Disk instability model incorporating a variable inner disk radius in SS Cyg and U Gem

Abstract

Previous theoretical studies indicate that the inner disk in dwarf novae evaporates into a high-temperature, optically thin, and geometrically thick accretion flow during quiescence, with the inner edge moving toward the white dwarf at the onset of an outburst. We incorporate this process into the numerical model developed by Kimura & Osaki (2023) and test the code on two representative dwarf novae, SS Cyg and U Gem. By modeling the inner accretion flow, we calculate the optical, ultraviolet (UV), and X-ray luminosities. Our results show that evaporation suppresses the inside-out outbursts without requiring a radially dependent viscosity parameter in the cold state. The observed time delay between the rise in UV luminosity and the onset of the optical outburst is more than one day, which is successfully reproduced when the inner disk is truncated at several $\times 10^{9}$ cm in the standard evaporation model. However, while the modeled accretion rate at the inner disk edge in U Gem accounts for its quiescent X-ray luminosity, the rate in SS Cyg remains insufficient. This discrepancy in SS Cyg suggests that SS Cyg may require either more efficient evaporation or an additional mass supply into the coronal cavity via gas-stream overflow. By accounting for disk evaporation, our simulations offer a refined version of the disk instability model for dwarf nova outbursts that naturally explains the observed multiwavelength light curves.