GCN Circular 402

J. Kemp & J. Halpern (Columbia U.) report on behalf of the
MDM Observatory GRB follow-up team's southern extension:

"We observed the optical counterpart of GRB 990712 in the R band 
on Aug. 16.32 UT using the CTIO 0.9m.  A total of 50 minutes exposure
was obtained in seeing of 2."0.  An object of R = 21.73 +/- 0.06 is
detected at position (J2000) RA 22:31:53.00, Dec -73:24:28.7 in the 
USNO-A2.0 reference system, with uncertainty of 0".4.  This is consistent
with the OT position as measured by Bakos et al. (GCN #387) and 
Hjorth et al. (GCN #389).  The magnitude is referenced to the secondary 
standards used by Bakos et al. (IAUC #7225), although our own calibration
using Landolt standards (cf. GCN #395) is about 0.08 mag brighter.
The object appears only marginally extended in these images of relatively
poor seeing.  Revising the decay model of Hjorth et al. (GCN #389), we
find a slope of -1.05 and a constant contribution of R = 21.78 from the
host galaxy.  An extrapolation of the OT decay with a slope of -1.05 would
have predicted R = 25.5 at this time. In this model, the observed light is
now dominated by the host galaxy.  HST observations are needed to follow 
the afterglow decay, and to locate it within its relatively low-redshift 
host galaxy (z=0.43, Galama et al. GCN #388).  An HST observation is 
scheduled for Aug. 29.  If the OT decay continues with a slope of -1.05,
it will be found at R = 25.9 on Aug. 29.

The CTIO image and revised decay curve are posted at 
http://www.astro.bio2.edu/grb/

This message may be cited."