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)
Subjects: X-ray, Binary, Black Hole, Neutron Star, Star
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
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).