TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1443 SUBJECT: GRB 020531: optical photometry of Chandra sources DATE: 02/06/27 09:07:40 GMT FROM: Isabel Salamanca at U. of Amsterdam Isabel Salamanca, Evert Rol (University of Amsterdam), Nial Tanvir (University of Hertfordshire), Lex Kaper (UoA), on behalf of a larger collaboration, report: We have performed differential photometry of the sources detected by Chandra (GCN #1399) in the field of view of GRB 020531 (GCN #1399, #1402). The observations were done with the WFC on the INT at La Palma. For more details see GCN 1433. Of the 13 sources detected by Chandra, we detect 5 in both epochs: cx00, cx15, cx48, cx55, cx56. Four more sources are detected only in the second epoch: cx05,cx47,cx58 and cx52. We remark that the source cx00 is also visible in the DSS. We have perfomed differential photometry of the first 4 sources by comparing them with 12 stars in the field. The relative accuracy attained is 0.01 mag. The absolute photometry was done via the photometry performed by Henden etal. (GCN #1422). The estimated error in the zero point is higher, 0.4 mag, due to the fact that the observations were performed under non-photometric conditions. Below is a summary of the magnitudes of each object: Id V mag 31 May 2 June ===================================== 5 >24.7 24.5 15 21.73 21.80 --- --------------------------------- 28 >24.7 >25.2 36 >24.7 >25.2 41 >24.7 >25.2 47 >24.7 23.8 48 23.27 23.20 51 >24.7 >25.2 52 >24.7 24.0 55 23.15 23.19 56 21.71 21.60 58 >24.7 ~25.2 The proposed afterglow candidate, cx48 (GCN #1426, #1427 and #1428) is 0.07 +/- 0.01 mag brighter in the second epoch than in the first. This message may be cited. -- Dr. Isabel E. de Salamanca Anton Pannekoek Institute, UvA Amsterdam - The Netherlands.