Hubble Space Telescope observations of the black hole X-ray transient GRO J0422+32 near quiescence

Abstract

We present HST/FOS ultraviolet and optical spectroscopy of the black hole X-ray transient GRO J0422+32 shortly before the system reached quiescence. We find that the accretion spectrum from 2500 to 9000 A can be very well fitted by a self-absorbed synchrotron model, with superposed H I and Mg II emission lines. The explanations that we suggest for this spectrum are either that it is due to active coronal regions above a geometrically thin accretion disc, or that the disc is evaporated into an advective flow.