TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  824
SUBJECT: GRB 000926, Optical Observations
DATE:    00/09/30 22:04:07 GMT
FROM:    Jules Halpern at Columbia U.  <jules@astro.columbia.edu>

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."