TITLE:   GCN CIRCULAR
NUMBER:  10814
SUBJECT: GRB 100528A: NOT optical observation
DATE:    10/05/29 16:30:12 GMT
FROM:    Dong Xu at DARK,NBI  <dong@astro.ku.dk>

D. Xu (Weizmann Inst.), P. Jakobsson (Univ. Iceland), A. Irrgang (Karl 
Remeis Observatory & ECAP) report on behalf of a larger collaboration:

We observed the field of GRB 100528A (Del Monte et al., GCN 10810; Grupe 
et al., GCN 10813) with the NOT equipped with ALFOSC in the B, V, R, and 
i filters. Observations started on May 29 at 04:08 UT (26.34 hrs after 
the GRB). We obtained 3x180 s frames in each filter.

An optical source is found at the UVOT position (Grupe et al., GCN 
10813) and is clearly detected in the stacked frame in each filter. 
Preliminary analysis shows the following Vega magnitudes:

m(B)=21.71+/-0.06  (mid_time=26.4417 hr)
m(V)=20.95+/-0.03  (mid_time=26.6751 hr)
m(R)=20.48+/-0.02  (mid_time=26.9090 hr)
m(i)= 20.02+-0.02   (mid_time= 27.1415 hr),

which are mainly calibrated against the #1178-0655113 star in the USNO 
B1 catalog and the Galactic extinction is not taken into account.

The source is very likely the afterglow of GRB 100528A as there exists 
decay in the V filter compared with the measurement in Grupe et al. (GCN 
10813).