TITLE:   GCN CIRCULAR
NUMBER:  6898
SUBJECT: GRB 071011: Confirmation of P60 optical afterglow
DATE:    07/10/12 01:57:39 GMT
FROM:    Daniel Perley at U.C. Berkeley  <dperley@astro.berkeley.edu>

D. A. Perley, D. Kocevski, D. Poznanski, J. S. Bloom, and M. Modjaz (UC 
Berkeley) report:

We have performed further analysis of our LRIS imaging of GRB 071011
(Marshall et al., GCN 6882).

The afterglow candidate we reported previously (GCN 6883) does not vary 
by more than 0.1 magnitude between 13:01 and 14:22 UT, and is therefore 
unlikely to be the optical afterglow of GRB 071011.

Our earliest-time LRIS images do not cover the position of the X-ray 
source reported by Sbarufatti et al. (GCN 6891) due to the large offset 
between the initial BAT position and XRT position.  We returned to the 
field starting at 14:10:52 UT and acquired 2x120s in R-band and 2x120s 
in the RG850 filter through high airmass.  No new objects are detected 
within the XRT error circle to an approximate limiting magnitude (3 
sigma) of R~24.5.

We do detect a source at the location reported by Cenko et al.  Our 
position of this source (J2000), relative to the USNO B1.0 catalog, is:

RA = 00:33:32.731
dec = +61:08:04.57

The relative astrometric uncertainty is <0.5".

We calculate a preliminary magnitude of R = 20.4 +/- 0.1 relative to 
five nearby USNO stars.  Comparison to the magnitude reported by Cenko 
et al. (GCN 6895) indicates that the source has faded significantly and 
confirms that this is the optical afterglow of GRB071011.