An Accurate Geometric Distance to the Compact Binary SS Cygni Vindicates Accretion Disc Theory

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Science  24 May 2013:
Vol. 340, Issue 6135, pp. 950-952
DOI: 10.1126/science.1237145

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For Good Measure

SS Cygni is a well-studied binary star system in the northern constellation Cygnus, consisting of a white dwarf that accretes matter from its companion star. Miller-Jones et al. (p. 950; see the Perspective by Schreiber) used radio observations to derive a model-independent distance to this prototypical accreting white dwarf system. The measurement places the system significantly closer than previously determined, reconciling the observed properties of SS Cygni with our current understanding of accretion theory.


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 parsecs 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 substantially closer at 114 ± 2 parsecs. This reconciles the source behavior with our understanding of accretion disc theory in accreting compact objects.

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