TITLE: GCN CIRCULAR NUMBER: 24326 SUBJECT: LIGO/Virgo S190425z: MASTER explanation of the nature of an optical flash detected by a Swift as a short flare on a fast-moving WISE brown DATE: 19/04/28 11:05:43 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, N.Tyurina, E. Gorbovskoy, V.Kornilov, D.Vlasenko, V.Vladimirov, D.Zimnukhov, A.Kuznetsov, P.Balanutsa, I.Gorbunov, A. Chasovnikov, V.Grinshpun, F.Balakin, A.Chasovnikov (Lomonosov Moscow State University, SAI, Physics Department), R. Podesta, C. Lopez, C.Francile, F. Podesta (Observatorio Astronomico Felix Aguilar OAFA, San Juan National University), H.Levato (Instituto de Ciencias Astronomicas, de la Tierra y del Espacio ICATE,SJNU) R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres (The Instituto de Astrofisica de Canarias), D. Buckley (South African Astronomical Observatory), O. Gress, N.M. Budnev, O.Ershova (Applied Physics Institute, Irkutsk State University), V. Yurkov, A. Gabovich, Yu. Sergienko, D. Kobcev (Blagoveschensk EducationState University), A. Tlatov, V.Senik, A.V. Parhomenko, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), Global MASTER-Net (http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) started inspect of the LIGO/Virgo S190425z errorbox (Singer et al. GCN 24168) 3133 sec after trigger time at 2019-04-25 09:10:18 UT (Lipunov et al. GCN 24167). MASTER-IAC observed Swift OT (Breeveld et al., GCN 24296) at 2019-04-26 01:45:59 with mlim=20.0, and at 2019-04-26 01:57:23 with mlim=20.0 during automatic inspect of GW190425. There is optical transient with m_OT ~ 19.0 detected at both images. MASTER-OAFA observed these coordinates at 2019-04-26 05:24:35 during automatic GW190425 inspection with unfiltered mlim=19.0. There is no OT detection. MASTER-SAAO observed this area at 2019-04-27 21:15:24 and at 2019-04-27 21:26:02 with mlim=21.0 and at 2019-04-26 04:01:55 with mlim=19.3 during automatic GW190425 inspection . There is no OT detection. All instrumental unfiltered magnitude is W = 0.2B + 0.8R with respect to USNO B reference stars. The object detected by Swift possibly connected with the blue flare on fast moving cool AllWISE star (RA,DEC) = 255.5777822 -12.4869992 (2" per yr!). So this OT not connected with Gravitational Waves Event. You can find below the community observations of the Swift OT region: Telescope Apr Limit OT Filt GCN Zwicky 25.4 20.5 - R 24302 Zwicky 25.4 20.5 - g 24302 ASAS-SN 25.4 18.7 - g 24313 Gattini 25.5 15.2 - J 24306 Zwicky 25.5 20.5 - R 24302 KMTNet SSO 25.6 21.5 - R 24318 MASTER-IAC 26.1 20.0 19.0 W This MASTER-OAFA 26.2 19.0 19.0 W This Zwicky 26.3 20.5 - g 24302 Gattini 26.4 15.2 - J 24306 Zwicky 26.5 20.4 - R 24302 ASAS-SN 26.5 18.7 - g 24313 Swift 26.8 ? 17.7 U 24296 ASAS-SN 27.3 18.4 - g 24313 MASTER-SAAO 27.88 21.0 - W This LCO SAAO 27.9 21.5 - R 24307 Lulin 28.8 21.5 - R 24301 These data do not contradict the hypothesis of a short (10-15 minutes flash) on a red-brown dwarf. The message may be cited.