TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3416
SUBJECT: GRB 050509b: Keck/Chandra Cross-Correlation
DATE:    05/05/12 08:48:39 GMT
FROM:    Josh Bloom at Harvard/CFA  <jbloom@cfa.harvard.edu>

J. S. Bloom, R. J. Foley, D. Pooley, A. V. Filippenko, R. Chornock (UCB),
C. Blake (Harvard), J. X. Prochaska (UCSC), J. Hennawi (UCB), M. Gladders
(Carnegie), B. Koester (U Michigan) and H. W. Chen (MIT) report:

We observed the field of GRB 050509b (#3383,#3385) with Keck I (+LRIS)
with g' and R filters.  Starting at 20050511.25 UT, we took 5 dithered
images in each band for a total of 1660 and 1620 sec exposures in g and R,
respectively. At the location of the two reported Chandra sources we find
no significantly-detected flux in either band, however there are
indications of an R counterpart (at the few sigma level) to Chandra#8.
Moreover, these images do reveal a number of sources in the XRT fainter
than reported deep imaging with the VLT (3410) and Keck II (3401), which
we label J1-J4:

   J1 12:36:13.56 +28:59:03.4
   J2       13.44        04.7
   J3       13.41     58:56.3
   J4       13.47     59:06.2

(J2000; estimated errors of 0.3 arcsec in each direction).

We note that S5, which is less than 2 arcsec from the Chandra source #5,
appears bluer than neighboring S3 and S2.

The finding chart can be found at:
 http://astro.berkeley.edu/~jbloom/grb050509b-chandra-keck-combo.gif