Optical identification of the X-ray source GS 2023 + 338 AS V404 CYGNI

On 1989 May 22 the Japanese Ginga Team reported the discovery of a bright new X-ray transient and cataloged the source as GS 2023 + 338. We have identified this X-ray source with V404 Cygni, an optical nova whose last reported major outburst was in 1938. Photometry with the fine error sensor on the IUE satellite on 1989 May 27.3 gave V ~∼12.6, ∼ 6 mag brighter than the quiescent magnitude. Optical spectroscopy on 1989 May 27.4 revealed a strong and rich emission-line spectrum superposed on a reddened continuum. The outburst spectrum shows strong Balmer, He I, He II λ4686, and N II λλ4634-42 emission lines as well as numerous interstellar absorption lines. The near-ultraviolet O III Bowen fluorescence lines at 3429-3444 Å and possibly the complex between 3750-3790 Å are present in these early spectra. These emission features are characteristics of the optical counterparts of various X-ray sources including X-ray bursters and two other X-ray novae: A0620-00 (V616 Mon) and Cen X-4. Photometrically, the source exhibits fluctuations of a magnitude or more on time scales of days, minutes, and possibly seconds. The optical light curve exhibits a remarkable similarity to the X-ray light curve suggesting that X-ray heating of an accretion disk plays a role in producing the optical emission. The X-ray behavior and the presence of strong optical emission lines in V404 Cygni is in contrast to that of other X-ray transients and two other well-studied X-ray novae, V616 Mon and Cen X-4. A considerably higher outburst luminosity in V404 Cygni as compared with these other X-ray novae may account for this difference.