Abstract
Dwarf novae (DNs) are mass-exchanging binaries showing repeated outbursts, lasting from days to weeks and recurring on timescales from weeks to years, in which their accretion discs brighten by factors 20-100 either because of a thermal-viscous instability cycle in the accretion disc (the DI model) or as a consequence of an instability in the mass-donor star leading to a burst of enhanced mass-transfer (the MTI model). While the issue seemed to be settled in favor of the DI model, the last decade has progressively provided compelling evidence in support of the idea that there is a group of DN the outbursts of which are powered by MTI. V2051 Oph is one of the DNs yielding stronger evidence in favor of the MTI (Baptista et al. 2007). Here we report eclipse mapping analysis of velocity-resolved (|v|= 400-1000 km/s) Hβ, HeI λ 4922 and nearby continuum light curves of V2051 Oph on 4 consecutive nights along its 2002 July outburst, based on spectroscopy collected with the 1.5 m ESO telescope. The outburst starts with a ring of enhanced emission at the circularization radius, which spreads inwards and outwards with velocities of ≥ -0.9 km/s and +0.2 km/s, respectively, to form an extended bright disc in less than a day. The outburst maximum Hβ map shows two asymmetric arcs reminiscent of the spiral arms seen in other outbursting dwarf novae. Assuming a distance of 108 pc, the disc temperatures at outburst maximum barely reach the critical temperature above which the gas should be while in outburst according to DI model, and remain below that limit on all other nights. The results are at odds with predictions of the DI model, but are in good agreement with the expected response of a viscous disc to a burst of dense, enhanced mass-accretion through its sparse outer regions.
