Microquasar V404 Cyg /GS 2023+338: MASTER optical observations during the June and December 2015 super-outbursts.

• We present MASTER 2230 homogeneous WBVRI+polarization data of microquasar V404 Cyg. • We confirm MASTER polarization variability () and present new episodes. • We discuss the time delays of optical flares via X-ray one. We present several thousands homogeneous photometric (WBVRI and polarization) observations of the microquasar black hole binary V404 Cyg, obtained by the MASTER Global Robotic Network (Lipunov et al. 2010). MASTER were the first telescopes to optically monitor the microquasar after its gamma-ray onset from 18 h 34 m 09 s UT on June 15, 2015 (), until December 31, 2015. We report the results of the analysis of these observations and compare them with the observed X-ray and gamma-ray activity. We confirm the optical emission is correlated with the hard X-ray radiation on timescales of less than 1 min to several hundreds of seconds sec and some anomalously long timescales of up to 25–30 min (). We find no correlation between the delays and the orbital phase. We confirm polarization variability, published earlier (), and present new polarization variability episode. [ABSTRACT FROM AUTHOR]