An accurate geometric distance to the compact binary SS Cygni vindicates accretion disc theory

TL;DR

This paper reports a precise, model-independent distance measurement to SS Cygni, resolving previous discrepancies and confirming the validity of accretion disc theory for dwarf novae.

Contribution

It provides an accurate distance to SS Cygni using VLBI radio observations, supporting existing accretion disc models.

Findings

Distance to SS Cygni is 114 +/- 2 pc, closer than previous estimates.

Reconciles observed luminosity with accretion disc theory.

Supports the thermal-viscous instability explanation for dwarf novae outbursts.

Abstract

Dwarf novae are white dwarfs accreting matter from a nearby red dwarf companion. Their regular outbursts are explained by a thermal-viscous instability in the accretion disc, described by the disc instability model that has since been successfully extended to other accreting systems. However, the prototypical dwarf nova, SS Cygni, presents a major challenge to our understanding of accretion disc theory. At the distance of 159 +/- 12 pc measured by the Hubble Space Telescope, it is too luminous to be undergoing the observed regular outbursts. Using very long baseline interferometric radio observations, we report an accurate, model-independent distance to SS Cygni that places the source significantly closer at 114 +/- 2 pc. This reconciles the source behavior with our understanding of accretion disc theory in accreting compact objects.