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Accreting White Dwarfs: An Unreview

Simone Scaringi, Christian Knigge, Domitilla de Martino

Abstract

Accreting white dwarfs (AWDs) are among the best natural laboratories for understanding disk accretion. Their proximity, brightness, and purely classical nature make them ideal systems in which to probe the fundamental physics that governs the transport of angular momentum, the generation of outflows, and the coupling between disks, magnetospheres, and accretors. Yet despite decades of study, many critical questions remain unresolved. In this ``unreview'', we therefore focus not on what is known, but on what is unknown. What drives viscosity and sustains accretion in largely neutral disks? How are powerful winds launched, and how do they feed back on the disk and binary evolution? Why do so many systems show persistent retrograde precession, and what drives bursts in magnetic AWDs? By identifying these open problems -- and suggesting ways to resolve them -- we aim to motivate new observational, numerical, and theoretical efforts that will advance our understanding of accretion physics across all mass scales, from white dwarfs to black holes.

Accreting White Dwarfs: An Unreview

Abstract

Accreting white dwarfs (AWDs) are among the best natural laboratories for understanding disk accretion. Their proximity, brightness, and purely classical nature make them ideal systems in which to probe the fundamental physics that governs the transport of angular momentum, the generation of outflows, and the coupling between disks, magnetospheres, and accretors. Yet despite decades of study, many critical questions remain unresolved. In this ``unreview'', we therefore focus not on what is known, but on what is unknown. What drives viscosity and sustains accretion in largely neutral disks? How are powerful winds launched, and how do they feed back on the disk and binary evolution? Why do so many systems show persistent retrograde precession, and what drives bursts in magnetic AWDs? By identifying these open problems -- and suggesting ways to resolve them -- we aim to motivate new observational, numerical, and theoretical efforts that will advance our understanding of accretion physics across all mass scales, from white dwarfs to black holes.
Paper Structure (10 sections, 11 equations, 5 figures)

This paper contains 10 sections, 11 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Which angular momentum transport (or loss) mechanisms drive disk accretion in AWDs?
  3. How do accretion disk winds fit into our physical picture of AWDs?
  4. Do accreting white dwarfs drive (radio) jets?
  5. What triggers and sustains the misalignment and retrograde precession of disks in AWDs?
  6. Do AWDs contain a hot "corona"?
  7. Where and how are disks truncated by the effects of spinning magnetospheres in AWDs?
  8. What triggers outbursts in Intermediate Polars?
  9. Can AWDs be used as universal accretion laboratories?
  10. Conclusion

Figures (5)

  • Figure 1: Schematic taxonomy of accreting white dwarfs (AWDs). The diagram outlines the main subclasses: dwarf novae, nova-like variables, and intermediate polars. Systems discussed in this review are shown in red. These represent the "vanilla" disk-accreting white dwarfs with main-sequence donors and no steady nuclear burning, serving as benchmark laboratories for accretion physics.
  • Figure 2: The Mach number as a function of disk radius in several simple accretion disk models with parameters appropriate for AWDs. For the steady-state (Shakura-Sunyaev) disks, we show the Mach numbers corresponding to both the central and effective disk temperature. In the isothermal models, the temperature is assumed to be constant throughout the disk.
  • Figure 3: The C iv$\lambda\lambda 1550$ line in the NL variable RW Sex. This P Cygni profile, with its strong, blue-shifted absorption, was one of the first clear indications that high-state AWDs must drive powerful disk winds greenstein82
  • Figure 4: Comparison between Type-I X-ray bursts and micronovae. The top two panels (a & b) show TESS lightcurves the rapid bursts observed in TV Col and ASASSN$-$19bh scaringi22a. The bottom two panels (c & d) show X-ray lightcurve of 4U 1636$-$536 observed with EXOSAT-ME and SAX J1808.4$-$3658 observed with RXTE-PCA X-ray. Similarities in the lightcurve morphology between the TV Col and 4U 1636-536 (e.g. multi-peaked burst) and between ASASSN$-$19bh and SAX J1808.4$-$3658 (e.g. pre-cursor) are obvious. scaringi22a
  • Figure 5: Observed PSD break frequencies for accreting black holes and white dwarfs compared to the semi-empirical scaling from Eq. \ref{['eq:3']} (black line). The consistency across ten orders of magnitude in mass may suggest a near-universal relation linking variability timescales and accretion power. Figure adapted from scaringi15.