Episodic absorption in the outflow of the old nova V603 Aquilae

We report on the time-dependent behaviour of ultraviolet spectral lines in Hubble Space Telescope Goddard High-Resolution Spectrograph data of the cla ssical nova V603 Aql. In particular, episodic blueshifted absorption (exten ding to similar to -2500 km s(-1)) is present, with a variability time-scal e down to similar to1 min. The data provide a rare opportunity to study the rapid evolution of absorption structures that may be associated with accre tion-disc winds in cataclysmic variables. At least three absorption events are recorded (at blueward velocities only) over similar to5 h, each lasting similar to 10-15 min. The derived velocity, acceleration and optical depth properties provide an empirical picture of stochastically variable structu res in the outflow, with no evidence for short-term (less than similar to 1 h) cyclic or modulated behaviour in the overall absorption properties. In c ontrast, the emission components of the ultraviolet resonance lines are ver y stable in velocity and strength in this low-inclination system. On at lea st two occasions there is an intriguing short-term 'flare' in the ultraviol et continuum flux (of up to similar to 40 per cent). Though there is no cle ar one-to-one relation in these data between the continuum fluctuations and the occurrence of the absorption events, the time-scales for the two varia ble phenomena are essentially the same. The irregular absorption episodes i n the ultraviolet data of V603 Aql presently defy a clear physical interpre tation. Their overall characteristics are discussed in the context of insta bilities in radiation-pressure-driven disc winds.