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GCN Circular 5597

GRB 060923A: Keck and Gemini Observations
2006-09-23T18:17:24Z (17 years ago)
Derek Fox at PSU <>
D.B. Fox (Penn State), A. Rau (Caltech), & E.O. Ofek (Caltech) report
on behalf of a larger collaboration:

"We have imaged the Swift XRT localization region for GRB060923A
(Trigger 230662; Stamatikos et al., GCN 5583) with Keck-I + LRIS
(I-band) and Gemini-North + NIRI (J- and K-band).  We identify the
candidate afterglow of Tanvir et al. (GCN 5587) in our K-band images
as the only point source consistent with the XRT localization, with a
brightness estimated as follows:

   K-band:  Ks~20.6 mag at 07:04 UT

We estimate its brightness (without correcting for Galactic
extinction) by reference to the 2MASS star at R.A. 16:58:31.463,
Dec. +12:22:12.88 (J2000), which has J=15.175 mag, H=14.585 mag, and
Ks=14.457 mag in that catalog.

The position of the source by reference to NOMAD catalog sources in
the vicinity is:

    R.A. 16:58:28.15, Dec. +12:21:38.9 (J2000)

with an estimated uncertainty of less than 0.5" in each coordinate.

The source is undetected in our earlier J-band and I-band images.  Our
upper limits on the brightness of the source in these bands are:

   I-band:  I>23.5 mag at 06:11 UT
   J-band:  J>23.7 mag at 06:42 UT

The I-band photometry is referenced to the SDSS imaging of this field
(Cool et al., GCN 5585) while the J-band zero-point is derived from
the 2MASS star referenced above.  Magnitudes have not been corrected
for Galactic extinction.

If this source is the afterglow of GRB060923A, the red colors are
suggestive of a highly extinguished or high-redshift burst.  In
particular, correcting for the Schlegel, Finkbeiner & Davis (1998)
estimate of E(B-V)=0.058 mag Galactic extinction, and assuming a
temporal decay of t^(-1) and an intrinsic afterglow spectrum of F_nu ~
nu^(-1), these limits indicate suppression of the afterglow flux
(relative to K-band) by a factor of >18 at 0.9 micron (I-band) and by
a factor of >14 at 1.23 micron (J-band).
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