RADIO OBSERVATIONS OF THE 1989 TRANSIENT EVENT IN V404 CYGNI (=GS 2023+338)

The bright transient X-ray source GS 2023 + 338, now identified with the 1938 " nova " designated V404 Cyg, was observed between 1989 May 30 and 1991 May 31 with the VLA at 1.49, 4.9, 8.4, and 14.9 GHz. A strong synchrotron-radiation source was initially present in optically thin decay, followed by a relatively slower decay, for approximately 150 days, proportional to approximately(t - t0)-beta, with beta = 1.2 and 0.83 for 1.49 GHz and three high frequencies, respectively. The radio spectrum evolved from an optically thin synchrotron radio source during the first 2 days to flat and then "inverted," with a strong overall tendency to become more and more inverted during the remaining observations. The linear polarization detected during the first 50 days of observation confirms that the radiation is synchrotron emission. The mostly constant polarization angles indicate the stability of the magnetic field configuration for the emitting regions. Most observations showed significant variations on times scales of a few minutes to hours. Evolution between flat and inverted spectrum states occurred on time scales of hours. The one continuous observation made at 14.9 GHz shows sinusoidal variations with a period of approximately 22 minutes, while five continuous observations at 8.4 GHz are dominated by variations on 54-120 minute time scales. A neutral hydrogen absorption experiment performed 1989 June 3, when the source was 0.072 Jy, determined an H I column density of almost-equal-to 5 x 10(21) atoms per cm-2. From H I absorption and the reddening of the optical spectrum, the distance of source is approximately 3 kpc. The unique aspect of this V404 Cyg event is the slow decay component observed over a span of 2 yr, with coupled radio, optical, and X-ray emission and very localized surface brightness fluctuations.