TITLE: GCN CIRCULAR NUMBER: 824 SUBJECT: GRB 000926, Optical Observations DATE: 00/09/30 22:04:07 GMT FROM: Jules Halpern at Columbia U. J. P. Halpern, N. Mirabal (Columbia U.), D. Turnshek & J. Busche (U. Pittsburgh) report on behalf of the MDM Observatory GRB follow-up team: "We processed and calibrated the MDM R and B data reported previously in GCN #806, now using Landolt standard stars, and we acquired new R and B observations on the MDM 2.4. The comparison star at (J2000) RA 17:04:01.21, Dec +51:46:19.9, previously assumed to have R = 17.1, is now measured at R = 17.06, B = 18.50. In this new system, we find the following magnitudes for the OT: Sep. 28.183 B = 20.89 +/- 0.04 Sep. 28.177 R = 19.86 +/- 0.02 Sep. 29.176 B = 22.11 +/- 0.09 Sep. 29.196 R = 21.14 +/- 0.03 Sep. 30.179 B = 23.07 +/- 0.06 Sep. 30.211 R = 22.11 +/- 0.06 Correcting the observed B-R for the Galactic extinction A_B = 0.101, A_R = 0.063 of Schlegel et al., we find that B-R = 0.95 +/- 0.05 consistently for these three nights. This color corresponds to spectral slope beta = -1.51 +/- 0.12, which indicates that there may be intrinsic extinction. Our latest R-band measurement confirms the steepening of the decay reported by Fynbo et al. (GCN #820) and Veillet (GCN #823). For the first 2 days the decay slope was consistent with alpha = -1.65 +/- 0.02. After 2.2 days, alpha = -2.51 +/- 0.17. Images of this field and an updated decay curve are posted at http://www.astro.columbia.edu/~jules/grb/000926 This message may be cited."