TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  1893
SUBJECT: GRB 030226 : Predicted light curve
DATE:    03/02/27 03:25:19 GMT
FROM:    James Rhoads at STScI  <rhoads@stsci.edu>

J. E. Rhoads, J.M. Castro Cerón, J. Gorosabel, A. Fruchter, 
and C. Kouveliotou report:

Using the redshift lower limit (z > 2; Ando et al, GCNC 1884)
and the GRB fluence (5.7e-6 erg/cm2, 30 keV < E < 400 keV; 
Suzuki et al, GCNC 1888), we can predict the time of the break
in GRB 030226 light curve, assuming the standard burst energy
of Frail et al (2001) (or more precisely, exploiting their empirically
demonstrated correlation between break time and apparent isotropic
gamma ray energy).

Based on the relations in Frail et al, we obtain
 t_jet = 10.6 days
for a redshift z=2.0 (and t_jet < 10.6 days for z > 2.0).
Prior to that time the current slow decay may continue;
thereafter a much steeper decay, t^-2 or faster, may
occur (Rhoads 1999).

The primary uncertainty in the predicted break time arises from
the observed scatter in the E_isotropic - t_jet correlation.  The
quoted one-sigma scatter in gamma ray energy in Frail et al (2001)
is a factor of 2.  We infer a corresponding scatter of a factor
of 2.5 in t_jet, placing a one-sigma lower limit  t_jet > 4 days.

Together with the current decay slope (0.88; Price & Warren, GCNC 1890)
and the t=3.5 hour flux (R=18.44; Garnavich et al, GCNC 1885)
this suggests that the optical afterglow of GRB 030226 will be rather
bright during the interval 1 day < t < t_jet, relative to most afterglows
at the same observed age.  Representative fluxes using this naive
extrapolation are

       GRB age (days): 0.75  1.0   1.5   2.0   3.0   4.0   5.0  7.5   10.0
Projected R magnitude: 20.0  20.3  20.7  20.9  21.3  21.6  21.8 22.2  22.5

We stop our projections at the nominal 10 day jet break time, and
further caution even a modest steepening of the decay index early on
would substantially reduce the fluxes at a few days.