GCN Circular 214

E. Falco, C. Petry, C. Impey, A. Koekemoer, and J. Rhoads report on behalf
of the KPNO GRB Followup team:

Falco, Petry, and Impey have observed the optical counterpart of GRB 990123
in U band on 1999 January 24.497 UT using the 4 meter Mayall telescope and
CCD Mosaic Camera at Kitt Peak National Observatory.  Conditions were
non-photometric with 1.3 arcsecond seeing.
 
The counterpart (cf. Odewahn et al, GCN 201
Not found
 
 
) is clearly detected.  Absolute
flux calibration is not yet possible, but the transient is approximately
1.1 magnitude fainter than the reference object at position RA=15:25:32.7,
dec=+44:44:29.7 (J2000).  The statistical uncertainty in the flux
should be of order 3%.  The counterpart is a point source in our data,
suggesting that host galaxy flux contributes only a small fraction of
the current U band brightness.  We suggest that the afterglow may be bluer
than the host galaxy; if so, U band would be a good wavelength to follow
its light curve to late stages.
 
In addition, Koekemoer has observed the counterpart at K band on 1999
January 24.546 UT using the 2.1 meter telescope and ONIS near-infrared
camera at Kitt Peak National Observatory.  Total exposure time was 600
seconds.  The transient is not readily apparent in the coadded image.
The rough limiting magnitude of the K band image can be estimated from
the USNO-A1 catalog, which gives Red and Blue magnitudes 14.4 and 15.5
for a star with signal to noise ratio 10 in the combined K band image.
More careful processing might improve this limit modestly.
Combining these two measurements will yield a limit on the spectral slope
of the afterglow.
 
A section of the U band image is available at
http://www.noao.edu/noao/grb/990123.html .
 
This report is citable.