TITLE: GCN CIRCULAR NUMBER: 15220 SUBJECT: GRB 130907A: MASTER refined analysis DATE: 13/09/12 18:46:58 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs E. Gorbovskoy, V. Lipunov, D.Denisenko, V.Kornilov, A.Belinski, N.Tyurina, D.Kuvshinov, N.Shatskiy, P.Balanutsa, A.Kuznetsov, V.V.Chazov, A.Sankovich, D.Zimnukhov Sternberg Astronomical Institute, Moscow 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 K.Ivanov, S.Yazev, N.M.Budnev, O.Gres, O.Chuvalaev, V.A.Poleshchuk Irkutsk State University V.Krushinski, I.Zalozhnih, A. Popov 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) MASTER II robotic telescope (MASTER-Net: http://observ.pereplet.ru) located in Kislovodsk was pointed to the GRB130907A 18 sec after notice time and 43 sec after GRB time at 2013-09-07 21:42:00 UT in two polarizations. The observations were performed at low elevation (zenith angle 84 deg)and under the changing transparency conditions. The OT (Page et. al. GCN15183) appears on the 4th image in the series and is visible on four consecutive images (20, 30, 40 and 50 sec exposures) in both tubes with the signal-to-noise ratio from 3 to 6. Starting with T0+5 min, the image quality was further deteriorating with the OT visible only on the sum of two pairs of synchronous images. Our photometry results are presented in Table 1. Table 1. ------- Start date and time exptime T_start-T_trig T_mid-T_trig P| mag. err_P P- mag err_P- P| + P- err. (P| + P-) -----------------------+--------+----------------+--------------+--------+---------+----------+---------+----------+--------------- 2013-09-07 21:42:00.31 10 47 52 <10.5 --- <10.5 --- <11.0 --- 2013-09-07 21:42:22.79 10 69 74 <10.5 --- <10.5 --- <11.0 --- 2013-09-07 21:42:43.59 20 91 101 <11.0 --- <11.0 --- <11.5 --- 2013-09-07 21:43:16.66 20 123 133 12.2 0.2 13.0 0.3 12.5 0.2 2013-09-07 21:43:47.44 30 154 169 13.3 0.4 13.6 0.6 13.5 0.4 2013-09-07 21:44:29.95 40 197 217 13.7 0.5 13.7 0.6 13.9 0.5 2013-09-07 21:45:21.88 50 249 274 13.7 0.5 14.4 0.8 14.0 0.6 2013-09-07 21:46:23.87 60+80 311 387 <13.8 --- <13.7 --- 13.9 0.6 *) All time intervals in seconds. P| and P- is an unfiltered magnitude with RA and DEC orientated polaroid correspondingly. The sign '<' before magnitude means the value is upper limit. The light curve and movie available here http://master.sai.msu.ru/static/GRB/GRB130907A/grb130907A.html The epoch of our observations completely covers Swift BAT weak, soft peak from T+210 sec to T+240 sec (Cummings et. al. GCN15202). Our first automatic and not so exact (due to difficult observing conditions) estimation of the OT magnitude, given in the previous telegram (Gorbovskoy et. al GCN15184), belongs to epoch of the 5th exposure with t_mid=217 sec. after the trigger. This fact isn't clear from the telegram text that could mislead some researchers. We apologize for this inaccuracy. The message may be cited.