TITLE:   GCN CIRCULAR
NUMBER:  11508
SUBJECT: GRB 101225A: optical break from NOT observation
DATE:    10/12/31 01:43:13 GMT
FROM:    Dong Xu at Weizmann Inst  <dong@astro.ku.dk>

D. Xu (Weizmann Inst.), P. Hakala (Turku U.), J.P.U. Fynbo (DARK/NBI)
report on behalf of a larger collaboration:

We imaged the filed of GRB(or XRF) 101225A (Racusin et al., GCN 11493)
at the Nordic Optical Telescope (NOT) equipped with either StanCam or
ALFOSC in B, V, R, and I filters. The R-band optical afterglow  (Xu et
al., GCN 11495) light curve was found decaying very slowly from
m(R)=22.6 +/- 0.2 at 5.47167 hrs after the BAT trigger to m(R)=22.8
+/- 0.2 at 55.2 hrs after the BAT trigger (Xu et al., GCN 11496;
Wiersema et al., GCN 11502; Thoene et al., GCN 11503; Cenko, GCN
11506).

Two 450 s R-band images were obtained with NOT/ALFOSC, starting at
21:50:46.9 UT, 30/12/2010 (i.e., 123.217 hrs after the BAT trigger)
under the conditions of seeing ~1". The afterglow was clearly detected
in the stacked image, largely pointlike, and had m(R)=23.3+/-0.2. This
indicates that R-band afterglow is now decaying faster compared with
its behavior during the past days, and that the host hasn't dominated
the brightness, being consistent with the featureless spectroscopy
(Wiersema et al., GCN 11502; Chornock et al., 11507).