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GCN Circular 257

GRB990123, Optical Observation
1999-02-15T05:21:39Z (25 years ago)
Jules Halpern at Columbia U. <>
J. P. Halpern, Y. Yadigaroglu, K. M. Leighly, & J. Kemp (Columbia U.)
report on behalf of the MDM Observatory GRB follow-up team:

"We imaged the field of GRB 990123 in the R band on Feb. 14.50 UT using
the MDM Observatory 2.4m telescope.  A total of 100 minutes exposure was
obtained in seeing of 0.9 arcsec.  An object is detected at a position
consistent with that of the GRB host galaxy as measured in the HST STIS
image (Bloom et al. astro-ph/9902182).  Its magnitude is R = 23.77 +/- 0.10 
(referenced to the comparison star of GCN #207), equivalent to Gunn r = 24.10.
Since this is consistent with the estimate of r = 24.0-24.2 for the host
galaxy alone by Bloom et al., we conclude that the optical transient is no
longer making a detectable contribution to the total light in ground-based

We also report a revised magnitude on Feb. 3.54 of R = 23.55 +/- 0.24,
after more detailed analysis of those data.

Further HST observations are therefore needed to follow the unusual afterglow 
decay of this most energetic GRB.

The latest MDM image will be posted at

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