TITLE: GCN CIRCULAR NUMBER: 23023 SUBJECT: GRB 180720B: MASTER Global Net OT observations DATE: 18/07/23 11:57:20 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, E. Gorbovskoy, N.Tiurina, D.Vlasenko, V.Kornilov, A.Kuznetsov, V.Chazov, I. Gorbunov, D.Zimnukhov, D.Kuvshinov, P.Balanutsa, V.Vladimirov, Lomonosov Moscow State University,SAI D. Buckley, South African Astronomical Observatory (SAAO) R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres The Instituto de Astrofisica de Canarias (IAC) A. Tlatov, V.Senik, D. Dormidontov Kislovodsk Solar Station of the Pulkovo Observatory R. Podesta, F. Podesta, C. Lopez, C.Francile Observatorio Astronomico Felix Aguilar (OAFA) H.Levato, Instituto de Ciencias Astronomicas, de la Tierra y del Espacio (ICATE) O. Gres, N.M.Budnev , Yu.Ishmuhametova Irkutsk State University (ISU) A. Gabovich, V. Yurkov, Yu. Sergienko Blagoveschensk Educational State University (BSPU) MASTER-Amur robotic telescope (MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Blagoveschensk State Pedagogical University) was pointed to the Swift GRB180720B (Siegel et al., GCN Circ #22973) 113 sec after trigger time at 2018-07-20 14:23:37 UT. So as the GRB was 4.6 degrees above the horizont, the good observations started on 2018-07-20 15:21:50UT. Galaxy latitude b = -63.07 degree. The moon (58 % bright part) is 9 degrees above the horizon. The distance between moon and object was 143. The sun altitude was -17.19 degree. The object observed till 2018-07-20 15:47:58 MASTER-SAAO robotic telescope (MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical observatory) was pointed to GRB180720B 27260 sec after trigger time at 2018-07-19 20:56:09 UT as MASTER's LAT (Bissaldi et al GCN22980)alert inspection with polarization filters. MASTER-SAAO auto-detection system ( Lipunov et al., "MASTER Global Robotic Net", Advances in Astronomy, 2010, 30L ) discovered bright OT source at (RA, Dec) = 00h 02m 06.92s -02d 55m 05.6s on 2018-07-19 20:56:09 UT. The OT magnitude is ~16.8m . There is no minor planet at this place. We have reference image on 2016-05-04.12293 UT with unfiltered mlim = 18.1m. The observations made on zenit distance = 16.7 degrees, galaxy latitude b = -74.37 degree. The moon (61 % bright part) was 24 degrees above the horizon.The sun altitude was -63.7 degree. MASTER-SAAO reobserved this area on 2018-07-21 22:28:10-22:54:51UT as MASTER's MAXI (Negoro et al GCN 22993) alert inspection with mlim=20.2 and unfiltered m_OT~19.1 The discovery and reference images are http://master.sai.msu.ru/static/OT/MASTERGRB180720B.jpg MASTER-Kislovodsk robotic telescope (MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Pulkovo Solar Station) was pointed to the GRB180720B (Siegel et al., GCN Circ #22973) 27457 sec after trigger time at 2018-07-20 21:59:21 UT . On our first (180s exposure) set MASTER-Kislovodsk auto-detection system discovered OT with m_OT~17.0, automatical mlim~17.6. The observations made on zenit distance = 25.66 degrees, galaxy latitude b = -63.07 degree. The moon (61 % bright part) is below the horizon(The altitude of Moon is -8). The distance between moon and object is 138 The sun altitude is -24.87 degree. The object observed till 2018-07-20 22:19:52 MASTER-IAC robotic telescope (MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Spain (IAC Teide Observatory) was pointed to the GRB180720B (Siegel et al., GCN Circ #22973) 44691 sec after trigger time at 2018-07-21 02:46:35 UT . On our first (60s exposure) MASTER-IAC auto-detection system discovered optical transient within Swift error-box with automatical magnitude ~18.0. The observations made on zenit distance = 43.01 degrees, galaxy latitude b = -63.07 degree. The moon (63 % bright part) is below the horizon(The altitude of Moon is -15). The distance between moon and object is 136 The sun altitude is -36.01 degree. The object observed till 2018-07-21 03:03:36 The visibility GRB error box (coord:0.5279, -2.9170 error_box: 0.05) at trigger time at different MASTER sites: obj: -47.70 sun: 29.73 - Tavrida (Crimea, Russia) obj: -41.62 sun: 72.28 - IAC, Teide, (Tenerife, Spain) obj: -54.25 sun: 15.39 - SAAO (Sutherland, SA) obj: -46.85 sun: 23.59 - Kislovodsk (Russia) obj: -29.98 sun: 15.28 - Ural(Kourovka, Russia) obj: -10.74 sun: -8.46 - Tunka (near Baykal Lake, Russia) obj: 4.55 sun: -17.19 - Amur(Blagoveschensk) obj: 6.72 sun: 27.64 - OAFA (Argentina) The message may be cited.