Accretion disc outbursts: a new version of an old model

Abstract

We have developed 1D time-dependent numerical models of accretion discs, using an adaptive grid technique and an implicit numerical scheme, in which the disc size is allowed to vary with time. The code fully resolves the cooling and heating fronts propagating in the disc. We show that models in which the radius of the outer edge of the disc is fixed produce incorrect results, from which probably incorrect conclusions about the viscosity law have been inferred. In particular we show that outside-in outbursts are possible when a standard bimodal behaviour of the Shakura-Sunyaev viscosity parameter α is used. We also discuss to what extent insufficient grid resolutions have limited the predictive power of previous models. We find that the global properties (magnitudes, etc. ...) of transient discs can be addressed by codes using a high, but reasonable, number of fixed grid points. However, the study of the detailed physical properties of the transition fronts generally requires resolutions which are out of reach of fixed grid codes. It appears that most time-dependent models of accretion discs published in the literature have been limited by resolution effects, improper outer boundary conditions, or both.

Echo outbursts in KS 1731–260

“…An increase ofṁ tr is expected to give rise to a decrease of T C accompanied by an increase of the peak luminosity of the outburst (e.g. the model by Hameury et al 1998). This contradicts our observations.…”

“…The disk viscosity is lower in the cold state than in the hot state (e.g. Hameury et al 1998;Dubus et al 2001). A larger zone brought to the cool state thus requires more matter to accumulate in the disk to reach Σ max .…”

Hydrogen-poor Disks in Compact X-Ray Binaries

“…We neglect X-ray irradiation Ϫ Q when calculating the thermal equilibria (see § 5). The thermal equilibria are calculated from an extended grid of detailed models for the disk vertical structure, for different values of the surface density S and the central temperature , at a given radius R T c from the central object of mass (see Hameury et al 1998 for M 1 details on the numerical technique employed). Once a solution to the vertical structure (corresponding to a given S, , and R) T c is known, the disk effective temperature is uniquely deter-T eff mined.…”

“…[22] of Hameury et al 1998). We note, however, that Gu, Vishniac, & Cannizzo (2000) have argued for weak convection in thin accretion disks because of the action of shearing and turbulent radial mixing.…”

“…It has long been known that such hydrogen-rich disks are subject to a thermal-viscous instability occurring when hydrogen becomes partially ionized at central (midplane) Meyer & Meyer-Hofmeister 1981;see, e.g., Cannizzo 1993a, Ludwig, Meyer-Hofmeister, & Ritter 1994, and Hameury et al 1998 for detailed calculations).…”