TITLE:   GCN CIRCULAR
NUMBER:  35172
SUBJECT: GRB 231117A: Mondy optical observations
DATE:    23/11/23 23:07:40 GMT
FROM:    Alexei Pozanenko at IKI, Moscow <apozanen@iki.rssi.ru>

N. Pankov (HSE, IKI),  A. Pozanenko (IKI), E. Klunko (ISTP), S. Belkin (IKI) report on behalf of GRB IKI FuN:

We observed the field of GRB 231117A (Laha et al., GCN 35071; Navaneeth et al., GCN 35072; Beardmore et al., GCN3574; Cattaneo et al., GCN 35075; Svinkin et al., GCN 35079; Cheung et al., GCN 35081; Dafcikova et al., GCN 53095; Busman et al. GCN 35138; O'Connor et al., GCN 35139; Watson et al., GCN 35153, Kuin et al., GCN 35154; Dichiara  et al., GCN 35160; Fong et al., GCN 35163) with AZT-33IK telescope of Mondy observatory  in  R-filter on 2023-11-17, 2023-11-18 and 2023-11-20. Using image subtraction and the image on 2023-11-20 as a template we obtained preliminary photometry of the optical transient

Date       UT start  t-T0     Exp.   Filter   OT     Err.  
                   (mid, days)  (s)
2023-11-17 11:45:33  0.38349  30x120 R        21.00  0.16   
2023-11-18 12:09:46  1.41142  46x120 R        21.59  0.16  

The photometry is based on nearby USNO-B1.0 R2 stars. The index of power law (t-T0)^-alpha between  the two epochs is alpha = 0.8 (+0.13 -0.2).
However, photometry may be biased due to the template contamination by a still decaying optical transient, so we caution against using this index for any physical assessments.