel #1114; C. B. Markwardt (CRESST/U. Md./GSFC) and H. A. Krimm (CRESST/USRA/GSFC)
on 16 Jun 2007; 01:14 UT
Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)

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

Referred to by ATel #: 1128, 1129, 1133

We report on high time resolution properties of the 182 Hz X-ray pulsar SWIFT J1756.9-2508 (ATEL #1105, #1107, #1108, #1111).

The RXTE PCA instrument has performed several observations of the source since its discovery for a total of 13 ksec. Over 1.3 days, the source has faded from 20 to 17 mCrab (2-10 keV).

Pulsations continue to be detectable in the PCA data, and the clear signatures of orbital Doppler modulation are present. The orbital period is 54 minutes, and the pulsar mass function is (1.66 +/- 0.25) x 10^{-7} M_sun (i.e. minimum companion mass of ~0.007 M_sun). The orbital period and mass function place J1756 as one of the "ultra-compact" X-ray binary systems, and very similar to the two other short orbital-period millisecond X-ray pulsars XTE J0929-314 and XTE J1807-294.

No strong high or low frequency QPOs are detected.