Аннотация:MASTER Global robotic net (MASTER-Net:http://observ.pereplet.ru Lipunov etal.,2010, Advances in Astronomy,2010,30L) started GRB 210420B (Swift trigger 1044382 Moss et al. GCN 29844, Ttrigger=2021-04-20 18:20-50UT) at MASTER-Kislovodsk at 2021-04-20 20:01:24UT There is optical counterpart (changing) at RA, Dec(2000)= 16h 57m 18.23s ,+42d 34m 11s.8 +-1” MASTER OT J165718.23+423411.8 found by MASTER auto-detection system with m_OT\raisebox-0.5ex 19 at 2021-04-20 20:05:23TU Observation started at alert altitude 45deg.