Repetitive rebrightening of EG Cancri: Evidence for viscosity decay in the quiescent disk?

Abstract

A WZ Sge-type dwarf nova, EG Cancri, exhibited six consecutive mini-outbursts with a mean interval of about seven days after the end of the main outburst in 1996/1997. Most unusual was that the star abruptly entered into a deep faint minimum after such frantic activities. We propose that this peculiar phenomenon may be understood in terms of viscosity decay in the cold disk. In this picture, the viscosity is produced by MHD turbulence due to the magneto-rotational instability ("Balbus-Hawley instability") and dies down exponentially with time when the disk becomes cold because the magnetic fields decay due to finite conductivity in the cold disk (Gammie & Menou 1998). But the viscosity is refreshed to a high value every time when a mini-outburst occurs (i.e., the disk becomes hot again). It is argued that a sudden cessation of repetitive mini-outbursts may be brought about by a very small reduction in viscosity or a small increase in its decay rate, which may in turn be produced most likely by stochastic fluctuations of magnetic fields. Numerical simulations based on a simple model reproduce the observed light curve of EG Cancri very well. We discuss possible causes why the reflares after the main outburst occur mostly in the WZ Sge-type stars.