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Swift/BAT Discovery of the Orbital Period of IGR J16320-4751

ATel #649; R. Corbet (GSFC/USRA), L. Barbier (GSFC), S. Barthelmy (GSFC), J. Cummings (GSFC/NRC), E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (UMD), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/NRC), G. Sato (ISAS), J. Tueller (GSFC), on behalf of the Swift-Survey team
on 2 Nov 2005; 19:50 UT
Credential Certification: Robin Corbet (corbet@gsfc.nasa.gov)

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

Referred to by ATel #: 2588

The Swift/Burst Alert Telescope (BAT) light curve of the X-ray pulsar IGR J16320-4751 obtained between 2004 Dec 21 and 2005 Sep 17 shows a strong modulation at a period of 8.96 days. The mean BAT count rate of IGR J16320-4751 in the range 14 - 200 keV is 5.7E-4 counts/s which corresponds to approximately 13 mCrab. Most photons are detected below 50 keV. The Fourier amplitude of the modulation is 3.5(+/- 0.3)E-4 counts/s. A sine wave fit to the BAT light curve gives parameters of period = 8.96 +/- 0.01 days and epoch of maximum flux = MJD 53507.1 +/- 0.1.

We interpret this modulation as revealing the orbital period of a binary system. This source has a pulsation period of ~1300s (Lutovinov et al., 2005, A&A, 433, L41) and the location of this system on the pulse/orbital period diagram (Corbet, 1986, MNRAS, 220, 1047) suggests that it consists of a neutron star accreting from the wind of an early spectral type supergiant.