Optical studies of V404 Cyg, the X-ray transient GS2023 + 338 - I. The 1989 outburst and decline

Abstract

We present the results of optical spectroscopy and photometry of V404 Cyg, the optical counterpart of the newly discovered X-ray transient GS2023 + 338 by GINGA (Makino et al. 1989). The overall optical light curve is similar to that of an earlier outburst of this object, which was recorded as Nova Cyg 1938 and classified as a classical nova. However, it is not a classical or recurrent nova, as the large range in optical brightness and intense and variable X-ray emission at maximum indicate that the system must be a low-mass X-ray binary. The optical spectrum, however, is unlike any known LMXB or X-ray transient in that it displays strong, broad, variable and complex emission lines of H, He I and He ii. The reddened spectrum and interstellar absorption features indicate a probable distance in the range 1-3 kpc. CCD photometry reveals that the source is variable, but gives no indication of the orbital period, there being no eclipse or regular modulation. Multicomponent fitting to the complex He II λ4686 A line and subsequent analysis of various asymmetry parameters do not show any clear trend or periodicity.