TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  2110
SUBJECT: GRB 030329, color evolution
DATE:    03/04/07 11:36:10 GMT
FROM:    Sylvio Klose at TLS Tautenburg  <klose@tls-tautenburg.de>

A. Henden (USRA/USNO, Flagstaff), B. Canzian (USNO),
A. Zeh, S. Klose (Thueringer Landessternwarte Tautenburg),

on behalf of the FUN and another collaboration

report:


USNO Flagstaff has been observing the optical transient of GRB 030329
starting 0.65 days after the burst with high photometric accuracy. A
plot of the BVRcIc data obtained during the first 8 days after the
burst reveals short-term (during a night) and long-term (over days)
color fluctuations. In particular, the USNO data reveal a broad bump
in the B-Ic color of the  optical transient around day 5 when the
afterglow shows an excess of red light compared to the earlier light
curve. Before the occurrence of this bump the optical transient
reddened continuously but slowly.

Between day 5 and 7 the data reveal also an increase in the B-Rc color
but a decline of the V-Ic and Rc-Ic colors. On day 8, however, this
trend has stopped and the optical transient was considerably redder in
all colors (B-V, V-Rc, Rc-Ic). More precisely, the optical transient
was redder than ever before. Based on the USNO data alone one cannot
decide what the reason for this color evolution is (emission lines
from the underlying host galaxy, a dust echo, broad-band supernova
features, intrinsic afterglow physics, etc.). If it is a supernova
(Matheson et al., GCN 2107; Bersier et al., GCN 2109) then the color
changes much more rapidly than predicted by the simplest model (Zeh et
al., GCN 2081).