GCN Circular 248

I. A. Yadigaroglu & J. P. Halpern (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. 3.54 UT using
the MDM Observatory 2.4m telescope.  A total of 60 minutes exposure was
obtained in seeing of 1.2 arcsec.  An object is detected at magnitude 
R = 23.9 +/- 0.25 (referenced to the comparison star of GCN #207).
However, its position is approximately 0.6 arcsec north of the previous 
position.  The new position is (J2000) RA 15:25:30.343, Dec +44:45:59.86, 
whereas the position on Jan. 30.52 was (J2000) RA 15:25:30.330,
Dec +44:45:59.27.  These positions are measured with respect to the 
same set of comparison stars, and they each have a statistical uncertainty
of 0.3 arcseconds in radius, but negligible systematic difference.
A possible extension in a Jan. 27 K-band image 0.5 arcsec to the north 
of the OT was described by Djorgovski et al. (GCN #243).  Our new R-band
image is consistent with that report.

The optical transient has faded at an accelerated rate since our last
reported observation on Jan. 30.52 (GCN #242).  A continuation of the 
alpha_r = -1.13 decay (Bloom et al. GCN #240) would have predicted an
R magnitude of 23.47 on Feb. 3.54.  Since the position of the optical
centroid has shifted, we conclude that our measured R = 23.9 represents
an upper limit to the magnitude of both the OT and any coincident galaxy,
and that we are beginning to detect either an intervening galaxy or
the host galaxy of the burst.  We note that the accelerated decay of
the OT could be an indication that the cooling frequency has passed below
the optical band, or that a jet which initially was highly collimated 
toward us has begun to spread.  An alternative interpretation in which
the initial OT has disappeared, and at the same time been replaced by
a fainter lensed component, seems less likely.

The new MDM image will be posted in the near future at
http://cba.phys.columbia.edu/grb/

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