TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4138
SUBJECT: GRB 051021: MDM Upper Limit
DATE:    05/10/22 18:37:45 GMT
FROM:    Jules Halpern at Columbia U.  <jules@astro.columbia.edu>

J. Eastman, D. L. Depoy (Ohio State U.), A. P. Crotts, & D. J. Beirne
(Columbia U.) observed the afterglow (Fox et al., GCN 4120) of
HETE GRB 051021 (Yoshida et al., GCN 4116; Olive et al., GCN 4124)
in the R band using the MDM 1.3m, and in SDSS r using the MDM 2.4m.
On Oct. 22 04:45 UT, 15.4 hours after the burst, they report a 5-sigma
upper limit of R > 23.0.  Compared with the Lulin measurement of R=19.8
at 2.5 hours (Hsieh et al., GCN 4123), this indicates a mean temporal
decay index steeper than -1.6, possibly a jet break.  In combination
with the bright near-IR detection by Haislip et al. (GCN 4127), J=17.16
at 12.4 hours, and if the decay index between 12.4 and 15.4 hours is
not steeper than -3.5, the implied R-J color is greater than 5, which
requires a high redshift.