A review of the disc instability model for dwarf novae, soft X-ray transients and related objects (1910.01852v1)

Abstract: I review the basics of the disc instability model (DIM) for dwarf novae and soft-X-ray transients and its most recent developments, as well as the current limitations of the model, focusing on the dwarf nova case. Although the DIM uses the Shakura-Sunyaev prescription for angular momentum transport, which we know now to be at best inaccurate, it is surprisingly efficient in reproducing the outbursts of dwarf novae and soft X-ray transients, provided that some ingredients, such as irradiation of the accretion disc and of the donor star, mass transfer variations, truncation of the inner disc, etc., are added to the basic model. As recently realized, taking into account the existence of winds and outflows and of the torque they exert on the accretion disc may significantly impact the model. I also discuss the origin of the superoutbursts that are probably due to a combination of variations of the mass transfer rate and of a tidal instability. I finally mention a number of unsolved problems and caveats, among which the most embarrassing one is the modelling of the low state. Despite significant progresses in the past few years both on our understanding of angular momentum transport, the DIM is still needed for understanding transient systems.

• The paper reviews the Disc Instability Model (DIM), explaining its success in simulating outbursts in systems like dwarf novae and soft X-ray transients despite limitations in modeling angular momentum transport.
• The DIM posits thermal-viscous instabilities trigger outbursts, but requires enhancements like irradiation and struggles to fully reconcile magnetic angular momentum transport (MRI) with observations.
• Significant unresolved issues include understanding the low accretion state and improving DIM's physical basis by integrating advanced MHD simulations and incorporating disc winds and new observational data.

Overview of the Disc Instability Model in Astrophysical Transients

The paper by J.M. Hameury provides an extensive review of the Disc Instability Model (DIM) as it applies to dwarf novae, soft X-ray transients, and related astrophysical systems. DIM has been instrumental in understanding the behavior of cataclysmic variables (CVs) that show quasi-periodic outbursts, such as dwarf novae and certain low-mass X-ray binaries (LMXBs). Despite relying on the Shakura-Sunyaev viscosity prescription, which is not entirely accurate in its representation of angular momentum transport, DIM has shown surprising efficacy in simulating outburst behavior in these systems, provided certain complexities are incorporated into the model.

Key Components and Recent Developments

The DIM posits that outbursts occur due to thermal-viscous instabilities in the accretion disc, triggered when the disc's temperature reaches levels at which hydrogen ionizes. To accurately simulate observed phenomena, enhancements such as disc irradiation, mass transfer variations, and disc truncation are necessary. Furthermore, recent insights have suggested that winds and associated torques can significantly affect disc stability, demanding reconsideration of DIM's foundational assumptions.

A substantial issue is the efficiency of the Shakura-Sunyaev prescription in capturing the dynamics of angular momentum transport. Magnetic mechanisms like the magnetorotational instability (MRI) have been recognized as key to this process, but numerical simulations and the reproduction of observed viscous time scales present challenging discrepancies. These have prompted investigation into supplementary non-local transport processes such as spiral shocks.

Theoretical Implications and Unresolved Issues

While DIM has clarified much of the behavior of dwarf novae, various unsolved problems persist, especially concerning the low state of accretion discs. MRI's effectiveness appears diminished in these cooler states, prompting exploration into alternative physical processes, such as hydrodynamical instabilities and wind-driven angular momentum transport. The question of how angular momentum transport fundamentally operates within these systems remains an open field of research.

DIM also struggles to wholly elucidate specific behaviors observed in different CV sub-classes such as the IW Andromedae-like phenomenon and stunted outbursts, potentially requiring revisions to models of mass transfer variability and exploration of underlying mechanisms.

Constraints and Future Directions

Despite its successes, DIM's predictive power relies heavily on the free parameters introduced to accommodate observational attributes, such as disc irradiation and magnetic field effects. Speculative projections into the physical grounds of DIM suggest considerable room for refinement, particularly in:

• Integration of more nuanced magneto-hydrodynamic simulations
• Improved understanding of how magnetic fields and disc winds shape angular momentum profiles
• Enhanced computational models to simulate finer details of disc structure and its evolution

Progress in observational techniques, such as improved Doppler tomography, can further test DIM's predictive capacity by offering more precise insights into accretion disc dynamics. As data from missions like Kepler and others continue to provide high-resolution temporal observations of these systems, they present opportunities to refine existing frameworks or develop entirely new models for predicting disc behaviors.

In conclusion, while DIM has been pivotal in extending our understanding of transient accretion phenomena, ongoing research efforts must aim to resolve its fundamental physical uncertainties and address its computational constraints for it to maintain relevance in astrophysical modeling.