HST ultraviolet observations of rapid variability in the accretion-disc wind of BZ Cam

Abstract

We present results from time-resolved Hubble Space Telescope(HST) GHRS ultraviolet spectroscopy of the nova-like cataclysmic variable BZ Cam. Extensive changes are recorded in the extended absorption troughs of the UV resonance line profiles, which betray a highly unsteady and continuously variable supersonic outflow. The large-scale line-profile changes are almost exclusively confined to blueshifted velocities, although the Si iv and C iv emission components reveal orbital phase-linked velocity shifts. The character of the accretion-disc wind of BZ Cam can alter radically on time-scales down to 100 s or less. Correlated behaviour is noted in lines of low and high ionization stages, which may reflect localized density changes. Aside from the incidence of enormous line-profile changes on remarkably short time-scales, BZ Cam provides an important benchmark for the simultaneous presence of very strong absorption and well-developed emission components in Si iv, C iii and C iv, for an estimated low-inclination (i<40°) or possibly even face-on system. We illustrate the problems posed for pure-scattering line-synthesis models in matching the UV line profiles of BZ Cam, and set constraints for alternative thermal emission sources. A phenomenological interpretation is that the high-speed wind of BZ Cam is highly irregular in nature, and the outflow is riddled with structure arising from localized density enhancements. It is tempting to link our empirical results to recent hydrodynamic simulations of unsteady radiatively driven accretion-disc winds, such as those of Proga, Drew & Stone. Ultimately, line profiles synthesized from the hydrodynamic codes would have to match the substantial absorption equivalent width changes in the UV lines of BZ Cam, over the very short flow times across the line formation regions, i.e. the overall contribution of the, structured, disc-projected wind has to be substantially variable.