TITLE:   GCN CIRCULAR
NUMBER:  21210
SUBJECT: GRB 170531B: Zeiss-1000/CrAO optical observations
DATE:    17/06/07 21:38:36 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

E. Mazaeva (IKI), A. Pozanenko (IKI),  D. Chestnov (ISON), A. Novichonok 
(ISON), V. Agletdinov (KIAM), I. Nikolenko (CrAO),  A. Volnova (IKI), 
report on behalf of larger GRB follow-up collaboration:

We report refined analysis of Zeiss-1000/CrAO observations. Results 
reported below supersede GCN cir. 21174.
We observed the field of the GRB 170531B (Evans et al., GCN  21171) with 
Zeiss-1000/Koshka (CrAO) telescope  between May 31 (UT) 22:32:32 and 
June 01 (UT)  00:58:11. We took several unfiltered images with
exposure of 180 s. The afterglow  (Lipunov et al. GCN 21173;
Pozanenko et al. GCN 21174) is clearly visible in each image in first 
initial images, and detected later in combined images. The finding chart 
of the combined image can be found at
http://grb.rssi.ru/GRB170531B/GRB170531B_Koshka_170531_sources.png

The coordinates of the afterglow (source A in the finding chart) are 
(J2000) 19:07:32.10 -16:25:06.0 with uncertainty of 0.5 arcsec in both 
coordinates. The coordinates are consistent with the position reported 
by  de Ugarte Postigo et al., GCN 21177, Tyurina et al., GCN   21179, 
and Siegel et al., GCN 21188.


Preliminary photometry of the afterglow can be found in the light curve at
http://grb.rssi.ru/GRB170531B/GRB170531B_light_curve_v2.png

The photometry is based on nearby USNO-B1.0 stars, calibrated against to 
R2 magnitudes
USNO-B.1_id  R2
0736-0718845 16.77
0735-0729622 16.96
0735-0729439 16.59
0735-0729327 17.22

Due to presence of sources in near vicinity to the afterglow the 
photometry of wide FOV telescopes might be affected by the sources. In 
particular our photometry reported in GCN 21189 is apparently biased by 
the two sources C and D (see the finding chart). We report coordinates 
and brightness of the nearby sources.

Source  coordinates(J2000)          AG distance  filter  mag.   err.
C       19:07:32.34 -16:25:09.1     4.6"         CR      20.46  0.04
D       19:07:31.83 -16:25:05.6     3.9"         CR      20.47  0.04