Temporal resolution of a pre-maximum halt in a classical nova: V5589 Sgr observed with STEREO HI-1B

Classical novae show a rapid rise in optical brightness over a few hours. Until recently the rise phase, particularly the phenomenon of a pre-maximum halt, was observed sporadically. Solar observation satellites observing coronal mass ejections enable us to observe the premaximum phase in unprecedented temporal resolution. We present observations of V5589 Sgr with STEREO HI-1B at a cadence of 40 min, the highest to date. We temporally resolve a pre-maximum halt for the first time, with two examples each rising over 40 min then declining within 80 min. Comparison with a grid of outburst models suggests that this double peak, and the overall rise time-scale, is consistent with a white dwarf mass, central temperature and accretion rate close to 1.0M circle dot, 5 x 107 K and 10-10M circle dot yr(-1), respectively. The modelling formally predicts mass-loss onset at JD 245 6038.2391 +/- 0.0139, 12 h before the optical maximum. The model assumes a main-sequence donor. Observational evidence is for a subgiant companion, meaning that the accretion rate is underestimated. Post-maximum, we see erratic variations commonly associated with much slower novae. Estimating the decline rate is difficult, but we place the time to decline two magnitudes as 2.1 < t(2)(d) < 3.9, making V5589 Sgr a 'very fast' nova. The brightest point defines 'day 0' as JD 245 6038.8224 +/- 0.0139, although at this high cadence the meaning of the observed maximum becomes difficult to define. We suggest that such erratic variability normally goes undetected in faster novae due to the low cadence of typical observations, implying that erratic behaviour is not necessarily related to the rate of decline.