TITLE: GCN GRB OBSERVATION REPORT NUMBER: 134 SUBJECT: GRB980613 Optical Observations DATE: 98/07/07 18:13:02 GMT FROM: Jules Halpern at Columbia U. J. P. Halpern (Columbia), and R. Fesen (Dartmouth) report on behalf of the MDM Observatory GRB follow-up team that the optical afterglow of GRB 980613 discovered by Hjorth et al. (GCN #109) is also present and variable on MDM images at a level just below the limit of I = 22.3 previously quoted by us in GCN #106. Our J2000 position of the OT is (+/- 0.6"): RA = 10 17 57.82 Dec = +71 27 25.5 Measured magnitudes (at mean epoch) and 1-sigma statistical errors are: R = 22.96 +/- 0.09 (June 14.24 UT) I = 22.53 +/- 0.09 (June 14.20 UT) I = 22.83 +/- 0.15 (June 15.19 UT) Actual errors are larger, limited by systematic effects of fringing in the I band. Photometry was calibrated using Landolt standards. Our magnitudes of the five reference stars measured by Diercks et al. (GCN #108) are given in the following table, and are in agreement with their values, as well as those of Djorgovski et al. (GCN #117). Quoted uncertainties are 1-sigma statistical. Coordinates are measured with respect to the USNO A1.0 reference system. Star RA(2000) Dec(2000) R I -------------------------------------------------------------------- 1 10 17 47.52 +71 26 59.9 17.91 +/- 0.01 17.34 +/- 0.01 2 10 17 54.87 +71 27 39.8 19.20 +/- 0.01 18.76 +/- 0.01 3 10 18 06.61 +71 27 04.7 20.02 +/- 0.05 18.72 +/- 0.01 4 10 17 55.31 +71 28 16.3 19.54 +/- 0.02 18.61 +/- 0.01 5 10 17 41.44 +71 28 08.9 18.70 +/- 0.01 18.31 +/- 0.01 -------------------------------------------------------------------- Our measurement of R = 22.96 for the OT on June 14.24 is similar to that of Hjorth et al. (R = 22.9 +/- 0.2 on June 13.89), which may be consistent with a plateau in the first 24 hours after the burst, or perhaps just an effect of the large error bars. Our R-band measurement combined with the later detection by Djorgovski et al. (R = 24.5 +/- 0.5 on June 16.30) implies a power-law decay slope of 1.3. Our images are posted at http://cba.phys.columbia.edu/grb/980613/ This message may be cited.