GCN Circular 218

GRB 990123: New Optical Observations, Decay Measure

J. S. Bloom, S. R. Kulkarni, S. G. Djorgovski, S. C. Odewahn (CIT), R.
Sagar, A. K. Pandey, Neelakshi, R. K. S. Yadav (U. P. State Observatory,
India) report on behalf of the Caltech-NRAO-CARA GRB collaboration:

"On 24.0 Jan 1999 UT, we imaged the optical transient (GCN #201) of GRB
990123 (GCN #199) with the 1.04-m UPSO telescope, India.  In one 50-minute
B-band image the transient is well-detected at B = 20.16 +/- 0.15 (Jan
23.958 UT).  The transient is also well-detected in a 60-minute stacked
R-band exposure at gunn-r = 20.02 +/- 0.11.  The B-band image was
calibrated to the zeropoint of GCN #206 with a set of secondary stars in
the OT field.  The R-band photometry was tied the gunn-r system using the
Palomar 60-inch photometry (#207). The error associated with the
calibration between the two somewhat different bandpasses is small
compared to the statistical uncertainties.  These measurements have not
been corrected for Galactic extinction which would amount to A_B = 0.069
and A_r = 0.041 (GCN #207).  With the Galactic extinction correction, the
transient still blue (ie. has a positive spectral index beta ~= 0.3) at
this second epoch (see GCN #207).

Assuming a power-law decay (f_nu[nu] = const*t^alpha) we find

    alpha_B = -0.90 +/- 0.11    (between Jan 23.58 and Jan 23.95)
    alpha_r = -1.04 +/- 0.14    (between Jan 23.56 and Jan 24.00)

This decay is significantly flatter than alpha = -1.5 found at earlier
times (ie. t <= Jan 23.5; GCN #208). A recalibration of the transient
magnitude reported by Garnavich et al. (GCN #215) using the reference star
from Gal et al. (GCN #207), gives r = 21.0 (errors not reported) on Jan
24.547 UT.  This implies a decay of alpha = -1.26, consistent with that
found above.  

An updated light-curve may be obtained at 
http://astro.caltech.edu/~jsb/GRB/grb990123.html

This message may be cited."