TITLE: GCN CIRCULAR NUMBER: 17838 SUBJECT: GRB 150518A: MASTER-net before, early and after optical stereoscopic observations with possible MP discovery inside MAXI error box DATE: 15/05/21 13:09:21 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs E. Gorbovskoy, V. Lipunov, N.Tyurina, V.Kornilov, P.Balanutsa, A.Kuznetsov,D.Kuvshinov, Lomonosov Moscow State University, Sternberg Astronomical Institute D.Buckley, S. Potter, A.Kniazev, M.Kotze South African Astronomical Observatory A. Tlatov, A.V. Parhomenko, D. Dormidontov, V.Sennik Kislovodsk Solar Station of the Pulkovo Observatory K.Ivanov, O.Gres, N.M.Budnev, S.Yazev, O.Chuvalaev, V.A.Poleshchuk Irkutsk State University V.Yurkov, Yu.Sergienko, D.Varda, E.Sinyakov, A.Gabovich Blagoveschensk Educational State University, Blagoveschensk 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-Kislovodsk robotic telescope (MASTER-Net: http://observ.pereplet.ru, FOV = 2x2 square degrees in paralel mode ( Lipunov et al., 2010, MASTER Robotic Net, Advances in Astronomy, vol. 2010, pp. 1-7)) located in Kislovodsk was pointed to the MAXI/GSC GRB150518A (Kawamuro et al., GCN 17825) 21 sec after notice time and 3189 sec after trigger time at 2015-05-18 22:35:43 UT. On our first (180s exposure) image we do not found OTs within Swift/XRT erro-box (Sbarufatti et al., GCN 17827) brighter then 18.5 . But we see bright object at positon: 15h 36m 53.67s , +16d 19m 36s.5 with 17.5 unfiltered mag outside XRT and inside MAXI. MASTER-SAAO robotic telescope (MASTER-Net: http://observ.pereplet.ru, FOV = 2x2 square degrees in paralel mode ( Lipunov et al., 2010, MASTER Robotic Net, Advances in Astronomy, vol. 2010, pp. 1-7)) located in Souht Africa Astronomical Observatory was pointed to the MAXI/GSC GRB150518A (Kawamuro et al., GCN 17825) 13 sec after notice time and 3189 sec after trigger time at 2015-05-18 22:35:35 UT. On our first (180s exposure) image we not found OTs within Swift/XRT error-box (Sbarufatti et al., GCN 17827) brighter then 17.1 . But we see bright object at positon: 15h 36m 51.70s , +16d 19m 13s.6 with ~ 17.0 unfiltered mag outside Swift/XRT and inside MAXI. There is one asteroid MPC 2010 KL61 with poorly astrometrical efemerids (Leave for survey recovery) at MPC Data Base. The offset both Russian and South Africa OT positiion 36 arc sec. This corresponds to parallax effect for Solar System Body. We do not see any OT at the aftergow canditate position (Xu et al., GCNC 17829, 17832; Mao et al., GCNC 17830; Littlejohns et al., GCNC 17831; Moskvitin GCN 17834) with same limit (3 sigma). We have only one alert image in each telescope because MASTER telescope recieved new MAXI correction of the error box and X-ray Transient leave the MASTER FOV during alert observations. But we have observation this X-ray aftergrow position before last trigger activated by previouse Fermi (trigger number 451674501, 25 apr 2015) and during regular synoptic survey (MASTER-Tunka, ~ 2 days after trigger). Preliminary results: Id Pro.type Date time Exp.time Limit Filt. Site Starting (3sigma) 759597 SumSurvey 2015-05-20 17:02:11 480 20.1 W Tunka 1458076 MaxiAlert 2015-05-18 22:35:35 180 17.1 P/ SAAO 1458345 MaxiAlert 2015-05-18 22:35:43 180 18.5 P| Kislovodsk 823020 FermiInsp 2015-04-25 20:11:41 180 19.9 W Kislovodsk Where W - unfiltered, P - polarization unfiltered. All magnitude calculated with respect to hundrends USNO B stars with calibration: m = 0.2B + 0.8R . At the Tunka Survey we see marginaly faint (~2 sigma) object on the X-ray position (Sbarufatti et al., GCN 17827). But this is may be noice. We have a lot of images in MASTER NET Data Base imaging last several years with Swift XRT error box. No object we see on this place with 20.5 mag limit on first overview (since 2010). The message may be cited.