GCN Circular 240

GRB 990123:  Detection of the IR Transient, and the Light Curve Fits

J. S. Bloom, C. Koresko, S. R. Kulkarni,  S. G. Djorgovski, R. R. Gal, 
      and S. C.  Odewahn,  Caltech,
H. I. Teplitz, NASA Goddard Space Flight Center,
D. Koerner, Univ. of Pennsylvania,
D. Kirkpatrick, Infrared Processing and Analysis Center,
M. A. Malkan, and I. S. McLean, Univ. of California, LA,
D. A. Frail, National Radio Astronomy Observatory, 
report on behalf of the Caltech-CARA-NRAO GRB collaboration:

"The field of the optical transient of GRB 990123 (GCN 201
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 ) was observed
with the Near Infra-red Camera (NIRC) on the Keck I telescope on
24 January 1999 UT by Koerner and Kirkpatrick.  We report the discovery
of the IR counterpart of the afterglow with K = 18.3 +/- 0.03 mag (epoch 
24.6356 Jan 1999 UT).   The same field was observed by Malkan, Teplitz and
McLean on the nights of January 27.65 and 28.55 UT, and in both instances
the object was readily detected.  Fitting a power law to the three
magnitudes yields a power law exponent of alpha_K= -1.14 +/- 0.08.

Separately, we have carried out a program of recalibrating Gunn r and R 
band magnitudes reported by others via the GCN.  We used those measurements
which reported the magnitude of a reference star (or stars) and have put all 
the measurements on a common photometric system. A fit to these recalibrated 
magnitudes yields a power law slope, alpha_r = -1.13 +/- 0.03.  Within 
errors, the slope of the K band data is fully consistent with that obtained 
from the r/R band data. We adopt a mean alpha of -1.13.

The color difference between the decaying r and K fluxes is (r-K) = 2.75 mag. 
Converting the magnitudes to flux this color difference corresponds to 
beta = -0.82, where f_nu = const * nu^beta.  The ratio alpha/beta = 1.38. 
This suggests that the cooling frequency is higher than the central frequency 
of the r band (about 4x10^14 Hz) even two days after the burst.  We urge 
continued monitoring of the optical transient in the r/R band with the view 
of detecting the passage of the cooling break.

In contrast to the light curves measured in the r and K bands, the light
curve in the B band (calibrated on a uniform photometric system) is not well 
described by a power law.  The origin of this discrepancy is as yet unclear.
Further observations are in progress.

This report is citeable."