TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  1607
SUBJECT: GRB021004: optical observations and predicted break time
DATE:    02/10/08 19:58:09 GMT
FROM:    Daniele Malesani at Brera Astro. Obs.  <malesani@merate.mi.astro.it>

Malesani D., Covino S., Ghisellini G., (INAF-OAB), D. Lazzati (IOA,
Cambridge, UK), Cecconi M., Fugazza D., Guerra J.C. (INAF-TNG), Masetti
N. (IASF-CNR), Pian E. (INAF-OATs), on behalf of a larger Italian
collaboration, report:

On Oct 6.05 UT, we observed the optical counterpart of GRB 021004
(Shirasaki et al., GCN 1565; Fox, GCN 1564) using the 3.6m TNG telescope
in the Canary Islands, with an R and an I filter. With respect to the
comparison star used by Fox and calibrated by Henden (GCN 1583), the
source has the following magnitudes:

UT     Band  Mag    Error
6.062  R     19.72  0.04
6.058  I     19.22  0.04

Our values are in good agreement with the nearly simultaneous
measurements of Holland et al. (GCN 1585), Bersier et al. (GCN 1586),
and Halpern et al. (GCN 1593).


To estimate the break time t_break of the light curve, we can use the
correlation discovered by Frail et al. (2001, ApJ, 562:L55), upon which
the standard energy reservoir result is based; this can be expressed
through

t_break = t_0 * (1+z) / E_iso

where t_0 = 22(+34)(-16) days and E_iso is the isotropic energy in units
of 10^52 ergs. The large error for t_0 originates from the intrinsic
dispersion (a factor of 2) in the distribution of beaming-corrected
energies (Frail et al. 2001). This result, however, holds independently
of the unknown jet opening angle, external density and radiative
efficiency.

Using the HETE-2 fluence (Lamb et al., GCN 1600), and assuming a
bolometric correction k = 1.3+-0.2 (calculated following Bloom, Frail &
Sari 2001, AJ, 121:2879), we can predict the break time for this
afterglow; we quote the results for the two reported values of the
redshift z:

1) z = 1.60 (Fox et al., GCN 1569):
   E_iso = 3.1*10^52 ergs;
   7.5 days < t_break < 47 days;
   t_break_best = 18.8 days.

2) z = 2.32 (Chornock & Filippenko, GCN 1605):
   E_iso = 5.6*10^52 ergs;
   5.3 days < t_break < 34 days;
   t_break_best = 13.4 days.

Given the slow average decline shown by this burst, this event should
remain bright longer than usual. Further observations are hence
encouraged.


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