The eclipsing accreting white dwarf Z chameleontis as seen with TESS

Abstract

We present results from a study of TESS observations of the eclipsing dwarf nova system Z Cha, covering both an outburst and a superoutburst. We discover that Z Cha undergoes hysteretic loops in eclipse depth -out-of-eclipse flux space in both the outburst and the superoutburst. The direction that these loops are executed in indicates that the disk size increases during an outburst before the mass transfer rate through the disk increases, placing constraints on the physics behind the triggering of outbursts and superoutbursts. By fitting the signature of the superhump period in a flux-phase diagram, we find the rate at which this period decreases in this system during a superoutburst for the first time. We find that the superhumps in this source skip evolutionary stage "A" seen during most dwarf nova superoutbursts, even though this evolutionary stage has been seen during previous superoutbursts of the same object. Finally, O-C values of eclipses in our sample are used to calculate new ephemerides for the system, strengthening the case for a third body in Z Cha and placing new constraints on its orbit.

Discovery of a new WZ Sagittae-type cataclysmic variable in the Kepler/K2 data

“…13. We find a similar broken rise in and Z Cha from TESS (Court et al 2019). The detection of slow, linear precursors in these Figure 11.…”

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

“…During the latter stages of the outburst, the eclipses generally have lower eclipse fractions than during the outburst rise, resulting in a generally clockwise 5 hysteretic loop in the parameter space as defined here. Notably, this is different to the behaviour seen in Z Cha, in which eclipses execute a generally anticlockwise hysteretic loop in the same parameter space over the course of an outburst (Court et al 2019).…”

“…In Figure 2 we show how our algorithm decomposes the lightcurve from Sector 14 into smoothed and detrended components. Note that the eclipse-isolating algorithm we use here is different to the method used in Court et al (2019) to isolate eclipses in the AWD Z Cha. In that paper, the authors remove the eclipses from the lightcurve and fit splines across the resulting data gaps to interpolate what the flux would be at each eclipse midpoint if no eclipse occured.…”

“…However in EX Dra the hotspot varies significantly during quiescence (e.g. Golysheva et al 2015), as does the depth of the secondary eclipse and the brightness of the flux between the main and secondary eclipses, which would significantly contaminate our estimates for out-of-eclipse flux for each eclipse if we used the method of Court et al (2019).…”

“…In both Sectors, the eclipses show significant deviation below the line of maximum eclipse during outburst, although the magnitude of this deviation is significantly smaller than those seen in eclipses of the CVs KIS J192748.53+444724.5 and Z Cha (Scaringi et al 2013;Court et al 2019). In order to better quantify the hysteresis shown in this parameter space, we can divide the depth of each eclipse by the predicted maximum eclipse depth, thus obtaining the fractional depth ∆ of each eclipse:…”