[vsnet-grb-info 7062] GRB 081203A: optical observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8645
SUBJECT: GRB 081203A: optical observations
DATE:    08/12/10 02:07:24 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen at iki.rssi.ru>

V. Rumyantsev, K. Antonyuk (CrAO), M. Andreev(Terskol Branch of Institute 
of Astronomy), A. Pozanenko (IKI) on behalf of larger GRB follow up 
collaboration report:

We observed   the field of GRB 081203  (Parsons  et al, GCN 8595) with the 
AZT-11 telescope of CrAO observatory  starting  Dec. 03 (UT) 15:44:29 
under good weather conditions and seeing of about ~2 arcsec. Several 
exposures were obtained in VRI bands up to 17:13:56 and then between
Dec. 04 (UT) 01:19:13 - 02:47:51.

Coordinates of the afterglow are RA(J2000)= 15:32:07.58 Dec(J2000)= 
+63:31:14.8 with uncertainty of 0.2 arcsec which is compatible with the 
coordinates of the afterglow (Parsons  et al, GCN 8595).

Preliminary photometry based on SDSS star J153200.06+633229.9 
(RA(J2000)=15:32:00.01 Dec(J2000)=+63:32:30.4) assuming V=17.28   R=16.85 
I=16.46 is following

T0+  Filter Exp.  mag.
s           s

  6528 V     180   16.84 +/-0.02
  6712 R     180   16.63 +/-0.02
  6896 I     180   16.22 +/-0.02
11524 V     180   17.68 +/-0.05
11708 R     180   17.53 +/-0.03
11892 I     180   17.15 +/-0.04
41196 R   3x180   20.27 +/-0.15
46144 R   3x180   20.20 +/-0.14

While light curves in a time interval 6500 - 12000 s after burst onset in 
R and I bands look similar with a power law decay index of about 0.66, the 
power law decay index in V-band is ~0.59 and one can suggest a spectral 
variability of the afterglow. Also we observed steep decay between 12000 - 
46000 s with a power law index of about 1.9.