TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3409
SUBJECT: GRB 050509b: No Optical Variability in XRT Error Circle
DATE:    05/05/11 06:45:45 GMT
FROM:    S. Bradley Cenko at Caltech  <cenko@srl.caltech.edu>

S. Bradley Cenko, B. T. Soifer, Chao Bian, Vandana Desai, S. R. Kulkarni
(Caltech), Brian P. Schmidt (ANU), Arjun Dey and Buell T. Jannuzi (NOAO)
report on behalf of the Caltech-NRAO-Carnegie GRB collaboration:

We have continued to image the field of GRB 050509b with the Low
Resolution Imaging Spectrometer mounted on the 10-m Keck I telescope.  Our
latest observations consisted of 3 x 300 s images simultaneously in the g'
and R filters.  The mean epoch of these observations was approximately
5:56 UT 10 May (25.8, 25.9, and 26.0 hours after the burst, respectively).

We stacked all the R-band images obtained on 9 May (GCNs 3391, 3401) and
differenced this from the stack obtained on 10 May.  Adopting the value of
R = 18.5 for the USNO-A2 star located at RA 12:36:19.970, Dec +29:00:36.31
(J2000.0), we find no variation within the revised XRT error circle (GCN
3395) to a limiting magnitude of R = 25.6.  Specifically for the extended
source in the XRT error circle (S1 in GCN 3401), we find this object has
not varied by more than +/- 0.18 magnitudes from 9 May to 10 May.

It is therefore unlikely that any of the 4 sources identified in GCN 3401
is the optical afterglow of GRB 050509b.