[vsnet-grb-info 7369] GRB090305: GMOS-S Candidate Optical Afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8933
SUBJECT: GRB090305: GMOS-S Candidate Optical Afterglow
DATE:    09/03/05 10:32:47 GMT
FROM:    S. Bradley Cenko at Caltech  <cenko at srl.caltech.edu>

S. B. Cenko, B. E. Cobb, D. A. Perley, and J. S. Bloom (UC Berkeley)
report on behalf of a larger collaboration:

We have imaged the field of the short GRB 090305 (Beardmore et al., GCN
8932) with GMOS mounted on the Gemini South telescope beginning
approximately 27 minutes after the burst.  Observations were taken in
the Sloan g', r', and i' filters and cover nearly the entire BAT error
circle.

Inside the BAT error circle, we find a fading point source at the location
(J2000.0):

   RA: 16:07:07.61
   Dec: -31:33:21.9

Calibrating with respect to the USNO-B1 object at 16:07:06.58, -31:33:37.9
(R2=18.99), we measure an initial magnitude of R ~ 23.3.  The source fades
by approximately 1.1 mag over the next two hours, corresponding to a
power-law decay index of ~ 0.6 (assuming no contribution from an
underlying host galaxy).  At this point we tentatively consider this
source the afterglow of GRB090305, although we await confirmation from the
XRT.

We wish to thank the staff at Gemini South, in particular Peter Pessev,
for performing our target of opportunity observations.