TITLE: GCN CIRCULAR NUMBER: 21014 SUBJECT: Swift Trigger 748858: MASTER-OAFA early optical observations DATE: 17/04/19 13:58:58 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs R.Podesta, C.Lopez, F. Podesta Observatorio Astronomico Felix Aguilar (OAFA) , National University of San Juan, Argentina H. Levato, C. Saffe Instituto de Ciencias Astronomicas,de la Tierra y del Espacio (ICATE), San Juan, Argentina V. Lipunov, E. Gorbovskoy, V.Kornilov, D.Kuvshinov, N.Tyurina, A.V.Krylov, I.Gorbunov, P.Balanutsa, A.Kuznetsov, V.V.Chazov, D. Vlasenko Lomonosov Moscow State University, Sternberg Astronomical Institut of MSU K.Ivanov, S.Yazev, N.M.Budnev, O.Gres, O.Chuvalaev, V.A.Poleshchuk, O. Ershova Irkutsk State University V.Yurkov, Yu.Sergienko Blagoveschensk Educational State University, Blagoveschensk A. Tlatov, V.Senik, A.V. Parhomenko, D. Dormidontov Kislovodsk Solar Station of the Pulkovo Observatory 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 MASTER II robotic telescope (MASTER-Net: http://observ.pereplet.ru) located in OAFA was pointed to the possible grb's Swift Trigger 748858 17 sec after notice time and 93 sec after trigger time at 2017-04-19 07:45:09 UT. On our first (20s exposure) set we haven`t found optical transient within SWIFT error-box (ra=199.85 dec=15.4703 r=0.05) (M. H. Siegel et al. GCN 21011). The 5-sigma upper limit has been about 18.7 mag Maximal limit of coadded images starting at 2017-04-19 07:45:09 (93 s after trigger time, 5000s exposure) is 19.7 mag. The observations made on zenit distance = 71 degrees, galaxy latitude b = 76 degree. The moon (51 % bright part) is 47 degrees above the horizon. The distance between moon and object is 104 The sun altitude is -41.8 degree. The object observed till 2017-04-19 09:29:17 The message may be cited.