The Galactic low-mass X-ray binary GS 2023+338 was discovered during the burst on May 22, 1989 with the Japanise satellite Ginga (aka Galaxy). The flux reached 17 crabs (this unit of flux measurement is an bright X-ray source, Crab Nebula) at 10-35 keV. At the time, the object increased its brightness by about seven magnitudes and reached a brightness of 11.6mag in the filter V ; thus the object could be observed with an amateur telescope with a diameter of 20-30 cm.
    This object proved to be a recurrent nova V404 Cygni (a variable star in the Cygnus constellation) which has already the burst in 1938. The system is dynamically resolved due to optical spectroscopic measurements. The mass function turned out to be very high: f(M)=6.0, which at once sets the lower limit of six solar masses on the mass of an invisible compact object. There are two stellar objects in the binary system on orbit with a period of 6.5 days: a black hole with a mass of about 10.6 solar masses and an orange sub-giant, a K0 star with a mass of about 0.3 solar masses (Cherepashchuk et al.) and a photospheric temperature of about 4700 K.
    Apparently we deal with a system where a subgiant fills its Roche lobe (at a distance of about 60 million kilometres from the black hole) and transfers the matter onto the accreting black hole; as a result, the hot accretion disk forms and the system becomes very bright in the X-rays.
    The distance to the star of 2.39 kpc (7400 light-years) was exactly determined from the measurements of the annual parallax of the weak quiet radio emission (0.3 mJy) from V404 Cyg using the interferometric system (VLA+VLBA+GBT) in USA (Miller-Jones+, 2009).
    Even the measurements of 1989 clearly show that it is an X-ray binary with irregular relativistic ejections of the matter from the inner parts near the black hole (Han and Hjellming, 1992). These objects are called microquasars in modern classification. Korbel et al. (2008) showed that the X-ray and radio flux from microquasars are well correlated, thus we may expect high radio fluxes during the X-ray bursts and low fluxes between those bursts.
    According to the BAT-data from the Swift space observatory the X-ray flux in the range 15-50 keV reached 40 crabs in the period from June 15 and 26, 2015 for the first time from 1989; from the measurements of the INTEGRAL observatory (60-300 keV) it was even stronger, up to 50 crabs, i.e., several thousand times higher than in the quiescent state that lasted for 26 years.
    We have daily detected radio emission at five frequencies (2.3, 4.6, 8.5, 11.2, and 21.7 GHz) from June 18 to July 12 with the RATAN-600 radio telescope. No other radio telescope worldwide carried out multi-frequency measurements. Unfortunately, the VLBI observations were conducted before maximal radio brightness and did not reveal the object's extended structure. At the beginning of the activity period the measured fluxes varied from 50 to 500 mJy with the spectral index of synchrotron emission varying from +0.65 to -0.8 from day to day. On June 26, several hours after the bright X-ray outburst, the radio flux from V404 Cyg increased 30-40 times from 0.05 Jy in the previous day to almost 4 Jy exceeding the maximum flux in the burst of 1989 several times. It should be noticed that the level of 4 Jy is very high for this kind of objects and especially for radio stars. The bursts greater 4 Jy occur only in the microquasar Cyg X-3 (the record is 25 Jy). The radio spectrum of this radio burst corresponded to optically thin synchrotron radiation at frequencies higher than 4 GHz (the spectral index is -0.3) and to optically thick synchrotron radiation in selfabsorption (the spectral index is -2.5) at lower frequencies. Probably, we detected the very beginning of the formation the relativistic jet from the poles of accretion disk around the black hole. Upon 14 days the radio flux fell below the detection level at the RATAN-600 in relation to the decrease of the X-ray flux.
    It is remarkable that in December 2015 we again detected the radio flux at the level of 30-70 mJy from the object that became brighter first in the X-ray (to 5 crabs) and then by 2-3 magnitudes in the optical range.
    (Adapted from the Astronomer's Telegrams (ATels): #7667, #7716, #8454, and #8482 and from the report at the "High Energy Astrophysics" Conference, IKI, December 21-25, 2015).
S.A.Trushkin, N.A.Nizhelskij, P.G.Tsybulev