A 10 Day Period in IGR J16328-4726 from Swift/BAT Observations

ATel #2588; R. H.D. Corbet (UMBC/NASA GSFC), S. D. Barthelmy (NASA GSFC), W. H. Baumgartner (UMBC/NASA GSFC), H. A. Krimm (USRA/NASA GSFC), C. B. Markwardt (UMCP/NASA GSFC), G. K. Skinner (UMCP/NASA GSFC), J. Tueller (NASA GSFC)
on 28 Apr 2010; 15:54 UT
Credential Certification: Robin Corbet (Robin.Corbet@nasa.gov)

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IGR J16328-4726 is a little studied X-ray source. It is listed as a "blended" variable source in the 4th IBIS/ISGRI Catalog (Bird et al. 2010, ApJ Supp, 186, 1) but is not present in the Swift BAT 22 month all-sky survey (Tueller et al. 2010, ApJ Supp, 186, 378). Grupe et al. (2009, ATel #2075) report a flare detected with the Swift BAT which was followed up with Swift XRT observations.

We have analyzed the Swift BAT 58 month survey (Baumgartner et al. 2010, HEAD, 11, 1305) light curve of this source. The light curve covers the interval 2004-12-20 through 2009-09-30 (MJD 53,359 to 55,104) and we used an energy range of 15 - 100 keV. The power spectrum of this light curve reveals highly significant modulation at a period near 10 days.

In order to characterize the modulation we fitted a sine wave to the light curve and derived:
Tmax (MJD) = 54,256.08 (+/- 0.14) + n x 10.076 (+/- 0.003)
where Tmax is the time of maximum flux.

The mean flux is 5e-5 cts/s/detector element, equivalent to approximately 1.3 mCrab, and the flux modulation (semi-amplitude/mean) on the 10 day period is approximately 100%. We note that IGR J16328-4726 is only 0.2 degrees from the bright transient source 4U 1630-472 and 0.45 degrees from the X-ray pulsar IGR J16320-4751. However, there is no obvious increase in the brightness of IGR J16328-4726 during flares in 4U 1630-472, and the 8.96 day period of IGR J16320-4751 (ATel #649) is not present in the power spectrum of IGR J16328-4726.

The Grupe et al. observation began after the BAT trigger which occurred at MJD 54,992.329, equivalent to a phase of 0.07 when our ephemeris predicts a high flux. We searched for pulsations in this 5s time resolution XRT data set, and also in other XRT observations of IGR J16328-4726, but none were detected.

The 10 day period, which is likely to be the orbital period of IGR J16328-4726, is suggestive of a high-mass X-ray binary classification, particularly one powered by accretion from the wind of a supergiant. This classification would be consistent with the high level of absorption found by Grupe et al. (2009).