TESS Reveals Details of KU Cygni’s Accretion Disk

KU Cygni is a long-period (~38.4 days) interacting Algol-type eclipsing binary star known to exhibit strong double-peaked hydrogen emission lines due to its relatively persistent accretion disk. However, it has also been shown to undergo episodic dimming events arising from a thickening of the disk, followed by brightening as dust presumably migrates inward through the disk and evaporates. This makes KU Cyg an excellent natural laboratory for observing dynamic physical processes within its accretion disks and for investigating the disk’s physical properties such as viscosity.

We present high-cadence TESS photometry and identify rapid variability (flickering) that is likely due to transits of high density/opacity disk regions across the line of sight to the primary star, and we support these observations with coinciding ground-based spectroscopy and multi-band photometry. We show that the secondary star is not significantly variable and we identify a correlation between the observed photometric variations and the strengths of certain absorption lines associated with the primary star, such as Fe II.