TITLE:   GCN CIRCULAR
NUMBER:  21277
SUBJECT: GRB 170626A: SAO RAS confirmation of OT fading
DATE:    17/06/27 22:58:52 GMT
FROM:    Moskvitin Alexander at SAO RAS  <mosk@sao.ru>

A. Moskvitin (SAO RAS), A. Volnova (IKI RAS), A. Pozanenko (IKI RAS),
I. Reva (Fesenkov Astrophysical Institute),
report on behalf of larger GRB follow-up team:

We observed the field of the GRB 170626A (Marshall et al., GCNC 21264)
with the 1-m telescope of SAO RAS, Zeiss-1000 on June, 27.
The OT observed by numerous telescopes (de Ugarte Postigo et al.,
GCNC 21269; Melandri et al., GCNC 21270; Gres et al. GCNC 21272;
Izzo et al., GCNC 21273; Volnova et al., GCNC 21276)
is approx. 0.9 magnitude fainer than in previous SAO epoch.
F ~ (t-T0)^ alpha. The decay slope alpha is about -0.7.
ftp://ftp.sao.ru/pub/grb/GRB170626A/GRB170626A_2epochs.png

The result of the first SAO epoch (GCNC 21268) were checked with the
full 1.5 hour stack obtained during the first night.
The stacked frames of both epochs were re-calibrated
against all standards mentioned by Volnova et al. (GCNC 21276).
The results are presented in the following table.
The errors are statistical only.


Date       UT start   T_mid - T0, d  Filter Exp., s   mag_OT   Err.

2016.06.26 19:57:42   0.4695         Rc     18 x 300  19.83 +/- 0.04 (GCNC
21268)
2016.06.27 18:50:27   1.4079         Rc     12 x 300  20.68 +/- 0.07