Swift detection of a flare from IGR J16328-4726.

ATel #2075; D. Grupe (PSU), J. Kennea (PSU), P. Evans (U. Leicester), P. Romano (INAF-IASF Palermo), C. Markwardt (CRESST/GSFC/UMD), M. Chester (PSU)
on 10 Jun 2009; 15:31 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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At 07:54:27 UT on 2009 June 10, the Swift Burst Alert Telescope (BAT) triggered on a transient source coincident with the INTEGRAL source IGR J16328-4726 (Bird et al., 2007). The BAT on-board calculated location is RA, Dec 248.176, -47.391 which is
    RA(J2000) = 16h 32m 42s
    Dec(J2000) = -47d 23' 28"
with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The peak count rate was ~1273 counts/sec (15-350 keV), at ~183 sec after the trigger.

The XRT began observing the field of IGR 16328-4726 at 08:00:52.5 UT, 385.3 seconds after the BAT trigger. Using 2754 s of XRT Photon Counting mode data and 6 UVOT images, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching to the USNO-B1 catalogue): RA, Dec = 248.1579, -47.3951 which is equivalent to:
    RA (J2000): 16 32 37.88
    Dec (J2000): -47 23 42.4
with an uncertainty of 1.7 arcsec (90% confidence). Position enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, in press, arXiv:0812.3662). The XRT light curve for the first orbit of observations shows that the source brightness is steadily decaying.

The X-ray spectrum of the first orbit of data can be fit by an absorbed power law model with Gamma = 0.56 +0.75/-0.68 and an absorption column density N_H=(8.1 +5.7/-4.9) x 10²² cm^-2, which is shows some evidence for absorption in excess of the expected galactic value of 1.54 x 10²²Â cm^-2.

We note that the high absorption makes the detection of an optical counterpart with UVOT unlikely. Within the XRT error circle we find no USNO-B1 counterparts. We do find one 2MASS counterpart candidate: RA, Dec (J2000)= 16 32 37.91, -47 23 40.9 (J=14.631, H=12.423, K=11.275 mag).