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

Citation
Y. Osaki et al., Repetitive rebrightening of EG Cancri: Evidence for viscosity decay in thequiescent disk?, ASTRON ASTR, 370(2), 2001, pp. 488-495
Citations number
40
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Space Sciences
Journal title
ASTRONOMY AND ASTROPHYSICS
ISSN journal
0004-6361 → ACNP
Volume
370
Issue
2
Year of publication
2001
Pages
488 - 495
Database
ISI
SICI code
0004-6361(200105)370:2<488:RROECE>2.0.ZU;2-X
Abstract
A WZ Sge-type dwarf nova, EC Cancri, exhibited six consecutive mini-outburs ts with a mean interval of about seven days after the end of the main outbu rst in 1996/1997. Most unusual was that the star abruptly entered into a de ep faint minimum after such frantic activities. We propose that tills pecul iar phenomenon may be understood in terms of viscosity decay in the cold di sk. 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 fi elds 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-ou tburst occurs (i.e., the disk becomes hot again). It is argued that a sudde n cessation of repetitive mini-outbursts may be brought about by a very sma ll reduction in viscosity or a small increase in its decay rate, which may in turn be produced most likely by stochastic fluctuations of magnetic fiel ds. Numerical simulations based on a simple model reproduce the observed li ght curve of EC Cancri very well. We discuss possible causes why the reflar es after the main outburst occur mostly in the WZ Sge-type stars.