GCN Circular 6465

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
Not found
 
 
), the long burst located approximately 45" in 
projection from a nearby early-type galaxy (GCN 6275
Not found
 
 
), 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
Not found
 
 
) 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