GCN Circular 11191

M. Andreev, A. Sergeev (Terskol Branch of Institute of Astronomy) and 
A.Pozanenko (IKI) on behalf of larger GRB follow up collaboration report:

We are continuing observations of the Swift GRB 100901A (Immler  et al., GCN 
11159
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) with the Z-600 telescope of Mt.Terskol observatory in R-filter 
(Andreev  et al., GCN 11166
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). In a combined image at mid time Sep. 03 (UT) 
00:36 we detect  afterglow (Immler et al., GCN 11159
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; Guidorzi et al., GCN 
11160
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; Ivanov et al., GCN 11161
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; Klunko et al., GCN 11162
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).

In comparison with our previous photometry (Andreev  et al., GCNs 11166
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, 
11168
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) below we used several  reference stars of SDSS DR7 and Lupton 
transformation ugriz -> BVRI:

T0+     Filter, Exposure,  OT
(d)                 (s)

1.4599 R     10x120        19.22 +/- 0.05

The photometry above is consistent with steady decay based on early 
photometry (Kann et al, GRB 11187). We note however the systematic faintness 
of about ~0.25m in SDSS DR7 in comparison with USNO-B1.0 R2 which can 
explain possible contradiction of MITSUME observation calibrated against 
SDSS (Kuroda et al, GCN  11189
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) and photometry reported in Yoshida et al 
(GCN 11190
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). Detailed calibration of the field stars is necessary.