EX draconis: Using eclipses to separate outside-in and inside-out outbursts

Abstract

We present a study of the eclipses in the accreting white dwarf EX draconis (EX Dra) during TESS Cycles 14 and 15. During both of the two outbursts present in this data set, the eclipses undergo a hysteretic loop in eclipse-depth/out-of-eclipse-flux space. In each case, the direction in which the loops are executed strongly suggests an outburst that is triggered near the inner edge of the accretion disc and propagates outwards. This in turn suggests that the outbursts in EX Dra are 'inside out' outbursts; events predicted by previous hydrodynamic studies of dwarf nova accretion discs and confirmed spectroscopically in a number of other accreting white dwarf systems. We therefore propose that the direction of the loop executed in eclipse-depth/out-of-eclipse flux space be used as a test to phenomenologically distinguish between 'inside out' and 'outside in' outbursts in other eclipsing dwarf novae; a reliable and purely photometric test to differentiate between these phenomena.