GCN Circular 18511

D.Buckley, S. Potter, A.Kniazev, M.Kotze
South African Astronomical Observatory

R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres
The Instituto de Astrofisica de Canarias

E. Gorbovskoy, V. Lipunov,    N.Tyurina, V.Kornilov, 
P.Balanutsa, A.Kuznetsov, D.Kuvshinov, D. Vlasenko, E.Popova
Lomonosov Moscow State University, Sternberg Astronomical Institute

K.Ivanov, S.Yazev, N.M.Budnev, O.Gres, O.Chuvalaev, V.A.Poleshchuk
Irkutsk State University

A. Tlatov, A.V. Parhomenko, D. Dormidontov, V.Sennik
Kislovodsk Solar Station of the Pulkovo Observatory

V.Yurkov, Yu.Sergienko, D.Varda, E.Sinyakov
Blagoveschensk Educational State University, Blagoveschensk

V.Krushinski, I.Zalozhnih
Ural Federal University, Kourovka

Hugo Levato and Carlos Saffe
Instituto de Ciencias Astronomicas, de la Tierra y del Espacio (ICATE)

Claudio Mallamaci, Carlos Lopez and Federico Podest
Observatorio Astronomico Felix Aguilar (OAFA)


Two MASTER II  robotic telescope (MASTER-Net: http://observ.pereplet.ru) 
located in SAAO and IAC was pointed to the  GRB151027B (Ukwatta et. al GCN 
18499
Not found
 
 
) 25 sec after notice time and 156 sec after trigger time at 
2015-10-27 22:43:19 UT. On our first (30s exposure)  set we haven`t found 
optical transient (Malesani et. al. GCN 18501
Not found
 
 
, Xu et. al. GCN 18505
Not found
 
 
, 
Wiseman et. al GCN 18507
Not found
 
 
)  within SWIFT error-box. The 2-sigma upper limit 
has been about 17.7 mag. The observations were significantly complicate 
the closest distance to the Moon (~ 35 d). The Moon background light is 
picked up the sky background on our cameras more than 10 times. Therefore, 
we could not get deep enough image. Nevertheless, we detect OT on a 
several coadd images in consequent time  in  SAAO.
The first time we see  OT 475 seconds after the trigger with magnitude ~ 
18.9.  OT had a slight brightening 1736 seconds after the trigger to 18.5 
mag.
The preliminary photometry available at table below.

Table 1. GRB151027B MASTER-SAAO Early photometry.

T-Ts    T-Tm     Exptime   Mag.(P/+P\)   Coadd
159      174     2 x 30     >17.7          1
201      221     2 x 40     >18.0          1
252      277     2 x 50     >18.3          1
313      343     2 x 60     >18.3          1
384      424     2 x 80     >18.4          1
159      311     2 x 260    >19.0          5
475      842     2 x 690     18.9          5
1235    1736     2 x 900     18.5          5
2264    3280    2 x 1800     18.9         10
3499    4515    2 x 1800     19.1         10

T - is trigger time, Ts - exposure start time, Tm - exposure middle time.


We see OT in separate polarizations on s/n~5 confidence lewel. Table 1 
shows the averaged data from two mutually perpendicular polarization.


The message may be cited.