TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1434 SUBJECT: Fading Optical Source in the Field of GRB020531 DATE: 02/06/20 02:49:35 GMT FROM: George Ricker at MIT Fading Optical Source in the Field of GRB020531 A. Dullighan, G. Monnelly, N. Butler, R. Vanderspek, P. Ford, G. Ricker (MIT); H. Ebeling, R. Wainscoat (U. Hawaii); N. Kawai, A. Yoshida (RIKEN, Japan) write: We have observed the error box of the short-hard burst source GRB020531 (GCN #1399, #1402) with the Baade 6.5m telescope at the Magellan Observatory on June 1.16 and June 10.13 UT, and with the Subaru 8.2m telescope at Mauna Kea Observatory on June 5.43 UT. The R band images were calibrated using the data provided by Henden et al.(GCN #1422). Our observations were as follows: 2002 Date Instrument, Exposure Time Limiting R Mag. UT Telescope (sec) (3 sigma) June 1.16 LDSS2, Baade 180 23.6 June 5.43 SuprimeCam, Subaru 420 25.5 June 10.13 LDSS2, Baade 360 x 2 24.0 We detect a fading optical counterpart for Chandra Source #5 (GCN #1415) in all three epochs. 2002 Date R Mag Error UT of #5 June 1.16 23.12 0.20 June 5.43 23.79 0.06 June 10.13 23.96 0.20 The decline in brightness of the counterpart between the measurements is consistent with a power law decay with alpha = 0.35 +/- 0.05. The relative X-ray brightness of Chandra Source #5 at the two epochs of the Chandra observations is consistent with the optical decay index. However, the independent significance of any X-ray decline is low (1.4 sigma level-of-confidence). Since Source #5 lies just outside the current error box for GRB020531 (GCN #1407), its possible association, if any, with GRB020531 is unclear. This message may be cited.