X-ray spectral evolution of GS 2023+338 (V404 Cyg) during decline after outburst

We have reanalysed archival Ginga data of the soft X-ray transient source G S 2023+308 (V404 Cyg) covering the decline phase of its 1989 May outburst. Our spectral modelling includes the relativistically smeared Compton-reflec ted continuum and iron K alpha fluorescent line near 6.5 keV produced by X- ray illumination of the accretion disc. This gives a powerful diagnostic of the accretion geometry, with the amplitude of the reprocessed components s howing the solid angle subtended by the disc, while the detailed shape of t he relativistic smearing shows how close this material is to the black hole event horizon. We find that reflection is always significantly present in the spectra, but that its fractional contribution decreases as the decline progresses. The amount of relativistic smearing is also consistent with decreasing during t he decline, although this is poorly constrained except for the brightest sp ectra. One plausible scenario explaining this evolution is of an optically thick disc with inner radius increasing as a function of time, with the X-r ay source in the form of a central corona. This is similar to the evolution inferred for other X-ray transient sources, such as Nova Muscae, except th at the underlying power-law spectrum of GS 2023+338 stayed constant as the disc geometry changed. This challenges the underlying assumption of almost all models for the spectra of accreting black holes, namely that the hard X -rays are formed by Comptonization of seed photons from the accretion disc.