GCN Circular 208

GRB 990123: Optical Decay Slope

J. S. Bloom, S. R. Kulkarni, S. G. Djorgovski, S. C. Odewahn, R. R. Gal,
L. M. Lubin (CIT), and D. A. Frail (NRAO) note on behalf of the
Caltech-CARA-NRAO GRB collaboration:

"After an initial rise to about 1 Jy, the early-time optical light-curve
of the transient afterglow of GRB 990123 (as reported by the ROTSE Team;
GCN #205) displayed a power-law decay.  We note here that the decay
appears to be well-continued by the later time (~3 hours) optical fluxes
of the transient discovered at Palomar (GCN #201; GCN #206; GCN #207).  
The decay constant is approximately alpha = -1.5 (defined as F_nu (t) =
const * t^alpha). This is consistent with the decay constants measured at
late-times in other GRB afterglows.

This suggests a strong phenomenological connection between the emission 
mechanism(s) at very early times (from tens of seconds to few minutes after 
the burst) and at later times (>~ few hours).  The continuity of the power-law
light curve represents a strong constraint for theoretical models of optical 
emission from GRBs and their afterglows.

A lightcurve, which will be actively updated, may be obtained at
http://astro.caltech.edu/~jsb/GRB/grb990123.html

This message may be cited."