TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3391
SUBJECT: GRB 050509b: Possible Optical Counterpart
DATE:    05/05/09 09:36:49 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), Edo Berger (Carnegie), Arjun Dey and Buell T. Jannuzi (NOAO)
report on behalf of a larger collaboration:

We have imaged the error circle of the Swift GRB 050509b (GCN 3381) with
the Keck Low Resolution Imaging Spectrometer (LRIS) mounted on the Keck I
telescope.  We have obtained two epochs of simultaneous imaging in the g'
and R filters.  The first epoch consisted of 4 x 300 s images taken at a
mean time of approximately 7:00 UT 9 May (~ 3 hours after the burst).  The
second epoch consisted of 3 x 300 s images taken approximately 1.5 hours
later.

Inside the XRT error circle, we find four sources, three of which are
marginal detections.  The brightest source is located at (J2000.0):

	RA: 12:36:13.7
	Dec: 28:58:57.3

The source appears to brighten slightly from our first to our second
epoch.  Under close inspection, it also appears to be embedded in an
extended nebular galaxy.  An image can be found at:

http://www.srl.caltech.edu/~cenko/grb050509b/050509.jpg

Further observations are planned.