Testing the line‐driven disc wind model: time‐resolved ultraviolet spectroscopy of IX Vel and V3885 Sgr

To confront the predictions of the most recent line‐driven disc wind models with observational evidence, we have obtained Hubble Space Telescope ( HST ) STIS (1180–1700 Å) echelle spectra of the nova‐like variables IX Vel and V3885 Sgr at three epochs. The targets were observed in time‐tag mode for ∼2000 s on each occasion, allowing us to study the spectral time evolution on time‐scales down to ∼10 s. The mean ultraviolet (UV) spectra are characterized by the wind signature of broad blueshifted absorption in Ly α , N  v   λ 1240, Si  iv   λ 1398, C  v   λ 1549 and He  ii   λ 1640. There is very little redshifted emission other than in C  iv . Narrow blueshifted absorption dips, superposed on the broad absorption at around −900 km s −1 , accompany periods of well‐developed wind activity. The continuum level and mean line profiles vary markedly from observation to observation – with the wind signatures almost disappearing in one epoch of observation of IX Vel. The strong positive correlation between UV brightness and wind activity predicted by line‐driven disc wind models is disobeyed by both binaries. The wind signatures in the UV spectrum of IX Vel are revealed to be remarkably steady on time‐scales ranging from ∼10 to ∼1000 s. More variability is seen in V3885 Sgr, the binary with the lower opacity outflow. However, there is only one epoch in which the line profile changes significantly in ∼100 s or less. Narrow absorption dips, when present, show only smooth, small changes in velocity. We surmise that these may trace the orbital motion of the white dwarf. The near‐absence of line profile variability on the shorter 10‐ to 100‐s time‐scales, and the lack of correlation between wind activity and luminosity, could both arise if a non‐radiative factor such as the magnetic field geometry controls the mass loss rate in these binaries.