TITLE:   GCN CIRCULAR
NUMBER:  18175
SUBJECT: GRB 150818A: Mondy optical observations
DATE:    15/08/19 16:25:12 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

E. Mazaeva (IKI), E. Klunko (ISTP), A. Volnova (IKI), I. Korobtsev 
(ISTP),  A. Pozanenko (IKI) report on behalf of larger GRB follow-up 
collaboration:

We observed the field of Swift GRB 150818A (D'Elia et al., GCNC 18152) 
with AZT-33IK telescope of Sayan observatory (Mondy) starting on Aug., 
18 (UT) 15:07:42. We obtained several images in R-filter of 120 s 
exposure. The source previously mentioned (McCauley et al., GCN 18161; 
Schulze  et al., GCN 18164) is clearly visible. Coordinates of the 
source are (J2000) 15:21:25.56 +68:20:32.0 with uncertainty of 0.3 
arcsec in both coordinates. Preliminary photometry of the source is 
following

Date       UT start   t-T0     Filter   Exp.    OT    Err.
                       (mid, days)       (s)

2015-08-18 15:07:42   0.16843  R        31*120  20.25 0.08

Photometry is based on USNO-B1.0 stars:

USNO-B.1_id  R2
1583-0162302 16.65
1583-0162268 17.48
1583-0162214 16.71
1583-0162248 19.43

We can suggest the source as shallow fading afterglow because the source 
is significantly faded between our observations (R=20.25+/-0.08 at 4 hrs 
after burst trigger) and NOT observations at 9.62 hrs (Schulze  et al., 
GCN 18164).