TITLE:   GCN CIRCULAR
NUMBER:  6465
SUBJECT: GRB 070412:  Deep Keck imaging
DATE:    07/05/28 00:31:03 GMT
FROM:    Daniel Perley at U.C. Berkeley  <dperley@astro.berkeley.edu>

D. A. Perley, J. S. Bloom, R. J. Foley, and D. Kocevski (UC Berkeley) 
report:

On the night of 2007-04-16 (UT) we performed imaging on the field of 
GRB070412 (GCN 6273), the long burst located approximately 45" in 
projection from a nearby early-type galaxy (GCN 6275), using Keck I 
(/LRIS) under poor seeing conditions.  We acquired a total of 113 
minutes of usable exposure time in I-band and 132 minutes in V-band, 
with a mid-exposure time of approximately 11:27 UT, 106 hours after the 
BAT trigger.

Using the astrometrically-corrected XRT position [1] (RA=12:06:10.18, 
dec=+40:08:35.3, uncertainty=2.2"), we observe a faint source at the 
edge of the uncertainty region in I-band.  This source is not detected 
in V-band.  Calibrating with respect to nearby stars from SDSS, we 
calculate a magnitude of:

I = 24.6 +/- 0.3

The V-band limiting magnitude is V > 26.5.

We note that the same source is also apparent in LBT r' imaging from 
Prieto et al. (GCN 6374) in both their epochs of observation (2.8 hours
and 8.2 days after the burst), suggesting the source is not transient, 
in which case we place an I-band limiting magnitude of I > 25.0 on any 
transient source in the XRT error circle.  The red color is suggestive 
of a high-redshift potential host galaxy.  The LBT r' detection would 
place a firm upper limit on the redshift to be z < 5.9.

Images of the field are located at:

http://lyra.berkeley.edu/~dperley/070412/070412keck.png
http://lyra.berkeley.edu/~dperley/070412/070412keckzoom.png

The low-redshift galaxy has been subtracted using a Sersic model fit in 
the first image and a median filter in the second (zoom) image.


[1] http://astro.berkeley.edu/~nat/swift/xrt_pos.html