TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  1911
SUBJECT: GRB 030226: Steep Decline
DATE:    03/03/01 13:39:35 GMT
FROM:    Shri Kulkarni at Caltech  <srk@astro.caltech.edu>

S. R. Kulkarni, D. W. Fox, E. Berger and A. M. Soderberg,
California Institute of Technology, Pasadena, CA 91125, USA
report

"We have continued observations of the optical afterglow (Fox, Chen and
Price GCN 1879) of GRB 030226 (Suzuki et al. GCN 1888) with the
Echellette Spectrograph Imager (ESI)  mounted at the Cassegrain focus
of of the Keck-II telescope. Over the last 24 hours (from epoch day 2.3
to day 3.3) the R band flux has faded by 1.6 +/- 0.1 magnitudes.  If
interpreted as a power law decay the power law index, alpha, is about 4
(flux, f(t) proportional to t^-alpha).

Such a steep decline is unprecedented.  Even a density discontinuity
does not produce such a steep decrease (see Panaitescu & Kumar ApJ 541,
L51, 2000).  Substructure (in the ejecta or circumburst medium) or
copious radiative losses can potentially account for the steep
decline.

Separately, we note that GRB 030226 with it's early steepening (``jet
break time'' of 0.5 day; see Greiner et al. GCN 1894 and Zeh et al. GCN
1898) joins GRB 980519 and GRB 980326. These latter GRBs are notable
for being under-energetic when compared to the known GRB sample and may
form an interestind sub-class of cosmlogical GRBs (see Bloom, Frail and
Kulkarni, astro-ph/0302210).

The diversity both in energetics and circumburst density demonstrate
that bursts are those which deviate from the sample are likely to be
very interesting. Extrapolations of based on mean values of
the current sample of GRBs can thus be misleading and 
lead observers astray."