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) 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 will be available shortly at http://www.noao.edu/noao/grb/990123.html .
This report is citable.
Any scientific use of this data should credit E. Falco, C. Petry, C. Impey, and J. Rhoads, on behalf of the KPNO GRB Followup Team.
Further observations may be obtained.
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