Mass-transfer Outburts reborn: Modeling the light curve of the dwarf nova EX Draconis

Abstract: EX Draconis is an eclipsing dwarf nova that shows outbursts with moderate amplitude ($\simeq 2$ mag) and a recurrence timescale of $\simeq 20$-30 d. Dwarf novae outbursts are explained in terms of either a thermal-viscous instability in the disc or an instability in the mass transfer rate of the donor star (MTIM). We developed simulations of the response of accretion discs to events of enhanced mass transfer, in the context of the MTIM, and applied them to model the light curve and variations in the radius of the EX Dra disc throughout the outburst. We obtain the first modeling of a dwarf nova outburst by using $\chi^2$ to select, from a grid of simulations, the best-fit parameters to the observed EX Dra outbursts. The observed time evolution of the system brightness and the changes in the radius of the outer disc along the outburst cycle are satisfactorily reproduced by a model of the response of an accretion disc with a viscosity parameter $\alpha = 4.0$ and a quiescent mass transfer rate $\dot{M}_2 (\textrm{quiescence}) = 4.0 \times 10^{16}$ g/s to an event of width $\Delta t = 6.0 \times 10^5$ s ($\sim 7$ d) where the mass-transfer rate increases to $\dot{M}_2 (\textrm{outburst}) = 1.5 \times 10^{18}$ g/s.