Revisiting the accretion disc spectra of Dwarf Novae and Novalike variables: implications for the standard disc model

TL;DR

This paper re-analyzes UV spectra of Cataclysmic Variables to investigate differences in their accretion disc spectra, revealing persistent and significant discrepancies that challenge the standard disc model and have broader implications.

Contribution

It provides a revised analysis of UV spectra using updated calibrations, confirming persistent spectral differences between dwarf novae and nova-like CVs, questioning the standard accretion disc model.

Findings

Spectral energy distributions differ significantly between CV types.

Re-analysis confirms the spectral differences are statistically robust.

Implications extend to other accreting systems like X-ray binaries and AGN.

Abstract

Accretion discs are fundamental to much of astronomy. They can occur around stars both young and old, around compact objects they provide a window into the extremes of physics, and around supermassive black holes in galaxy centres they generate spectacular luminosities that can outshine the entire galaxy. However, our understanding of the inner workings of accretion discs remains far from complete. Here we revisit a conundrum in the observations of some of the simplest accreting systems; the Cataclysmic Variables (CVs). The high-accretion-rate states of (non-magnetic) CVs can be divided into the short-lived outbursts ($\sim$ a week) typical of dwarf novae (DNe) and the long-lived (and sometimes perpetual) high states of nova-like (NL) CVs. Since both sorts of high-state occur in approximately steady-state accretion discs with similar properties and accretors, we would expect them to display similar spectral energy distributions. However, previous analyses based on UV spectra from the {\it International Ultraviolet Explorer} have shown that their spectral energy distributions are different. We perform a re-analysis of the data using up to date calibrations and distance (and thus dereddening) estimates to test whether this difference persists and whether it is statistically significant over the sample. We find that it does persist and it is statistically significant. We propose routes to investigating this discrepancy further and discuss the implications this has for other accreting systems, such as X-ray binaries, Active Galactic Nuclei and protoplanetary discs.