[vsnet-grb-info 4703] GRB 070521: Keck imaging

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6451
SUBJECT: GRB 070521: Keck imaging
DATE:    07/05/22 07:49:54 GMT
FROM:    Daniel Perley at U.C. Berkeley  <dperley at astro.berkeley.edu>

D. A. Perley, J. S. Bloom (UC Berkeley), J. X. Prochaska (UC Santa 
Cruz), S. Stanford (LLNL), M. Brodwin (JPL), and N. R. Butler (UCB) report:

Starting at 13:21:22 UT (2007-05-21), we imaged the field of GRB 070521 
(GCN 6431) in V and R band with the Keck I telescope (+ LRIS), in a 
series of five exposures of 300 seconds each.  The host galaxy candidate 
of Hattori et al. (GCN 6444, "S1") is clearly detected, as is a second 
source ("S2") at the edge of the XRT error circle (Guidorzi et al., GCN 
6431).  We also detect a third, extended source ("S3") further to the 
south, not consistent with the preliminary XRT position reported by 
Guidorzi et al. but consistent with our preliminary re-analysis of the 
XRT data.

The positions of these sources (J2000) are:

S1:  16:10:38.562 +30:15:27.50
S2:  16:10:38.734 +30:15:31.38
S3:  16:10:38.754 +30:15:21.57

The astrometric uncertainty is about 0.3" in each coordinate.

Preliminary aperture magnitudes of these sources, calibrated with 
respect to the SDSS magnitudes from Cool et al. (GCN 6432) using the 
Lupton transform [1], are:

S1:  V = 25.46 +/- 0.12 , I = 23.88 +/- 0.12
S2:  V = 25.98 +/- 0.20 , I = 24.26 +/- 0.16
S3:  V = 26.63 +/- 0.39 , I = 24.39 +/- 0.17

These magnitudes are not corrected for Galactic extinction, which the 
NED extinction calculator [2] estimates to be E(B-V) = 0.027, or A_V = 
0.09, A_I = 0.05, in this direction.

An image of the field is located at:

http://lyra.berkeley.edu/~dperley/070521/070521keckI.png
http://lyra.berkeley.edu/~dperley/070521/070521keckI_clean.png



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[1] http://www.sdss.org/dr5/algorithms/sdssUBVRITransform.html#Lupton2005
[2] http://nedwww.ipac.caltech.edu/forms/calculator.html