TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3357 SUBJECT: GRB050502b: Afterglow Confirmation DATE: 05/05/05 06:25:50 GMT FROM: S. Bradley Cenko at Caltech S. Bradley Cenko, Derek B. Fox (Caltech), Joshua Rich, Brian Schmidt (ANU), Jessie Christiansen (UNSW), and Edo Berger (Carnegie) report: We have further analyzed our images of the field of GRB050502b (GCN 3330) taken with the 1-m ANU telescope (GCN 3338). Additionally, we have imaged the field with the automated Palomar 60-inch telescope (P60). Observations at the P60 consisted of 10 x 120 s exposures in the Gunn i band taken at a mean epoch of 5:40 4 May 2005 UT (43.8 hours after the burst). Based on extrapolations from the 2MASS and USNO-B2 catalogs, we adopt values of V=16.7, I=15.8, R=16.1 for the star located at RA = 09:30:12.616, Dec = +17:00:05.55 (J2000.0). We find the magnitude of the afterglow candidate identified by Rich, Schmidt, and Christiansen (GCN 3338) as follows: Filter Facility UT Age Limit (3-sig) or measure ======================================================== V ANU1m 9:35 May 2 9.5 min V>20.7 V ANU1m 9:41 May 2 15 min V>20.2 V ANU1m 9:43 May 2 18 min V>20.8 V ANU1m 9:46 May 2 20 min V>21.8 I ANU1m 9:58 May 2 23 min I=19.84+/-0.09 i P60 5:40 May 4 43.8 hr i>21.7 We therefore identify this source as the afterglow of GRB050502b. To reiterate the coordinates (J2000.0): RA: 09:30:10.024 Dec: +17:00:05.55 Finally, we note the spectral slope at early times, beta > 2.9, indicates that GRB050502b may be either highly extinguished or at moderately high redshift.